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Polarography of aliphatic compounds
Talanta, 1965. Vol. 12, pp. 1065 to 1079.
Pergamon Press Ltd.
POLAROGRAPHY Department
Printed in Northern Ireland
OF ALIPHATIC COMPOUNDS
HENNING LUND of Chemistry, University of Aarhus, Aarhus, Denmark
(Received 25 January 1965.Accepted 14 April 1965) Summary_-A review of the polarography presented.
of aliphatic compounds
is
INTRODUCTION
A MAJORadvantage of the polarographic method is its selectivity, which often makes the determination of a compound in a mixture possible without prior separation. The selectivity of the polarographic determination is based on the differences in electrode potential necessary for the occurrence of different electrode reactions. In order to judge which compounds may be polarographically active and which substances may interfere with their determination, a knowledge of the electrode reactions, and the factors which influence these reactions, is necessary. In the following discussion of the polarography of aliphatic compounds emphasis is laid on the nature of the electrode reactions, and examples of compounds undergoing these reactions are mentioned. In some of the more important applications the merits of the polarographic determination compared to other methods are discussed. Often, however, no one of several applicable analytical methods is clearly preferable, and the choice will depend on the special circumstances or the personal preference of the analyst. ELECTRODE
REACTIONS
A polarographic reduction consists of a transfer of electrons from the cathode to the reducible system coupled with the uptake of the appropriate number of hydrogen ions. The high potential gradient,l about 108 V. cm-l, in the immediate vicinity of the electrode polarises the reducible molecule and makes the attack of one or more electrons on the reactive centre possible. Although the reaction results in a reduction at a certain place in the molecule it is the properties of the whole molecule which determines the energy necessary for the transfer of the electrons, Le., the reduction potential. The presence of multiple bonds makes the system more easily reducible, and aliphatic compounds are, therefore, generally more difficult to reduce than aromatic and heteroaromatic compounds. Under fixed conditions the electrode potential determines the course of the electrode reaction; a more negative potential may make further reduction possible. Other factors, however, also influence the electrode reaction and the necessary reduction potential. REACTION
CONDITIONS
Hydrogen ions are involved in most organic electrode reactions and pH may influence not only the reduction potential but also the course of the reaction. The change of electrode reaction with pH may be drastic as in the case of isonicotinic amidea (in acid solution the amide group is reduced to the aldehyde, whereas the pyridine nucleus is reduced in alkaline solution) or slight as in the reduction of 1065 2
H. LUMO
1066
androsta-l,4-dien-17/3-ol-3-one,3 which forms different stereoisomeric pinacols in acid and alkaline solution. Sufficient buffer capacity is, therefore, necessary to ensure that the consumption of hydrogen ions in the electrode reaction does not change the pH in the immediate vicinity of the electrode. In non-aqueous, aprotic media the scarcity of protons may change the reduction path. The reduction of carbon tetrachloride in acetonitrile4 thus yields dichloromethylene, whereas chloroform is formed in most media. The change in the electrode reaction with pH is often caused by a change in the species reduced. If the molecule has protolytic properties the form reduced in acid solution will carry more protons than the one reduced in alkaline solution. The protonated form is always more easily reducible than the unprotonated one. The pH around which the reaction changes from a reduction of the protonated to a reduction of the unprotonated form is often not at the pK of the reducible acid. In the acid-base equilibrium the more easily reducible acid is removed from the equilibrium at the electrode by reduction and the conjugate base combines with hydrogen ions to form the acid, which is then reduced. The height of the wave, i.e., the rate of the electrode reaction, is thus in a certain pH region determined partly by the rate of the recombination reaction at the electrode between the base and protons.” This is a special case of the general one where the electroactive species is formed in the vicinity of the electrode by a chemical reaction slow enough to be the rate controlling step for the over-all reaction. 6 Such waves are called kinetic waves and they can be identified as such by their independence of the height of the mercury reservoir and their high dependence on the temperature. This heavy dependence of the height of the wave on temperature and pH makes, in general, kinetic waves less suited for use in a quantitative determination than a diffusion controlled wave. The concentration of the electroactive compound may influence the electrode reaction. In some instances the reaction involves a slow step before a further reduction, and the intermediate may either be further reduced or react otherwise. The nonelectrochemical reaction is often a dimerisation. The rate of the dimerisation would as a second-order reaction increase faster with concentration than the first-order reduction.’ The polarographically visible result is that the height of the wave grows less with concentration than required by the IlkoviE equations*@ The course of an electrode reaction should preferentially be proved by a preparative electrolytic reduction at controlled potential, where the products can be identified by conventional means. Oftan the occurrence or absence of further reduction waves reveals the reduction path, but such evidence is not always reliable. For instance, a slow chemical step in the reaction may result in a diffusion from the electrode of a product which is then transformed to a reducible compound, which would have yielded a further wave in the polarogram if it had remained at the electrode.lO*ll In the following survey of types of polarographically reducible, aliphatic compound these are classified according to the type of bond suffering reduction in the electrode reaction. CARBON-CARBON
BONDS
Carbon-carbon triple bonds
Only few compounds containing the carbon-carbon triple bond are polarographically reducible. Some derivatives of acetylene-dicarboxylic acid are reduced in
Polarography of aliphatic compounds
1067
two steps, and from the half-wave potential of the second wave the reduction product from the diethyl ester was identified as diethyl fumarate.12 This means that a trans addition of hydrogen to the triple bond has taken place. In the reduction of the acid a dimerisation occurs and from a preparative reduction racemic dimethylsuccinic acid can be isolated.12 Unconjugated acetylenes are not polarographically reducible, but acetylene can on reaction with bromine in acetic acid be transformed into the reducible tetrabromoethane.18 Carbon-carbon double bonds
The isolated carbon-carbon double bond is not polarographically reducible under ordinary conditions, but by suitable conjugation a reducible system may be formed. Conjugation with an electron-withdrawing group, e.g., the cyano, carboxamide, carbethoxy, carboxyl or carbonyl group, renders the system reducible in a convenient potential region, whereas electron-donating groups, e.g., an alkyl group, make the reduction more difficult. Thus, derivatives of a&unsaturated acids, aldehydes and ketones can generally be determined polarographically. This includes compounds such as acrylonitrile,14*1s,16acrylamide,l’ acrylateP and homologues thereof, derivatives of unsaturated dicarboxylic acids, such as maleic and fumaric acids,19*20a&unsaturated carbonyl compounds, such as acrolein,21 citra1,22 a- and /?-ionone22 and methyl vinyl ketone.% The electrode reaction of unsaturated acid derivatives is in general a two-electron reduction of the double bond to the saturated acid. In some cases, e.g., acrylamidel’ and acrylonitrile, apthe number of electrons in the electrode reaction, n, is found to be about 0.2. This low value is explained by a polymerisation of unreduced acrylonitrile, in the vicinity of the electrode, initiated by the initial reduction product, the carbanion, CH,CHCN. Under certain conditions a reductive dimerisation of acrylonitrile to adipic nitrile takes place.26 The reduction of dimethylmaleic anhydride to racemic dimethylsuccinic acid and dimethylfumaric acid to meso dimethylsuccinic acid show that in both cases a trans addition of hydrogen to the double bond takes place.12 The determination of dissolved acrylonitrile in mixtures, e.g., with unsaturated hydrocarbons and saturated nitriles, is faster and more selective than an oxidative titration or a hydrolysis followed by a determination of ammonia and just as accurate. For an analysis of monomeric acrylonitrile in polymeric material the use of N,ZVdimethylformamide as a solvent and tetrabutylammonium bromide as supporting electrolyte is very convenient15 because of the high solubility of the organiccompounds in this solvent. The polarographic determination of a mixture of maleic and fumaric acid is in many cases preferable to other methods because it is more selective than the oxidative [KMnO,, Ce(IV), Br-Cl], conductometric and acidimetric methods, more accurate than the paper chromatographic one and demands less time and material than a method based on chemical separation. The polarographic method requires, however, a rather strict pH control. aa In the presence of interfering electroactive substances it may be necessary to precipitate the acids as their barium salts before the determination.19 The electrode reaction of a&unsaturated carbonyl compounds is a one-electron
1068
H. LUND
reduction to a radical which preferentially dimerises at the B-carbon atom to a saturated diketone, but may form a pinacol when a dimerisation at the /?+zrbon is stericaily unfavourable, In methyl vinyl ketone the radical reacts with mercury, thus forming di(3-ketobutyl) mercury.*s When considering the method of choice for analysis of these compounds it must be remembered that some of them, e.g,, methyl vinyl ketone, are rather volatile, and a deaeration cannot be performed without special pre~au~oRs.~ With the polarographic method for instance, a simultaneous determination of pseudo and &ionone and citraP in a mixture is possible, but if the special specificity of this method is not required a bromometrie titration may be preferable,
A carbon-carbon single bond is cleaved by reduction only under special circumstances. The only reported case is in the reduction of 4-cyanopyridine2*27 in alkaline solution, where cyanide and pyridine are formed. Here the stability of the cyanide ion and the e~~tron-attrac~ng properties of the pyridine ring may be responsible for the cleavage. CARBON-OXYGEN
BONDS
The carbon~xygen double bond is found in several po~ro~ap~~y reducible compounds, such as formaldehyde,as*SB acetaldehyde,” butyraldehyde,sl glyo~al,~~ sugars,ss*s4pyruvic acid,@ diacetyP and dehydroascorbic acid.s7 Two properties of the carbon-oxygen double bond are of major interest in the ~olaro~aphy of ~r~nyl compounds, i.e., the reducibility and the ease with which it adds nucieophilic reagents. Roughly speaking the two properties are modified in the same direction by a substituent, which is understandable because the electron can be regarded as a nucleophilic reagent and a reduction thus as an addition of a nu~~~phi~~ reagent. The reduction of formaldehyde proceeds easily with a half-wave potential ia alkaline solution at - 1.6 V vs. S,C.E. Substitution of one of the hydrogen atoms with an alkyl group, which is electron donating, lowers the half-wave potential to about - 1~8V vs. S.C.E. Substitution of both hydrogen atoms in formaldehyde with alkyd groups lowers the half-wave potential to about -23 V t,s. S.C.E., which can only be measured in a solution containing tetra~~~~rnon~urn ions as supporting electrolyte. An electron-attracting group, such as the carboxyl or carbonyl group a to a carbonyl group, raises the reduction potential, as in pyruvic acid or diacetyl. A hydroxyl group at the same carbon atom as the carbonyl group makes the compound unreducible unless an electron-at~~t~~g group, e.g,, carboxyl, is ~bstit~ted at the carbonyl group, as ia oxalic acid. Additions to the carbonyl group influence the polarographic behaviour of carbonyl compounds in two ways. One is the reversible addition of water or an alcohol to the carbonyf group, thus forming a hydrate or a semiacetal, which is non-reducible. ~orm~de~yde is very much hydrated, a~e~deh~de less, and acetone is practically unhydrated in aqueous solution. In sugars the semiacetal formation is very pronounced. The presence of electron-attracting groups, e.g., chlorine or carboxyl groups, favours hydration of the carbonyl group as in chloral and glyoxylic acid.* The
Polarography of aliphaticcompounds
1069
reduction wave of a hydrate-forming carbonyl compound is a kinetic wave, where the wave-height is partly determined by the rate of the dehydration. Another addition reaction is the acid and base catalysed aldol condensation, which is of importance for all aldehydes containing an a-hydrogen atom. The visible result of the condensation is a gradual diminishing of the wave-height of the aldehyde; in acid solution a wave of an a&?-unsaturated aldehyde may be formed because of a dehydration of the primarily formed condensation product. A further possible complication in the polarography of carbonyl compounds is enolisation.3g The enol-form is more difficult to reduce than the keto-form. The electrode reaction of saturated aldehydes is in most instancesS1 a twoelectron reduction to the alcohols; in some cases a pinacol formation is suspected.3a Reduction of oxalic acid@ gives glyoxylic acid, which is partly protected against further reduction by the formation of a hydrate. Similarly, diethyl oxalate is reduced to the semiacetal of glyoxylic acid ethyl ester.*l The determination of formaldehyde by polarography has been found preferable in some cases, e.g., in acetic acidzv or in the reaction mixture after the periodate oxidation of glycols, 42 but in many instances the calorimetric determination with chromotropic acid has advantages.4s Pyruvic acid is often preferentially determined by polarography; the choice between this method and a photometric one, using the 2,4_dinitrophenylhydrazone in alkaline solution or after reaction with salicylic aldehyde, depends on the kind of interfering compound present. The more difficultly reducible or highly hydrated carbonyl compounds are with advantage determined polarographically through their azomethine derivatives. Carbon-oxygen
single bonds
The reduction of a carbon-oxygen single bond has been found in aromatic and steroidal a-hydroxy- and a-acetoxyketones44*6*11 and in certain compounds such as o-benzoylbenzoic acid pseudo ethylester.4s CARBON-SULPHUR
BONDS
Carbon-sulphur double bonds
The carbo&ulphur double bond is more easily reduced than the carbon-oxygen double bond. Of aliphatic compounds, carbon disulphide,47,48 dithio-oxamide>r’ dithioformic acid41 and cyanothioformamide w have been found to be polarographically reducible. Carbon disulphide yields in alkaline solution two polarographic waves; the electrode reaction of the first wave is a two-electron reduction to dithioformic acid41 and the second one a two-electron reduction of this compound to hydrogen sulphide and an unidentified polymeric material. 41 In acid solution a single four-electron wave is found. The electrode reactions of dithio-oxamide and cyanothioformamide have not been reported. The polarographic determination of carbon disulphide, as such, is impracticible because the compound is too volatile for a deaeration of the solvent without loss. It is more feasible to let the compound react with a secondary amine and to determine the resulting dialkyldithiocarbamate by anodic polarographyl or by spectrophotometry.
1070
H. LUND
Curbon-sulphursingle bonds The carbon-sulphur single bond is reducible in some aromatic t~~y~ates,~*~ where the electrode reaction is a reductive cleavage with the formation of cyanide ionP or thiocyanate io& and in phenacylsulphonium salts.b4 In the latter case the carbon-sulphur bond was cleaved before the reduction of the carbonyl group. In aliphatic compounds this kind of reduction occurs at too negative a potential to be of analytical value. CARBON-NITROGEN
BONDS
Carbon-nitrogen triple bonds The carbon-~~ogen triple bond is more difficultly reducible than the carbonnitrogen double bond. No aliphatic nitrile has been reported to be reduced in the cyanide group, and the only proved example of a polarographic reduction of a nitriie group is the reduction of Ccyanopyridine to 4-aminomethyl pyridine in acid solution?s7 Carbon-nitrogen double bonds The carbon-nitrogen double bond is generally reduced at a less negative potential than the carbon-oxygen double bond, which makes it possible to determine many carbonyl compounds through their azomethine derivatives. Unconjugated ketones, for instance, are not normally reducible in the common supposing electrolytes, but they can be determined as their azomethine derivatives. Such derivatives are semicarbazones,” different kinds of hydrazone,66*67*68 oximes60~80and imines formed from ammonia,61 n-butylamine,s2 ethylenediaminesS or hexamethylenediamine.8S The transformation of a carbonyl group into a stable azomethine compound also traps the carbonyl group in a derivative, which is somewhat less apt to hydration, ring formation or condensation. The electrode reaction of imines is a simple two-electron reduction of the carbonnitrogen double bond to an amine. The oximes and most hydrazones and semicarbazones are reduced in a four-el~tron reduction, where the first step is a reductive cleavage of the nitrogen-oxygen or nitrogen-nitrogen single bond.@aM Carbon-nitrogensingle bonds The reductive cleavage of a carbon-nitrogen single bond is only possible under special conditions where two el~~on-attracting groups are bonded to the same carbon atom. In aliphatic compounds this type of reduction is found in trimethylaminoacrolein perchlorates and 2,2-dinitropropane. BBIn the latter case the reduction in alkaline solution produces 2nitropropane and nitrite ion, which also is the first step in acid solution. The reaction is here complicated by the reaction between 2-nitropropane and nitrite which forms a reducible “pseudonitrolic acid”. CARBON-HALOGEN
BONDS
The reduction of a carbon-halogen bond is important in organic polarography because it occurs in polyhalogenated compounds, e.g., carbon tetrachloride,8B chloro-, bromo-r0 and iodoform, 2,3-dibromobutane, 89 1,1,1-~chloro-2-methyl-2-propano171 and in a-halogenated carbonyl compounds, such as bromacetic acid72 and ch.loral.73*74 Unsaturated compounds may be transformed into reducible derivatives on addition of bromine.le
Polarography of aliphatic compounds
1071
The ease of reduction of a carbon-halogen bond can be judged from its bondenergy, the carbon-iodine being the easiest and the carbon-fluorine bond the most Odifficultlyreducible. The latter has only been found reducible when it is a to a carbony1 gro~p~~*~~or in an activated trifluoromethyl gro~p.~’ When placed in a ring with fixed conformation, a carbon-chlorine bond in the more stable equatorial position is more difficult to reduce than when placed in the axial position.76*7s The presence of more halogens at the same carbon atom lowers the reduction potential, and often a step-wise reduction of the carbon-halogen bonds is found. A double bond in an allylic position favours a reduction, whereas a vinylic halogen is more difficult to reduce than the saturated halogen compound. This parallels the reactivity of the halogen in a polar substitution reaction; in a reduction the electrode may be regarded as the attacking nucleophilic reagent. The electrode is a “bulky” attacking reagent, and steric hindrance of the attack on the reactive centre makes the reduction more difficult, i.e., requires a more negative reduction potential. 79 The attack of one electron results in the formation of a halide ion and a radical which may dimerises or take up one further electron forming a carbanion. The carbanion can be stabilised in different ways. The most common way is to abstract a proton from the medium. Such is the case in the reduction of methyl iodide to methane ,@ of ally1 bromide to propyleneae and of carbon tetrachloride to chloroform and this further to methylene chloride.69 Another possibility for the carbanion is to expel a stable anion, which in a polyhalogenated compound may be done by elimination of a p-halogen as halide ion. The less negative reduction potential generally found for trans vie-dihalogenethylenes compared to the cis-compounds 81**2 reflects the easier trans-elimination. A similar behaviour is found in polar elimination reactions. This kind of reduction is common in vicinal halogen compounds and results in the formation of a double bond; 2,3-dibromobutane is thus reduced to butylene, 89 dibromomaleic acid to acetylenedicarboxylic acid12 and meso-a, a -dibromosuccinic acid to fumaric acid.ls Also, in the reduction of carbon tetrachloride in acetonitrile* the lack of available protons forces the carbanion, formed initially, to expel a chloride ion in an a-elimination, thus forming dichloromethylene. In some instances competition between an elimination of halogen and a substitution with hydrogen may be expected. A third kind of reduction route has been found in certain trihalogenmethyl derivatives, which in a six-electron reduction form a methyl compound.74*77 The polarographic analysis of the components in a mixture of organic halogen compounds will often be preferable to the less selective methods, e.g., the determination of the halide ions obtained by alkaline hydrolysis or reduction of the halogen compounds, which require a separation of the components before the determination. OXYGEN-OXYGEN
BONDS
Compounds containing an oxygen-oxygen single bond are generally polarographically reducible. Thus, hydroperoxides,sa*8P peracids*5**6acylperoxides,s7 ozonides, aldehyde and ketone peroxide9 can be determined polarographically. The electrode reaction is a reductive cleavage of the peroxide bond to two hydroxyl groups. A convenient medium for the polarographic determination of water-insoluble peroxides is a 1: 1 methanol-benzene mixture containing lithium chloride.se The dialkyl peroxides are reduced at the most negative potentials with E, more negative
H. LUND
1072
than -1.0 V vs. S.C.E. in the above mentioned medium. The presence of many alkyl groups shifts the half-wave potential to more negative potentials because of the electron-donating power of these and their shielding of the reactive centre; di-tertbutyl peroxide is thus not reducible in the accessible potential region, Hydroperoxides are reduced in the interval -0.6 to -0*9 V vs. S.C.E., and the presence of electronwithd~wing groups, e.g., the acyl group, makes diacyl peroxides reducible in the region 0 to -0.2 V vs. S.C.E. The half-wave potential of an organic peroxide is thus valuable in the elucidation of its structure. The polarographic technique is often the method of choice for a qualitative and quantitative analysis of a mixture of peroxides?7,W-Qa A comparisons3 of this method with the commonly used iodometric and stannous chloride dete~inations in the analysis of some hydroperoxides showed that the three methods yielded the same results when applied to pure materials, but with impure products the polarographic technique probably gave more reliable results because it is more specific than the chemical methods. OXYGBN-SULPHUR
DOUBLE
BONDS
The oxygen-sulphur double bond is reducible in some sulphones@ and sulphoxides where the electrode reaction is, respectively, a four-electron or two-electron reduction to a sulphide. This kind of reduction is unimportant in aliphatic polarography. OXYGEN-NITROGEN
BONDS
Oxygen-nitrogen double bonds
The oxygen-nitrogen double bond is important in organic polarography, because it is easily reducible without requiring further conjugation. In aliphatic compounds it is found in mono~tro~es and -alkenes,86-87 dinitroalkanes,~ esters of nitriceg and nitrous acid,lW N-oxides, N-nitrosamineslol-lW and nitro- and nitroso derivatives of urealo and related compounds. 1os-107 All these compounds may be determined polarographically; a reduction of the oxygen-nitrogen double bond takes place in all the compounds, except in the nitrates, @*the ~-ni~osaminesl~ in alkaline solution, and in some dinitro~anes.~ The electrode reaction of alkyl nitrites is not known; they are hydrolysed too rapidly for a preparative reduction.41 Nitroalkanes, in acid solution, are reduced in a four-electron reaction to alkylhydroxylamines. s6 This wave is well-de%ed and very suitable for a quantitative deter~nation of such compounds. The half-wave potentials are in slightly acid solution about -0.8 V W. S.C.E. The hydroxylamine formed in the first reduction is in slightly acid solution reduced further at a more negative potential to alkylamines. In strongly alkaline solution primary and secondary nitroalkanes are transformed into the non-reducible a&form, and no polarographic wave is thus seen in this medium, The polaro~aphic determination of aliphatic nitroalkanes requires little or no separation before the measurement because only a few types of compound are reduced at the same potential. Polarography has thus been found advantageous in the determination of the toxic constituent of “creeping indigo”, 3-nitropropanoic acid,lM because an extract from the plant material could be used directly. Possibly the non-reducibility of nitroalkanes in strongly alkaline solution may be useful in their determination in a mixture containing interfering compounds. The polarographic method does not, however, distinguish primary nitroalkanes from secondary ones as some of the calorimetric methods are able to do.
Polarography of aliphatic compounds
1073
Compounds with a nitro group at a double-bonded carbon atom are reduced somewhat differently. g7 The four-electron reduction in acid solution produces an oxime of a saturated aldehyde. At a more negative potential a two-electron wave occurs, which was found to yield an alkylhydroxylamine. Generally, an oxime would be expected to yield an amine in a four-electron reduction. The polarographic behaviour of components with two nitro groups at the same carbon atom depends on whether or not a hydrogen atom is bonded to the same carbon atom. l,l-Dinitroethaneg8 shows in most solutions two waves; the over-all reduction in alkaline solution requires eight electrons and produces acetamide oxime. In chloropicrin the reduction probably involves both the nitro group and one or more of the carbon-chlorine bonds. N-Nitrosamines, in acid solution, are reduced polarographically in a four-electron reduction to unsymmetric dialkylhydrazines; in alkaline solution the electrode reaction consumes two electrons and an amine and nitrous oxide are formed by a reductive cleavage of the nitrogen-nitrogen single bond.lQQ The wave in acid solution, which at pH 1 is found at about -0.9 V us. S.C.E., is preferable for a quantitative determination of N-nitrosamines. Because secondary amines can be transformed quantitatively into N-nitrosamines the polarographic determination of these furnishes a convenient and selective method of determining secondary amines in the presence of aliphatic primary and tertiary amines.lOl Tertiary amines do not interfere, and some nitroalkane or nitrolic acid, resulting from a reaction between nitrite and a carbonium ion formed by reaction between the primary amine and nitrous acid, can be destroyed by dithionite. Compounds such as N,N-dialkylanilines may be nitrosated at carbon, but such nitroso derivatives are reduced at less negative potentials than aliphatic iV-nitrosamines. iV-Nitrosamines have also been determined by ultraviolet spectroscopy and by volumetric gas analysis after reduction to hydrazine followed by oxidation to nitrogen; however, the polarographic method has been found to be the most convenient.lo2 With respect to selectivity the polarographic determination of secondary amines through their N-nitrosamines compares well with other methods, e.g., the determination through the dithiocarbamates formed on reaction with carbon disulphide; most methods require a prior removal of primary amines by reaction with, for example, salicylaldehyde.
Oxygen-nitrogen single bonds Polarographically reducible compounds containing an oxygen-nitrogen single bond include alkyl nitratesP such as butyl nitrate, nitroglycerine,lQQ*l10pentaerythritol trinitrate,ul and derivatives of hydroxylamine like alkylhydroxylamines, oximes,sQ amidoximesQs*llQand hydroxamic acids.lla The aikyl nitrates are reduced to nitrite ion and alcohol in a two-electron reduction.QQ The amidoximes are reduced to amidines,QQ which may be further reducible, and the first step in the reduction of aid- and ketoximes is also a cleavage of the nitrogen-oxygen bond .66*86The electrode reaction of aliphatic hydroxamic acids has not been proved, but because p-cyanobenzhydroxamic acid and isonicotinichydroxamic acid are both reduced to the amide,41 which is then further reduced, the aliphatic hydroxamic acids are included in this section.
1074
H. LUND
Alkyl nitrates are reduced at about -0.7 V us. S.C.E. in neutral, aqueous ethanol; the most likely interference would thus be from nitro compounds. The polaro~ap~c method has, for example, been found advantageous in the determination of butyl nitrate in diesel oil,lla where a 2 : 1 mixture of benzene-methanol with dissolved lithium chloride was used as medium, and, combined with spectrophotometry, in the determination of pentaerythritol trinitrate in a mixture containing nitroglycerine, 2-nitro~phenyla~ne and dibutylphthalate.~ The determination of aliphatic esters as their hydroxamic acids by polarography seems, in most instances, less preferable than other methods. The alkylhydroxylamines are reducible only in a narrow pH interval and their determination through this wave is unlikely to be widely used. Hydroxyla~ne and ~-methy~y~oxyla~e, however, exhibit an anodic wave at different potentials in alkaline solution,” and such anodic waves are more likely to be of value for analytical purposes. SULPHUR-SULPHUR
SINGLE
BONDS
A s~phur-s~ph~ single bond is generally reducible, and compo~ds such as di-, tri- and tetrasulphides,l16 thiolsulphonates116 and disulphone@ can be determined polarographically. Important examples from this group are cystimF and its disulphoxide,ll* tetramethylthiuram disulphide, llD thioethanolamine disulphidp and di~io~yco~~ acid. The electrode reaction is in all cases a simple cleavage of the s~phur~ulph~ bond with the formation of mercaptans and sulphinic acids. The latter are formed from disulphones94 and thiolsulphonates.lle The polarographic method is used as a routine together with other methods in the qualitative and q~ti~tive analysis of different kinds of sulphur compound in naphthasB1 The polarographic determination of a disulphide is especially valuable, when the mercaptan is also present.lN It is less suited for secondary and tertiary disulphides,12s because their reduction potentials are rather negative, so the diffusion plateau of the waves are poorly developed. SULPHUR-NITROGEN
BONDS
Sulphur-nitrogendouble bonds A reduction of a sulphur-nitrogen double bond occurs in sulphilimines. The electrode reaction is a two-electron reduction to a sulphide and a s~phona~de, which were isolated from a preparative reduction.& Sulphur-nitrogensingle bonds A sulphur-nitrogen single bond, e.g., in sulphonamides, is generally not reducible. However, 4,4’-di~iod~o~ho~e is polaro~p~c~y reduciblep’ and the products isolated from a preparative two-electron reduction, in slightly acid solution, are morpholine and sulphur. Probably a reduction of the sulphur-sulphur bond occurs primarily, and the thus formed nitrogen-sulphhydryl compound decomposes. Such an instability may explain that no nitrogen-sulphhy~yl compound has ever been isolated. SULPHUR-HALOGEN
BONDS
A sulphur-halogen bond is reducible in sulphochlorides.lss In some instances the electrode reaction is reported to be a two-electron reduction to sulphinic acid,lss*rM
Polarography of aliphatic compounds
but in other cases a more complicated reduction has been claimed.126*126
route involving dimerisation
NITROGEN-NITROGEN
Nitrogen-nitrogen
1075
and further
BONDS
double bonds
Aliphatic compounds containing a nitrogen-nitrogen double bond are not as common as the corresponding aromatic ones. Azomethane, prepared by anodic oxidation of N,N’ -dimethylhydrazine in alkaline solution, is reducible in a twoelectron reduction to NJ -dimethylhydrazine. 41 The two compounds do not form a reversible system at the dropping mercury electrode, as their aromatic counterparts do, because there is a difference of about 1 V between the reduction potential of azomethane and the oxidation potential of N,N’-dimethylhydrazine.” In alkaline solution diazirines are reduced to diaziridines. Nitrogen-nitrogen single bonds
The nitrogen-nitrogen single bond is often reducible when a neighbour to a double bond. The reduction is thus found in N-nitrosamines in alkaline solutionlo* and as the first step in the four-electron reduction of phenylhydrazines, semicarbazones and possibly other types of hydrazone in acid solution. 6s In some cases, e.g., the dimethylglycylhydrazone of a&unsaturated steroids, 67 the height of the wave, which is twice that of the one-electron wave of the parent steroid, points to a two-electron reduction. The nitrogen-nitrogen bond of diaziridines is reducible in acid solution.“l ANODIC
WAVES
The dropping mercury electrode is less suited for oxidations than for reductions because the accessible potential region in the positive direction is limited by the potential at which mercury dissolves. The oxidation of the mercury anode occurs at a less positive potential when the medium contains anions forming insoluble salts with mercury. An extension of the accessible potential interval can be made by using a platinum electrode; the oxidation of the water is then the limiting process. By working in suitable non-aqueous solvents the useful potential region can be extended considerably in a positive direction ; for example, compounds as difficult to oxidise as benzenelz7 yield voltammetric waves at a platinum electrode in acetonitrile containing sodium perchlorate. Below, only compounds giving anodic waves at the dropping mercury electrode are discussed. These may be divided into classes according to the functional groups responsible for the electrode reaction. Here a division into oxygen-, nitrogen-and sulphur-containing compounds is made. Oxygen-containing compounds
The presence of an “enediol” group, -C(OH)=C(OH)-, makes a molecule subject to anodic oxidation. Enediols include compounds such as ascorbic acid,12* reductone,129 dihydroxyfumaric acid and dihydroxyacrylic acid. Two electrons per molecule are involved in the electrode reaction in which is produced an a-diketone. This diketone is often hydrated, and the hydrated form is not polarographically reducible. The system enediol-a-diketone is thus not polarographically reversible as is the corresponding aromatic system catechol-o-benzoquinone. The most important member of this group is ascorbic acid and because few
1076
H. LUND
compounds are oxidised as easily as ascorbic acid this compound can be determined in the presence of many different kinds of compounds without prior separation. The method is in general more specific than the calorimetric method which responds to many kinds of reducing compound. The sensitivity of a calorimetric method is, however, often higher than the polarographic one.
Many nitrogen compounds, such as ammoniamr and some amines,130show anodic waves, where the electrode reaction is a formation of a mercurous salt. Many other compounds forming insoluble mercury salts interfere with such a determination. Hydroxyla~nes and some hydrazines and hydrazides, aliphatic and aromatic, can be oxidised at the dropping mercury electrode. Pheny~ydroxylamine and nitrosobenzene form a reversible system, and the anodic oxidation of N,N-dibenzyE hydroxylamine produces N-benzylbenzaldoxime and some benzaldehyde,@ The electrode reactions of aliphatic hydroxylamines have not yet been proved. The anodic reaction of hydrazine at a mercury or oxide-coated platinum electrode is dependent on the concentration and produces nitrogen* and ammonia. A similar concentration dependence is found in the anodic reaction of isonicotinic hydrazide.2 NJ’-dimethylhydrazine is oxidised to azomethane,U but the electrode reaetion of ~,~-dimethy~ydr~e has not yet been proved. Both hydroxylamine and ~-me~y~y~oxyla~ne can be determined polarographically in a mixture, because their half-wave potentials differ by O-15V.” Similarly NJGdimethylhydrazine or NJ’-dimethylhydrazine can be determined in the presence of hydrazine, but the waves of the two dimethylhydrazines overlap too much for a dete~nation of the one in the presence of the other. More investigation is needed in order to evaluate the possibilities of the polarographic method compared with other methods in the determination of hydroxylamines and hydrazines. ~u~hur-costarring
compoernds
Molecules containing a thiol group or an enolisable thiocarbonyl group give anodic waves at the dropping mercury electrode and compounds such as mercaptans, e.g., cysteine,131 glutathionU2 and 2,3-dimercaptopropano1,13S dithiocarbamates,lls derivatives of thiourea,“* including thiobarbituratesla6 and some thioamides,les may be determined pol~o~ap~~y. Carbon sulphide can be determined as a dithiocarbamate after reaction with a suitable amine.” The electrode reaction consists of an oxidation of the mercury electrode with the formation of a mercurous salt. Part of the mercurous salt may be adsorbed on the electrode with the occurrence of an adsorption wave. Further complications may arise at higher concentrations. At a platinum electrode mercapto compounds are oxidised to disulphides at a considerably more positive potential. At this electrode, also, sulphides yield a voltammetric wave; sulphoxides result from the oxidation.la7 A polarographic determination of thiol compounds is faster than most methods, but may be less accurate; a higher accuracy may be obtained by a polarometrio (amperometric) titration. When a determination of both the mercapto compound and the disulphide is wanted, the direct polarographic method is often the method of choice.
Polarography
of aliphatic compounds
1077
Summarising it can be said that the polarographic method is useful in the quantitative and qualitative determination of many compounds and its proved and potential applications are in general not fully appreciated. Polarography is also valuable in the study of reaction rates,s as a tool in determination of structures, and for the establishment of the optimum conditions for electrosynthesis. ZnsannnenfassunS-Es wird eine &xsicht aliphatischer Verbindungen gegeben. R&nn&-On aliphatiques.
tiber die Polarographie
pr6sente une revue sur la polarographie
des composes
REFERENCES 1 G. J. Hoijtink, Rec. Trav. chim., 1957,76, 885. p H. Lund, Acta C/rem. SC&., 1963,17,2325. s Idem, ibid., 1957, 11,283. * S. Wawzonek and R. C. Duty, J. Electrochem. Sot., 1961, 108, 1135. s R. Brdicka and K. Wiesner, Coil. Czech. Chem. Comm., 1947,12,138. 6 J. Kouteck9, ibid., 1953,18,597. ’ L. Meites, J. Electroanulyt. Chem., 1963,5,270. 8 S. Karp and L. Meites, J. Amer. Chem. Sot., 1962,&i, 906. * H. Lund, Actu Chem. Stand, 1964,X$1984. lo P. Zuman and J. Michl, Nature, 1961, 192,655. l1 H. Lund, Acta C/tern. Scund., 1960,14,1927. la I. Rosenthal, J. R. Hayes, A. J. Martin and P. J. Elving, J. Amer. Chem. Sot., 1958,80,3050. I8 V. Medones, Coil. Czech. Chem. Comm., 1958,X$ 1465. l4 W. L. Bird and C. H. Hale, Analyt. Chem., 1952,24,586. I5 G. C. Claver and M. E. Murphy, ibid., 1959,31, 1682. lo A. S. Gorokhovskaya and B. E. Geller, Zavodska (I Lab., 1962,28,809. l’ E. M. Skobets, G. S. Nestyuk and V. I. Shapov J , Ukrain. Khim. Zhur., 1962,28,72. I* R. J. Lacoste, I. Rosenthal and C. H. Schmittinger, Analyt. Chem., 1956,28,983. l8 B. Warshowsky, P. J. Elving and J. Mandel, ibid., 1947.19, 161. 5oP. J. Elving and I. Rosenthal, ibid, 1954,26,1454. 81 M. Fields and E. R. Blout, J. Amer. Chem. Sot., 1948,70,930. **E. Knobloch, K. Hejno, Z. Arnold, K. Mhoukk and Z. Batik, Cesk. Farm., 1957, 6, 241. 111B. Budesinsky, K. MAou&k, F. Jana and E. Kraus, Coil. Czech. Chem. Comm., 1958,23,434. a*S. M. Murphey, M. G. Carangelo, S. M. B. Ginaine and S. M. C. Markham, J. Polymer Sci., 1961, 54,107. *5 M. M. Baizer, J. Electrochem. Sot., 1964, 111,215. 8EL. Holleck and D. Marquarding, Nuturwiss., 1962,49,468. *’ J. Volke and J. Holubek, Coil. Czech. Chem. Comm., 1963, 28, 1597. 88K. Veseli and R. BrdiEka, ibid., 1947, 12, 313. aDR. G. Peterson and M. A. Joslyn, J. Org. Chem., 1959,24, 1359. so P. J. Elving and E. Rutner, Znd. Eng. Chem., Analyt. Ed., 1946,18,176. 81Yu. V. Vodzinskii and I. A. Korshunov, Zhur.fiz. Khim., 1953,27, 63. 81 P. J. Elving and C. E. Bennett, J. Amer. Chem. Sot., 1954,76,1412. aa K. Wiesner, Coil. Czech. Chem. Comm., 1947,12, 64. 8* W. G. Overend, A. R. Peacocke and J. B. Smith, Chem. and Znd.. 1957, 113. 85 P. Zuman, Che&. Zvesfi, 1952,3, 191. s6 S. Harrison. CON. Czech. Chem. Comm.. 1950. 15.818. 81T. Wasa, M. Takagi and S. Ono, Bull. bhem..Sof. Japan, 1961, 34, 518. a8 J. K%a, Coil. Czech. Chem. Comm., 1959,24,2632. 119S. Ono, M. Takagi and T. Wasa, ibid., 1961, 26, 141. 40J. K%ta, ibid., 1957,22, 1677. 41 H. Lund, unpublished results. kBP. Zuman, CON. Czech. Chem. Comm., 1958,23,598. 4a C. E. Bricker and W. A. Vail, Analyt. Chem., 1950,22,720. 44 M. Fedorohko, Chem. Analit., 1958,3,573. *5P. Kabasakalian and J. McGlotten, Analyt. Chem., 1959,31,1091. 4(1 H. Lund, Actu Chem. Stand., 1960,14,359.
1078
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LUND
(’ J. Heyrovsky, Analyt. Chem., 1952,24,915. IS I. M. Kohhoff. P. E. Torren and R. W. Ramette, i&f&.,1952,24,1037. es Kh. Ya. Levitman, N. I. Karpovich, and E. I. Rutskaya, Sbornik Nauch, Rabat, Beforass. Politekh. Inst., 1956, No 55,112; Chem. Abs., 1958,52,13541. 6o K. Suchy, Mqyar Kern. Folyoirat, 1958,&I, 46. 61 P. Zuman, R. Zumanovd and B. So&k, Coil. Czech. Chem. Comm., 1953,18,632. 6aL. J. Brady, Anaiyt. Chem.,1948,u), 512. 1sK. Schwabeand J. Voigt, 2. Elektrochem., 1952,56,44. 6p P. Zuman and S. Tang, Coil. Czech. Chem. Comm., 1963,28,829. 66P. Souchay and M. Graizon, Chim. ana&., 1954,36,85. 60J. K. Wage, E. B. He&berg and L. F. Fieser, J. B&l. Chem., 1940,X36,653. 5* M. Biezina. V. Volkova and J. Volke. Coil. Czech. Chem. Comm.. 1954.19. 894. ’ ’ ’ sBJ. W. Haas’and C. C. Lynch, Analyt.‘Cbem., 1957,2Q, 479. KSP. Souchay and S. Ser, J. Chim.phys., 195549, C172. @ J. W. Haas, J. D. Storey and 6. C. Lynch, Analyt. Chem., 1962,34,14X 61 P. Zuman, Coil. Czech. Chem. Comm., 1950,15,839. 6aR. E. Van Atta and D. R. Jamieson, Analyt. Chem., 1959,31, 1217. Es M. E. Hall, ibid., 1959,31,2007. O4D. K. Banerjee, G. C. Riechmann and C. C. Budke, ibid., 1964,36,2220. 66H. Lund, Acta Chem. Scam& 1959,13,249. 8dZdem, ibid., 1964,18,563. (17 P. Zuman and V. Ho&k, Co& Czech. Chem. Comm., 1961,26,176. OSM. Masui and H. Sayo, J. Chem. Sot., 1961,4773. I0 M. V. Stackelberg and W. Stracke, Z. Elektrochem., 1949,53, 118. 7DV. Volkova and F. fcha, Cesk. Farm., 1957,6,141. ‘l J. Bimer, Analyt. Chem., 1961,33,1955. 1p P. J. Elving, J. M. Markowitz and I. Rosenthal, J. Eie~tr~~rn. Sot., 1954,101,195. 18P. J. Elving and C. E. Bennett, Anafyt. Chem., 1954,26,1572. 74Zdem, J. Electrochem. Sot., 1954,101, 520. U P. J. Elving and J. T. Leone, J. Amer. C&m. Sot., 1957, 79, 1546. TOA. M. Wilson and N. L. Allinger, ibid., 1961,83,1999. ‘t7H. Lund, Acta Chem. SC&., i959,13,192. 18P. Kabasakalian and J. McGlotten. Analvt. Chem.. 1962.34.1441. ‘* F. L. Lambert and K. Kobayashi, j. A&r. Chem.Soc., i96b, 82,5324. 8oP. J. Elving, I. Rosenthal, J. R. Hayes and A. J. Martin, Analyt. Chem., 1961, 33, 330. *l S. G. Mairanovskii and L. D. Bergel’son, Zhur.fir. Khim., 1960,34,236. @*W. A. Jura and R. J. Gaul, J, Amer. Chem. Sot., 1958,80,5402. 8s D. A. Skoog and A. B. H. Lauwzecha, Anafyr. Chem., 1956,28,825. ** M. L. Whisman and B. H. Eccleston, ibid., 1958,30,1638. *5W. E. Parker, C. Ricciuti, C. L. Ogg and D. Swern, J. Amer. Chem. Sot,, 1955,77,4037. 8e W. E. Parker, L. P. Witnauer and D. Swam, ibid, 1957,79, 1929. B7D. Swem and L. S. Silbert, Analyt. Chem., 1963,35,880. 88B. N. Moryganov, A. I. Kalinin and L. N, Mikhotova, Zhur. obschei. Khim., 1962,32,3476. ** W. R. Lewis, F. W. Quackenbush and T. DeVries, Anulyt. Chem., 1949,21,762. *OW. R. Lewis and F. W. Quackenbush, J. Amer. OiI Chemists’ Sot., 1949,26,53. Pi C. 0. Willits, C. Ricciuti, C. L. Ogg, S. G. Morris and R. W. Riemenschneider, ibid., 1953,30,420. *a C. 0. Willits, C. Ricciuti, H. B. Knight, D. Swern, Analyt. Chem., 1952,24,785. aaC. Ricciuti, J. E. Coleman and C. 0. Willits, ibid., 1955,27,405. O4S. G. Mairanovskii and M. B. Neiman, Doklady Akad. Nauk. S.S.S.R., 1952,87,805. OKF. Petru, Coil. Czech. Chem. Comm., 1947,12,620. gs A. P. Ballad, S. I. Moichanova, I. V. Pa&e&h, A. V. Topchiev and V. Ya. Shtem, Z/&r. am&. Z&im., 1959,14,188. (i7M. Masui and H. Sayo, Yakugaku Zasshi, 1958,78,703. DeZdem, J. Chem, Sot., 1961,5325. a9 F. Kaufman, H. J. Cook and S. M. Davis, J. Amer. Chem. Sot., 1952,74,4997. loo I. V. Patsevich, A. V. Topchiev and V. Ya. Shtem, Zhur. analit. Khim., 1958,13,608. lox F. L. English, Amzlyt. Chem., 1951,23,344. lop E. A. M. F. Dahmen, D. Vader and J. D. van der Laarse, Z. analyt. C&m., 1962,186,161. 1o8 H. Lund, Acta Chem. Scar& 1957, 11,990. lo4 K. Namba and K. Suzuki, Bull. Chem. Sot. Japan, 1955,28,620,623. los P. Lanza, A. Delmarco, A. F. McKay and G. Semerano, Ricerca xi., 1956,26, S116. Ia6 M. Yamashita and K. Sugino, J. Electrochem. Sot., 1957, 104, 100. lo7 G. C. Whitnack and E. St. C. Gantz, ibid., 1959, 106,422.
Polarography of ahphatic compounds
1079
lo*M. M. Frodyma, L. H. Muramoto, D. J. Williams and H. Matsumoto, Anatyt. Chem., 1963, 3fr 1403. loa G. C. Whitnack, M. M. Mayfield and E. St. C. Gantz, ibid., 1955,27,899. 110A. F. Wiliiams and D. Kenyon, Tukzntu, 1959,3,16O. I11W. M. Ayres and G. W. Leonard, Anatyt. Chem., 1959,31,1485. 11*J. Molhn and F. Kdp&rek, Colt. Czech. Chem. Comm., 1961,26,2438. lla T. Bsterud and M. Prytz, Actu Chem. &and., 1959,13,2114. al4 G. S. Shimonaev and L. S. Stephanova, Khim. i Tekhnot. T@tiv i Maset, 1962,7, 67; Chem. Abs., 1963,58,1282. I15J. H. Karchmer and M. T. Walker, Analyt. Chem., 1954,26,271. 116D. Barnard, M. B. Evans, G. M. C. Higgins and J. F. Smith, Gem. urtd Znd., 1961,20. 11’I. M Kolthoff, W. Stricks and N. Tanaka, J. Amer. Chem. Sot., 1955,77,4739. 118J. Fondari, J. L. Grand and P. Dubouloz, Analyt. Chim. Acto, 1964,31,97. 119E. C. Gregg and W. F. Tyler, J. Amer. Chem. Sot., 1950,72,4561. la0 E. C. Chevalier and W. C. Purdy, Anatyt. Chim. Actu, 1960,23,574. la1 J. H. Karchmer, Anutyt. Chem., 1958,30,80. 1*2R. L. Hubbard, W. E. Haines and J. S. Hall, ibid., 1958,3O, 91. Ia8S. G. Mairanovskil and M. B. Neiman, Doktauy Akad. Nauk. S.S.S.R., 1951,79,85. ***L. Horner and H. Nickel, Chem. Ber., 1956,89, 1681. Is5 N. Urabe and K. Yasukochi, Denki Kuguku, 1957,25,17. Ia8Zdem, J. Etectrochem. Sot. Japan, Overseas Ed., 1959, 27, 201. la7 H. Lund, Actu Chem. Stand., 1957, 11, 1323. l1 M. Biezina and P. Zuman, Die Potarographie in der Medizin, Biochemie und Pharmazie. Akadem-
ische Verlagsgesellschaft, Leipzig, 1956. laDR. BrdiEka and P. Zuman, Colt. Czech. Chem. Comm., 1950, 12,766. Ia0C. J. Nyman and R. A. Johnson, Anatyt. Chem., 1957,29,483. la1 I. M. Kolthoff and C. Barnum, J. Amer. Chem. Sot., 1940,62,3061. la1 I. Tachi and S. Koide, Proc. First Intern. Potarog. Congr. Prague, 1951, Pt. 1, 1951,450. Ia8P. Zuman, R. Zumanova and J. Teisinger, Colt. Czech. Chem. Comm., 1955,2O, 139. la4 R. L. Ed&erg, Analyt. Chem., 1954,26,724. Ias P. &man, Colt. Czech. Chem. Comm., 1955,20,646. la6 H. Lund, ibid., 1960, 25, 3313. 18’ M. M. Nicholson, J. Amer. Chem. Sot., 1954, 76, 2539.
Pergamon Press Ltd.
POLAROGRAPHY Department
Printed in Northern Ireland
OF ALIPHATIC COMPOUNDS
HENNING LUND of Chemistry, University of Aarhus, Aarhus, Denmark
(Received 25 January 1965.Accepted 14 April 1965) Summary_-A review of the polarography presented.
of aliphatic compounds
is
INTRODUCTION
A MAJORadvantage of the polarographic method is its selectivity, which often makes the determination of a compound in a mixture possible without prior separation. The selectivity of the polarographic determination is based on the differences in electrode potential necessary for the occurrence of different electrode reactions. In order to judge which compounds may be polarographically active and which substances may interfere with their determination, a knowledge of the electrode reactions, and the factors which influence these reactions, is necessary. In the following discussion of the polarography of aliphatic compounds emphasis is laid on the nature of the electrode reactions, and examples of compounds undergoing these reactions are mentioned. In some of the more important applications the merits of the polarographic determination compared to other methods are discussed. Often, however, no one of several applicable analytical methods is clearly preferable, and the choice will depend on the special circumstances or the personal preference of the analyst. ELECTRODE
REACTIONS
A polarographic reduction consists of a transfer of electrons from the cathode to the reducible system coupled with the uptake of the appropriate number of hydrogen ions. The high potential gradient,l about 108 V. cm-l, in the immediate vicinity of the electrode polarises the reducible molecule and makes the attack of one or more electrons on the reactive centre possible. Although the reaction results in a reduction at a certain place in the molecule it is the properties of the whole molecule which determines the energy necessary for the transfer of the electrons, Le., the reduction potential. The presence of multiple bonds makes the system more easily reducible, and aliphatic compounds are, therefore, generally more difficult to reduce than aromatic and heteroaromatic compounds. Under fixed conditions the electrode potential determines the course of the electrode reaction; a more negative potential may make further reduction possible. Other factors, however, also influence the electrode reaction and the necessary reduction potential. REACTION
CONDITIONS
Hydrogen ions are involved in most organic electrode reactions and pH may influence not only the reduction potential but also the course of the reaction. The change of electrode reaction with pH may be drastic as in the case of isonicotinic amidea (in acid solution the amide group is reduced to the aldehyde, whereas the pyridine nucleus is reduced in alkaline solution) or slight as in the reduction of 1065 2
H. LUMO
1066
androsta-l,4-dien-17/3-ol-3-one,3 which forms different stereoisomeric pinacols in acid and alkaline solution. Sufficient buffer capacity is, therefore, necessary to ensure that the consumption of hydrogen ions in the electrode reaction does not change the pH in the immediate vicinity of the electrode. In non-aqueous, aprotic media the scarcity of protons may change the reduction path. The reduction of carbon tetrachloride in acetonitrile4 thus yields dichloromethylene, whereas chloroform is formed in most media. The change in the electrode reaction with pH is often caused by a change in the species reduced. If the molecule has protolytic properties the form reduced in acid solution will carry more protons than the one reduced in alkaline solution. The protonated form is always more easily reducible than the unprotonated one. The pH around which the reaction changes from a reduction of the protonated to a reduction of the unprotonated form is often not at the pK of the reducible acid. In the acid-base equilibrium the more easily reducible acid is removed from the equilibrium at the electrode by reduction and the conjugate base combines with hydrogen ions to form the acid, which is then reduced. The height of the wave, i.e., the rate of the electrode reaction, is thus in a certain pH region determined partly by the rate of the recombination reaction at the electrode between the base and protons.” This is a special case of the general one where the electroactive species is formed in the vicinity of the electrode by a chemical reaction slow enough to be the rate controlling step for the over-all reaction. 6 Such waves are called kinetic waves and they can be identified as such by their independence of the height of the mercury reservoir and their high dependence on the temperature. This heavy dependence of the height of the wave on temperature and pH makes, in general, kinetic waves less suited for use in a quantitative determination than a diffusion controlled wave. The concentration of the electroactive compound may influence the electrode reaction. In some instances the reaction involves a slow step before a further reduction, and the intermediate may either be further reduced or react otherwise. The nonelectrochemical reaction is often a dimerisation. The rate of the dimerisation would as a second-order reaction increase faster with concentration than the first-order reduction.’ The polarographically visible result is that the height of the wave grows less with concentration than required by the IlkoviE equations*@ The course of an electrode reaction should preferentially be proved by a preparative electrolytic reduction at controlled potential, where the products can be identified by conventional means. Oftan the occurrence or absence of further reduction waves reveals the reduction path, but such evidence is not always reliable. For instance, a slow chemical step in the reaction may result in a diffusion from the electrode of a product which is then transformed to a reducible compound, which would have yielded a further wave in the polarogram if it had remained at the electrode.lO*ll In the following survey of types of polarographically reducible, aliphatic compound these are classified according to the type of bond suffering reduction in the electrode reaction. CARBON-CARBON
BONDS
Carbon-carbon triple bonds
Only few compounds containing the carbon-carbon triple bond are polarographically reducible. Some derivatives of acetylene-dicarboxylic acid are reduced in
Polarography of aliphatic compounds
1067
two steps, and from the half-wave potential of the second wave the reduction product from the diethyl ester was identified as diethyl fumarate.12 This means that a trans addition of hydrogen to the triple bond has taken place. In the reduction of the acid a dimerisation occurs and from a preparative reduction racemic dimethylsuccinic acid can be isolated.12 Unconjugated acetylenes are not polarographically reducible, but acetylene can on reaction with bromine in acetic acid be transformed into the reducible tetrabromoethane.18 Carbon-carbon double bonds
The isolated carbon-carbon double bond is not polarographically reducible under ordinary conditions, but by suitable conjugation a reducible system may be formed. Conjugation with an electron-withdrawing group, e.g., the cyano, carboxamide, carbethoxy, carboxyl or carbonyl group, renders the system reducible in a convenient potential region, whereas electron-donating groups, e.g., an alkyl group, make the reduction more difficult. Thus, derivatives of a&unsaturated acids, aldehydes and ketones can generally be determined polarographically. This includes compounds such as acrylonitrile,14*1s,16acrylamide,l’ acrylateP and homologues thereof, derivatives of unsaturated dicarboxylic acids, such as maleic and fumaric acids,19*20a&unsaturated carbonyl compounds, such as acrolein,21 citra1,22 a- and /?-ionone22 and methyl vinyl ketone.% The electrode reaction of unsaturated acid derivatives is in general a two-electron reduction of the double bond to the saturated acid. In some cases, e.g., acrylamidel’ and acrylonitrile, apthe number of electrons in the electrode reaction, n, is found to be about 0.2. This low value is explained by a polymerisation of unreduced acrylonitrile, in the vicinity of the electrode, initiated by the initial reduction product, the carbanion, CH,CHCN. Under certain conditions a reductive dimerisation of acrylonitrile to adipic nitrile takes place.26 The reduction of dimethylmaleic anhydride to racemic dimethylsuccinic acid and dimethylfumaric acid to meso dimethylsuccinic acid show that in both cases a trans addition of hydrogen to the double bond takes place.12 The determination of dissolved acrylonitrile in mixtures, e.g., with unsaturated hydrocarbons and saturated nitriles, is faster and more selective than an oxidative titration or a hydrolysis followed by a determination of ammonia and just as accurate. For an analysis of monomeric acrylonitrile in polymeric material the use of N,ZVdimethylformamide as a solvent and tetrabutylammonium bromide as supporting electrolyte is very convenient15 because of the high solubility of the organiccompounds in this solvent. The polarographic determination of a mixture of maleic and fumaric acid is in many cases preferable to other methods because it is more selective than the oxidative [KMnO,, Ce(IV), Br-Cl], conductometric and acidimetric methods, more accurate than the paper chromatographic one and demands less time and material than a method based on chemical separation. The polarographic method requires, however, a rather strict pH control. aa In the presence of interfering electroactive substances it may be necessary to precipitate the acids as their barium salts before the determination.19 The electrode reaction of a&unsaturated carbonyl compounds is a one-electron
1068
H. LUND
reduction to a radical which preferentially dimerises at the B-carbon atom to a saturated diketone, but may form a pinacol when a dimerisation at the /?+zrbon is stericaily unfavourable, In methyl vinyl ketone the radical reacts with mercury, thus forming di(3-ketobutyl) mercury.*s When considering the method of choice for analysis of these compounds it must be remembered that some of them, e.g,, methyl vinyl ketone, are rather volatile, and a deaeration cannot be performed without special pre~au~oRs.~ With the polarographic method for instance, a simultaneous determination of pseudo and &ionone and citraP in a mixture is possible, but if the special specificity of this method is not required a bromometrie titration may be preferable,
A carbon-carbon single bond is cleaved by reduction only under special circumstances. The only reported case is in the reduction of 4-cyanopyridine2*27 in alkaline solution, where cyanide and pyridine are formed. Here the stability of the cyanide ion and the e~~tron-attrac~ng properties of the pyridine ring may be responsible for the cleavage. CARBON-OXYGEN
BONDS
The carbon~xygen double bond is found in several po~ro~ap~~y reducible compounds, such as formaldehyde,as*SB acetaldehyde,” butyraldehyde,sl glyo~al,~~ sugars,ss*s4pyruvic acid,@ diacetyP and dehydroascorbic acid.s7 Two properties of the carbon-oxygen double bond are of major interest in the ~olaro~aphy of ~r~nyl compounds, i.e., the reducibility and the ease with which it adds nucieophilic reagents. Roughly speaking the two properties are modified in the same direction by a substituent, which is understandable because the electron can be regarded as a nucleophilic reagent and a reduction thus as an addition of a nu~~~phi~~ reagent. The reduction of formaldehyde proceeds easily with a half-wave potential ia alkaline solution at - 1.6 V vs. S,C.E. Substitution of one of the hydrogen atoms with an alkyl group, which is electron donating, lowers the half-wave potential to about - 1~8V vs. S.C.E. Substitution of both hydrogen atoms in formaldehyde with alkyd groups lowers the half-wave potential to about -23 V t,s. S.C.E., which can only be measured in a solution containing tetra~~~~rnon~urn ions as supporting electrolyte. An electron-attracting group, such as the carboxyl or carbonyl group a to a carbonyl group, raises the reduction potential, as in pyruvic acid or diacetyl. A hydroxyl group at the same carbon atom as the carbonyl group makes the compound unreducible unless an electron-at~~t~~g group, e.g,, carboxyl, is ~bstit~ted at the carbonyl group, as ia oxalic acid. Additions to the carbonyl group influence the polarographic behaviour of carbonyl compounds in two ways. One is the reversible addition of water or an alcohol to the carbonyf group, thus forming a hydrate or a semiacetal, which is non-reducible. ~orm~de~yde is very much hydrated, a~e~deh~de less, and acetone is practically unhydrated in aqueous solution. In sugars the semiacetal formation is very pronounced. The presence of electron-attracting groups, e.g., chlorine or carboxyl groups, favours hydration of the carbonyl group as in chloral and glyoxylic acid.* The
Polarography of aliphaticcompounds
1069
reduction wave of a hydrate-forming carbonyl compound is a kinetic wave, where the wave-height is partly determined by the rate of the dehydration. Another addition reaction is the acid and base catalysed aldol condensation, which is of importance for all aldehydes containing an a-hydrogen atom. The visible result of the condensation is a gradual diminishing of the wave-height of the aldehyde; in acid solution a wave of an a&?-unsaturated aldehyde may be formed because of a dehydration of the primarily formed condensation product. A further possible complication in the polarography of carbonyl compounds is enolisation.3g The enol-form is more difficult to reduce than the keto-form. The electrode reaction of saturated aldehydes is in most instancesS1 a twoelectron reduction to the alcohols; in some cases a pinacol formation is suspected.3a Reduction of oxalic acid@ gives glyoxylic acid, which is partly protected against further reduction by the formation of a hydrate. Similarly, diethyl oxalate is reduced to the semiacetal of glyoxylic acid ethyl ester.*l The determination of formaldehyde by polarography has been found preferable in some cases, e.g., in acetic acidzv or in the reaction mixture after the periodate oxidation of glycols, 42 but in many instances the calorimetric determination with chromotropic acid has advantages.4s Pyruvic acid is often preferentially determined by polarography; the choice between this method and a photometric one, using the 2,4_dinitrophenylhydrazone in alkaline solution or after reaction with salicylic aldehyde, depends on the kind of interfering compound present. The more difficultly reducible or highly hydrated carbonyl compounds are with advantage determined polarographically through their azomethine derivatives. Carbon-oxygen
single bonds
The reduction of a carbon-oxygen single bond has been found in aromatic and steroidal a-hydroxy- and a-acetoxyketones44*6*11 and in certain compounds such as o-benzoylbenzoic acid pseudo ethylester.4s CARBON-SULPHUR
BONDS
Carbon-sulphur double bonds
The carbo&ulphur double bond is more easily reduced than the carbon-oxygen double bond. Of aliphatic compounds, carbon disulphide,47,48 dithio-oxamide>r’ dithioformic acid41 and cyanothioformamide w have been found to be polarographically reducible. Carbon disulphide yields in alkaline solution two polarographic waves; the electrode reaction of the first wave is a two-electron reduction to dithioformic acid41 and the second one a two-electron reduction of this compound to hydrogen sulphide and an unidentified polymeric material. 41 In acid solution a single four-electron wave is found. The electrode reactions of dithio-oxamide and cyanothioformamide have not been reported. The polarographic determination of carbon disulphide, as such, is impracticible because the compound is too volatile for a deaeration of the solvent without loss. It is more feasible to let the compound react with a secondary amine and to determine the resulting dialkyldithiocarbamate by anodic polarographyl or by spectrophotometry.
1070
H. LUND
Curbon-sulphursingle bonds The carbon-sulphur single bond is reducible in some aromatic t~~y~ates,~*~ where the electrode reaction is a reductive cleavage with the formation of cyanide ionP or thiocyanate io& and in phenacylsulphonium salts.b4 In the latter case the carbon-sulphur bond was cleaved before the reduction of the carbonyl group. In aliphatic compounds this kind of reduction occurs at too negative a potential to be of analytical value. CARBON-NITROGEN
BONDS
Carbon-nitrogen triple bonds The carbon-~~ogen triple bond is more difficultly reducible than the carbonnitrogen double bond. No aliphatic nitrile has been reported to be reduced in the cyanide group, and the only proved example of a polarographic reduction of a nitriie group is the reduction of Ccyanopyridine to 4-aminomethyl pyridine in acid solution?s7 Carbon-nitrogen double bonds The carbon-nitrogen double bond is generally reduced at a less negative potential than the carbon-oxygen double bond, which makes it possible to determine many carbonyl compounds through their azomethine derivatives. Unconjugated ketones, for instance, are not normally reducible in the common supposing electrolytes, but they can be determined as their azomethine derivatives. Such derivatives are semicarbazones,” different kinds of hydrazone,66*67*68 oximes60~80and imines formed from ammonia,61 n-butylamine,s2 ethylenediaminesS or hexamethylenediamine.8S The transformation of a carbonyl group into a stable azomethine compound also traps the carbonyl group in a derivative, which is somewhat less apt to hydration, ring formation or condensation. The electrode reaction of imines is a simple two-electron reduction of the carbonnitrogen double bond to an amine. The oximes and most hydrazones and semicarbazones are reduced in a four-el~tron reduction, where the first step is a reductive cleavage of the nitrogen-oxygen or nitrogen-nitrogen single bond.@aM Carbon-nitrogensingle bonds The reductive cleavage of a carbon-nitrogen single bond is only possible under special conditions where two el~~on-attracting groups are bonded to the same carbon atom. In aliphatic compounds this type of reduction is found in trimethylaminoacrolein perchlorates and 2,2-dinitropropane. BBIn the latter case the reduction in alkaline solution produces 2nitropropane and nitrite ion, which also is the first step in acid solution. The reaction is here complicated by the reaction between 2-nitropropane and nitrite which forms a reducible “pseudonitrolic acid”. CARBON-HALOGEN
BONDS
The reduction of a carbon-halogen bond is important in organic polarography because it occurs in polyhalogenated compounds, e.g., carbon tetrachloride,8B chloro-, bromo-r0 and iodoform, 2,3-dibromobutane, 89 1,1,1-~chloro-2-methyl-2-propano171 and in a-halogenated carbonyl compounds, such as bromacetic acid72 and ch.loral.73*74 Unsaturated compounds may be transformed into reducible derivatives on addition of bromine.le
Polarography of aliphatic compounds
1071
The ease of reduction of a carbon-halogen bond can be judged from its bondenergy, the carbon-iodine being the easiest and the carbon-fluorine bond the most Odifficultlyreducible. The latter has only been found reducible when it is a to a carbony1 gro~p~~*~~or in an activated trifluoromethyl gro~p.~’ When placed in a ring with fixed conformation, a carbon-chlorine bond in the more stable equatorial position is more difficult to reduce than when placed in the axial position.76*7s The presence of more halogens at the same carbon atom lowers the reduction potential, and often a step-wise reduction of the carbon-halogen bonds is found. A double bond in an allylic position favours a reduction, whereas a vinylic halogen is more difficult to reduce than the saturated halogen compound. This parallels the reactivity of the halogen in a polar substitution reaction; in a reduction the electrode may be regarded as the attacking nucleophilic reagent. The electrode is a “bulky” attacking reagent, and steric hindrance of the attack on the reactive centre makes the reduction more difficult, i.e., requires a more negative reduction potential. 79 The attack of one electron results in the formation of a halide ion and a radical which may dimerises or take up one further electron forming a carbanion. The carbanion can be stabilised in different ways. The most common way is to abstract a proton from the medium. Such is the case in the reduction of methyl iodide to methane ,@ of ally1 bromide to propyleneae and of carbon tetrachloride to chloroform and this further to methylene chloride.69 Another possibility for the carbanion is to expel a stable anion, which in a polyhalogenated compound may be done by elimination of a p-halogen as halide ion. The less negative reduction potential generally found for trans vie-dihalogenethylenes compared to the cis-compounds 81**2 reflects the easier trans-elimination. A similar behaviour is found in polar elimination reactions. This kind of reduction is common in vicinal halogen compounds and results in the formation of a double bond; 2,3-dibromobutane is thus reduced to butylene, 89 dibromomaleic acid to acetylenedicarboxylic acid12 and meso-a, a -dibromosuccinic acid to fumaric acid.ls Also, in the reduction of carbon tetrachloride in acetonitrile* the lack of available protons forces the carbanion, formed initially, to expel a chloride ion in an a-elimination, thus forming dichloromethylene. In some instances competition between an elimination of halogen and a substitution with hydrogen may be expected. A third kind of reduction route has been found in certain trihalogenmethyl derivatives, which in a six-electron reduction form a methyl compound.74*77 The polarographic analysis of the components in a mixture of organic halogen compounds will often be preferable to the less selective methods, e.g., the determination of the halide ions obtained by alkaline hydrolysis or reduction of the halogen compounds, which require a separation of the components before the determination. OXYGEN-OXYGEN
BONDS
Compounds containing an oxygen-oxygen single bond are generally polarographically reducible. Thus, hydroperoxides,sa*8P peracids*5**6acylperoxides,s7 ozonides, aldehyde and ketone peroxide9 can be determined polarographically. The electrode reaction is a reductive cleavage of the peroxide bond to two hydroxyl groups. A convenient medium for the polarographic determination of water-insoluble peroxides is a 1: 1 methanol-benzene mixture containing lithium chloride.se The dialkyl peroxides are reduced at the most negative potentials with E, more negative
H. LUND
1072
than -1.0 V vs. S.C.E. in the above mentioned medium. The presence of many alkyl groups shifts the half-wave potential to more negative potentials because of the electron-donating power of these and their shielding of the reactive centre; di-tertbutyl peroxide is thus not reducible in the accessible potential region, Hydroperoxides are reduced in the interval -0.6 to -0*9 V vs. S.C.E., and the presence of electronwithd~wing groups, e.g., the acyl group, makes diacyl peroxides reducible in the region 0 to -0.2 V vs. S.C.E. The half-wave potential of an organic peroxide is thus valuable in the elucidation of its structure. The polarographic technique is often the method of choice for a qualitative and quantitative analysis of a mixture of peroxides?7,W-Qa A comparisons3 of this method with the commonly used iodometric and stannous chloride dete~inations in the analysis of some hydroperoxides showed that the three methods yielded the same results when applied to pure materials, but with impure products the polarographic technique probably gave more reliable results because it is more specific than the chemical methods. OXYGBN-SULPHUR
DOUBLE
BONDS
The oxygen-sulphur double bond is reducible in some sulphones@ and sulphoxides where the electrode reaction is, respectively, a four-electron or two-electron reduction to a sulphide. This kind of reduction is unimportant in aliphatic polarography. OXYGEN-NITROGEN
BONDS
Oxygen-nitrogen double bonds
The oxygen-nitrogen double bond is important in organic polarography, because it is easily reducible without requiring further conjugation. In aliphatic compounds it is found in mono~tro~es and -alkenes,86-87 dinitroalkanes,~ esters of nitriceg and nitrous acid,lW N-oxides, N-nitrosamineslol-lW and nitro- and nitroso derivatives of urealo and related compounds. 1os-107 All these compounds may be determined polarographically; a reduction of the oxygen-nitrogen double bond takes place in all the compounds, except in the nitrates, @*the ~-ni~osaminesl~ in alkaline solution, and in some dinitro~anes.~ The electrode reaction of alkyl nitrites is not known; they are hydrolysed too rapidly for a preparative reduction.41 Nitroalkanes, in acid solution, are reduced in a four-electron reaction to alkylhydroxylamines. s6 This wave is well-de%ed and very suitable for a quantitative deter~nation of such compounds. The half-wave potentials are in slightly acid solution about -0.8 V W. S.C.E. The hydroxylamine formed in the first reduction is in slightly acid solution reduced further at a more negative potential to alkylamines. In strongly alkaline solution primary and secondary nitroalkanes are transformed into the non-reducible a&form, and no polarographic wave is thus seen in this medium, The polaro~aphic determination of aliphatic nitroalkanes requires little or no separation before the measurement because only a few types of compound are reduced at the same potential. Polarography has thus been found advantageous in the determination of the toxic constituent of “creeping indigo”, 3-nitropropanoic acid,lM because an extract from the plant material could be used directly. Possibly the non-reducibility of nitroalkanes in strongly alkaline solution may be useful in their determination in a mixture containing interfering compounds. The polarographic method does not, however, distinguish primary nitroalkanes from secondary ones as some of the calorimetric methods are able to do.
Polarography of aliphatic compounds
1073
Compounds with a nitro group at a double-bonded carbon atom are reduced somewhat differently. g7 The four-electron reduction in acid solution produces an oxime of a saturated aldehyde. At a more negative potential a two-electron wave occurs, which was found to yield an alkylhydroxylamine. Generally, an oxime would be expected to yield an amine in a four-electron reduction. The polarographic behaviour of components with two nitro groups at the same carbon atom depends on whether or not a hydrogen atom is bonded to the same carbon atom. l,l-Dinitroethaneg8 shows in most solutions two waves; the over-all reduction in alkaline solution requires eight electrons and produces acetamide oxime. In chloropicrin the reduction probably involves both the nitro group and one or more of the carbon-chlorine bonds. N-Nitrosamines, in acid solution, are reduced polarographically in a four-electron reduction to unsymmetric dialkylhydrazines; in alkaline solution the electrode reaction consumes two electrons and an amine and nitrous oxide are formed by a reductive cleavage of the nitrogen-nitrogen single bond.lQQ The wave in acid solution, which at pH 1 is found at about -0.9 V us. S.C.E., is preferable for a quantitative determination of N-nitrosamines. Because secondary amines can be transformed quantitatively into N-nitrosamines the polarographic determination of these furnishes a convenient and selective method of determining secondary amines in the presence of aliphatic primary and tertiary amines.lOl Tertiary amines do not interfere, and some nitroalkane or nitrolic acid, resulting from a reaction between nitrite and a carbonium ion formed by reaction between the primary amine and nitrous acid, can be destroyed by dithionite. Compounds such as N,N-dialkylanilines may be nitrosated at carbon, but such nitroso derivatives are reduced at less negative potentials than aliphatic iV-nitrosamines. iV-Nitrosamines have also been determined by ultraviolet spectroscopy and by volumetric gas analysis after reduction to hydrazine followed by oxidation to nitrogen; however, the polarographic method has been found to be the most convenient.lo2 With respect to selectivity the polarographic determination of secondary amines through their N-nitrosamines compares well with other methods, e.g., the determination through the dithiocarbamates formed on reaction with carbon disulphide; most methods require a prior removal of primary amines by reaction with, for example, salicylaldehyde.
Oxygen-nitrogen single bonds Polarographically reducible compounds containing an oxygen-nitrogen single bond include alkyl nitratesP such as butyl nitrate, nitroglycerine,lQQ*l10pentaerythritol trinitrate,ul and derivatives of hydroxylamine like alkylhydroxylamines, oximes,sQ amidoximesQs*llQand hydroxamic acids.lla The aikyl nitrates are reduced to nitrite ion and alcohol in a two-electron reduction.QQ The amidoximes are reduced to amidines,QQ which may be further reducible, and the first step in the reduction of aid- and ketoximes is also a cleavage of the nitrogen-oxygen bond .66*86The electrode reaction of aliphatic hydroxamic acids has not been proved, but because p-cyanobenzhydroxamic acid and isonicotinichydroxamic acid are both reduced to the amide,41 which is then further reduced, the aliphatic hydroxamic acids are included in this section.
1074
H. LUND
Alkyl nitrates are reduced at about -0.7 V us. S.C.E. in neutral, aqueous ethanol; the most likely interference would thus be from nitro compounds. The polaro~ap~c method has, for example, been found advantageous in the determination of butyl nitrate in diesel oil,lla where a 2 : 1 mixture of benzene-methanol with dissolved lithium chloride was used as medium, and, combined with spectrophotometry, in the determination of pentaerythritol trinitrate in a mixture containing nitroglycerine, 2-nitro~phenyla~ne and dibutylphthalate.~ The determination of aliphatic esters as their hydroxamic acids by polarography seems, in most instances, less preferable than other methods. The alkylhydroxylamines are reducible only in a narrow pH interval and their determination through this wave is unlikely to be widely used. Hydroxyla~ne and ~-methy~y~oxyla~e, however, exhibit an anodic wave at different potentials in alkaline solution,” and such anodic waves are more likely to be of value for analytical purposes. SULPHUR-SULPHUR
SINGLE
BONDS
A s~phur-s~ph~ single bond is generally reducible, and compo~ds such as di-, tri- and tetrasulphides,l16 thiolsulphonates116 and disulphone@ can be determined polarographically. Important examples from this group are cystimF and its disulphoxide,ll* tetramethylthiuram disulphide, llD thioethanolamine disulphidp and di~io~yco~~ acid. The electrode reaction is in all cases a simple cleavage of the s~phur~ulph~ bond with the formation of mercaptans and sulphinic acids. The latter are formed from disulphones94 and thiolsulphonates.lle The polarographic method is used as a routine together with other methods in the qualitative and q~ti~tive analysis of different kinds of sulphur compound in naphthasB1 The polarographic determination of a disulphide is especially valuable, when the mercaptan is also present.lN It is less suited for secondary and tertiary disulphides,12s because their reduction potentials are rather negative, so the diffusion plateau of the waves are poorly developed. SULPHUR-NITROGEN
BONDS
Sulphur-nitrogendouble bonds A reduction of a sulphur-nitrogen double bond occurs in sulphilimines. The electrode reaction is a two-electron reduction to a sulphide and a s~phona~de, which were isolated from a preparative reduction.& Sulphur-nitrogensingle bonds A sulphur-nitrogen single bond, e.g., in sulphonamides, is generally not reducible. However, 4,4’-di~iod~o~ho~e is polaro~p~c~y reduciblep’ and the products isolated from a preparative two-electron reduction, in slightly acid solution, are morpholine and sulphur. Probably a reduction of the sulphur-sulphur bond occurs primarily, and the thus formed nitrogen-sulphhydryl compound decomposes. Such an instability may explain that no nitrogen-sulphhy~yl compound has ever been isolated. SULPHUR-HALOGEN
BONDS
A sulphur-halogen bond is reducible in sulphochlorides.lss In some instances the electrode reaction is reported to be a two-electron reduction to sulphinic acid,lss*rM
Polarography of aliphatic compounds
but in other cases a more complicated reduction has been claimed.126*126
route involving dimerisation
NITROGEN-NITROGEN
Nitrogen-nitrogen
1075
and further
BONDS
double bonds
Aliphatic compounds containing a nitrogen-nitrogen double bond are not as common as the corresponding aromatic ones. Azomethane, prepared by anodic oxidation of N,N’ -dimethylhydrazine in alkaline solution, is reducible in a twoelectron reduction to NJ -dimethylhydrazine. 41 The two compounds do not form a reversible system at the dropping mercury electrode, as their aromatic counterparts do, because there is a difference of about 1 V between the reduction potential of azomethane and the oxidation potential of N,N’-dimethylhydrazine.” In alkaline solution diazirines are reduced to diaziridines. Nitrogen-nitrogen single bonds
The nitrogen-nitrogen single bond is often reducible when a neighbour to a double bond. The reduction is thus found in N-nitrosamines in alkaline solutionlo* and as the first step in the four-electron reduction of phenylhydrazines, semicarbazones and possibly other types of hydrazone in acid solution. 6s In some cases, e.g., the dimethylglycylhydrazone of a&unsaturated steroids, 67 the height of the wave, which is twice that of the one-electron wave of the parent steroid, points to a two-electron reduction. The nitrogen-nitrogen bond of diaziridines is reducible in acid solution.“l ANODIC
WAVES
The dropping mercury electrode is less suited for oxidations than for reductions because the accessible potential region in the positive direction is limited by the potential at which mercury dissolves. The oxidation of the mercury anode occurs at a less positive potential when the medium contains anions forming insoluble salts with mercury. An extension of the accessible potential interval can be made by using a platinum electrode; the oxidation of the water is then the limiting process. By working in suitable non-aqueous solvents the useful potential region can be extended considerably in a positive direction ; for example, compounds as difficult to oxidise as benzenelz7 yield voltammetric waves at a platinum electrode in acetonitrile containing sodium perchlorate. Below, only compounds giving anodic waves at the dropping mercury electrode are discussed. These may be divided into classes according to the functional groups responsible for the electrode reaction. Here a division into oxygen-, nitrogen-and sulphur-containing compounds is made. Oxygen-containing compounds
The presence of an “enediol” group, -C(OH)=C(OH)-, makes a molecule subject to anodic oxidation. Enediols include compounds such as ascorbic acid,12* reductone,129 dihydroxyfumaric acid and dihydroxyacrylic acid. Two electrons per molecule are involved in the electrode reaction in which is produced an a-diketone. This diketone is often hydrated, and the hydrated form is not polarographically reducible. The system enediol-a-diketone is thus not polarographically reversible as is the corresponding aromatic system catechol-o-benzoquinone. The most important member of this group is ascorbic acid and because few
1076
H. LUND
compounds are oxidised as easily as ascorbic acid this compound can be determined in the presence of many different kinds of compounds without prior separation. The method is in general more specific than the calorimetric method which responds to many kinds of reducing compound. The sensitivity of a calorimetric method is, however, often higher than the polarographic one.
Many nitrogen compounds, such as ammoniamr and some amines,130show anodic waves, where the electrode reaction is a formation of a mercurous salt. Many other compounds forming insoluble mercury salts interfere with such a determination. Hydroxyla~nes and some hydrazines and hydrazides, aliphatic and aromatic, can be oxidised at the dropping mercury electrode. Pheny~ydroxylamine and nitrosobenzene form a reversible system, and the anodic oxidation of N,N-dibenzyE hydroxylamine produces N-benzylbenzaldoxime and some benzaldehyde,@ The electrode reactions of aliphatic hydroxylamines have not yet been proved. The anodic reaction of hydrazine at a mercury or oxide-coated platinum electrode is dependent on the concentration and produces nitrogen* and ammonia. A similar concentration dependence is found in the anodic reaction of isonicotinic hydrazide.2 NJ’-dimethylhydrazine is oxidised to azomethane,U but the electrode reaetion of ~,~-dimethy~ydr~e has not yet been proved. Both hydroxylamine and ~-me~y~y~oxyla~ne can be determined polarographically in a mixture, because their half-wave potentials differ by O-15V.” Similarly NJGdimethylhydrazine or NJ’-dimethylhydrazine can be determined in the presence of hydrazine, but the waves of the two dimethylhydrazines overlap too much for a dete~nation of the one in the presence of the other. More investigation is needed in order to evaluate the possibilities of the polarographic method compared with other methods in the determination of hydroxylamines and hydrazines. ~u~hur-costarring
compoernds
Molecules containing a thiol group or an enolisable thiocarbonyl group give anodic waves at the dropping mercury electrode and compounds such as mercaptans, e.g., cysteine,131 glutathionU2 and 2,3-dimercaptopropano1,13S dithiocarbamates,lls derivatives of thiourea,“* including thiobarbituratesla6 and some thioamides,les may be determined pol~o~ap~~y. Carbon sulphide can be determined as a dithiocarbamate after reaction with a suitable amine.” The electrode reaction consists of an oxidation of the mercury electrode with the formation of a mercurous salt. Part of the mercurous salt may be adsorbed on the electrode with the occurrence of an adsorption wave. Further complications may arise at higher concentrations. At a platinum electrode mercapto compounds are oxidised to disulphides at a considerably more positive potential. At this electrode, also, sulphides yield a voltammetric wave; sulphoxides result from the oxidation.la7 A polarographic determination of thiol compounds is faster than most methods, but may be less accurate; a higher accuracy may be obtained by a polarometrio (amperometric) titration. When a determination of both the mercapto compound and the disulphide is wanted, the direct polarographic method is often the method of choice.
Polarography
of aliphatic compounds
1077
Summarising it can be said that the polarographic method is useful in the quantitative and qualitative determination of many compounds and its proved and potential applications are in general not fully appreciated. Polarography is also valuable in the study of reaction rates,s as a tool in determination of structures, and for the establishment of the optimum conditions for electrosynthesis. ZnsannnenfassunS-Es wird eine &xsicht aliphatischer Verbindungen gegeben. R&nn&-On aliphatiques.
tiber die Polarographie
pr6sente une revue sur la polarographie
des composes
REFERENCES 1 G. J. Hoijtink, Rec. Trav. chim., 1957,76, 885. p H. Lund, Acta C/rem. SC&., 1963,17,2325. s Idem, ibid., 1957, 11,283. * S. Wawzonek and R. C. Duty, J. Electrochem. Sot., 1961, 108, 1135. s R. Brdicka and K. Wiesner, Coil. Czech. Chem. Comm., 1947,12,138. 6 J. Kouteck9, ibid., 1953,18,597. ’ L. Meites, J. Electroanulyt. Chem., 1963,5,270. 8 S. Karp and L. Meites, J. Amer. Chem. Sot., 1962,&i, 906. * H. Lund, Actu Chem. Stand, 1964,X$1984. lo P. Zuman and J. Michl, Nature, 1961, 192,655. l1 H. Lund, Acta C/tern. Scund., 1960,14,1927. la I. Rosenthal, J. R. Hayes, A. J. Martin and P. J. Elving, J. Amer. Chem. Sot., 1958,80,3050. I8 V. Medones, Coil. Czech. Chem. Comm., 1958,X$ 1465. l4 W. L. Bird and C. H. Hale, Analyt. Chem., 1952,24,586. I5 G. C. Claver and M. E. Murphy, ibid., 1959,31, 1682. lo A. S. Gorokhovskaya and B. E. Geller, Zavodska (I Lab., 1962,28,809. l’ E. M. Skobets, G. S. Nestyuk and V. I. Shapov J , Ukrain. Khim. Zhur., 1962,28,72. I* R. J. Lacoste, I. Rosenthal and C. H. Schmittinger, Analyt. Chem., 1956,28,983. l8 B. Warshowsky, P. J. Elving and J. Mandel, ibid., 1947.19, 161. 5oP. J. Elving and I. Rosenthal, ibid, 1954,26,1454. 81 M. Fields and E. R. Blout, J. Amer. Chem. Sot., 1948,70,930. **E. Knobloch, K. Hejno, Z. Arnold, K. Mhoukk and Z. Batik, Cesk. Farm., 1957, 6, 241. 111B. Budesinsky, K. MAou&k, F. Jana and E. Kraus, Coil. Czech. Chem. Comm., 1958,23,434. a*S. M. Murphey, M. G. Carangelo, S. M. B. Ginaine and S. M. C. Markham, J. Polymer Sci., 1961, 54,107. *5 M. M. Baizer, J. Electrochem. Sot., 1964, 111,215. 8EL. Holleck and D. Marquarding, Nuturwiss., 1962,49,468. *’ J. Volke and J. Holubek, Coil. Czech. Chem. Comm., 1963, 28, 1597. 88K. Veseli and R. BrdiEka, ibid., 1947, 12, 313. aDR. G. Peterson and M. A. Joslyn, J. Org. Chem., 1959,24, 1359. so P. J. Elving and E. Rutner, Znd. Eng. Chem., Analyt. Ed., 1946,18,176. 81Yu. V. Vodzinskii and I. A. Korshunov, Zhur.fiz. Khim., 1953,27, 63. 81 P. J. Elving and C. E. Bennett, J. Amer. Chem. Sot., 1954,76,1412. aa K. Wiesner, Coil. Czech. Chem. Comm., 1947,12, 64. 8* W. G. Overend, A. R. Peacocke and J. B. Smith, Chem. and Znd.. 1957, 113. 85 P. Zuman, Che&. Zvesfi, 1952,3, 191. s6 S. Harrison. CON. Czech. Chem. Comm.. 1950. 15.818. 81T. Wasa, M. Takagi and S. Ono, Bull. bhem..Sof. Japan, 1961, 34, 518. a8 J. K%a, Coil. Czech. Chem. Comm., 1959,24,2632. 119S. Ono, M. Takagi and T. Wasa, ibid., 1961, 26, 141. 40J. K%ta, ibid., 1957,22, 1677. 41 H. Lund, unpublished results. kBP. Zuman, CON. Czech. Chem. Comm., 1958,23,598. 4a C. E. Bricker and W. A. Vail, Analyt. Chem., 1950,22,720. 44 M. Fedorohko, Chem. Analit., 1958,3,573. *5P. Kabasakalian and J. McGlotten, Analyt. Chem., 1959,31,1091. 4(1 H. Lund, Actu Chem. Stand., 1960,14,359.
1078
H.
LUND
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Polarography of ahphatic compounds
1079
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