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Polarography of organic compounds in aprotic solvents
~LARU~~~
OF ORGANIC COMPOSTS IN APROTIC SOLVENTS
(I&T&IJ~~25 Jonumy 1%S, Accepted 5 April 196% S---A review of the polarogmphy of or@c aprotic solvents is presented.
compounds in
INTRODUCTION STUDIES of the polarographic
behaviour of organic compounds in aprotic solvents have been numerous in the past decade. The majority of the work had been carried out in ~methy~o~~de~ acetonitrile and dimethyl sulphoxide; pyridine91 liquid sulpbur dioxide,=+ me~yla~~ide,~ te~~e~yl~~~ and ~~orne~~~ have been examined briefly aud seem, with the exception of nitromethane, to offer no advantage. Nitromethane is relatively unaffected by Nchloro-compounds, which chlorinate many other common solvents, and has been used successfully in studies of these compounds. The three commonest solvents allow electron additions to be observed with many compounds, without accompanying complications due to protonation, and give more insight into the mechanism of the electrode process. This property leads to stepwise reductions of various functional groups with the formation of stable anion radicals, and in many systems, which are irreversible in solvents containing water or alcohols, become reversible under aprotic conditions. These solvents in oxidations at solid electrodes, offer a wider potential range for the study of anodic reactions than aqueous media. In aqueous media the range is limited to +l*O V, whereas in acetonitrile the range is+20 V (ZW.A@>. Additional advantages for these solvents are their synergistic elect on the reduction of halogen compounds, better solvent properties than aqueous systems and inertness towards easily hydrolysed compounds. This review will stress the investigations in which the results were either uot possible, or were better than those obtained in solvents containing water or alcohols. HYDROCARBONS
,
The first work7 dealt with the reduction of aromatic hydrocarbons in dimethylformamide and acetonitrile and it was found that it was possible ta form anion radicals from stilbene, anthracene and ~a~isyl~uo~ne
Studies of this type were extended to naphthalene, 1229
phenanthreue,
biphenyl,
1230
s.
WAWZONJX
terphenyls, phenylacetylenes8 and more complex aromatic hydrocarbons.Q-15 For a number of these examples only the first step was observed; the second reduction occurred at potentials beyond the usable range of the supporting electrolyte. Similar two-step reductions have been observed for anthracene and non-alternant hydrocarbons, in dioxan containing 4% water, but not for stilbene, which showed only a two-electron step under these conditions. l* Behaviour resembling that in solvents containing water can be obtained by adding proton donors, such as water,T phenolls and benzoic acid,7 to the aprotic solvent; the last two are better proton donors than water. The nature of the one-electron reduction step in aprotic solvents has been confhmed by large-scale electrolysis ,’ electron spin resonance studiesl’ and a.c_ polarography. lo The last of these has demonstrated that the first wave of stilbene in dimethylformamide is completely diffusion controlled and reversible. The polarographic results for these compounds parallel results obtained by the addition of alkali metals to aromatic hydrocarbons and other compounds. They suggest the possib~ity of a rapid method for dete~ng those compounds that will form anion radicals. The half-wave potentials obtained have been found to correlate with molecular orbital calculations.18 QUINONES
Quinones in these solvents are reduced first to the semiquinone anion and then to the dianonr6*1s-21 Q + e8 Q1st wave 2nd wave
Q- + err: Q”
The hydroquinones are not oxidised at the dropping mercury electrode, probably because of low ionisation; the dianion, if preformed with base, gives two anodic waves at the same potentials as those observed for the reduction of the quinone.22 The reduction of the semiquinone is not reversible at potentials more negative than approximately -1.4 V us. S.C.E and is shifted to more positive potentials by lithium ions. This phenomenon is ascribed to ion association and varies with the structure of the quinone.as The formation of the semiquinone anion has been demonstrated for anthraquinone by electron spin resonance studies,1’ and that of the dianion by the fo~ation of 9,Wdiethoxyanthracene in a large-scale electrolytic reduction in the presence of ethyl bromide.rQ The dianion can be seen around the dropping mercury electrode as a red colour. Addition of water and phenol as proton donors has no effect upon the first wave, but shifts the second wave to more positive potentials. In the presence of a suflkient amount of the proton donor, the two waves merge and give a single wave approximately equal in height to the sum of the two waves obtained in anhydrous media. This behaviour can be explained by the protonation of the quinone dianion, with the potential being determined by the rate of the reaction Q-+HA7+QII--l-A-. Benzoic acid as a proton donor behaves differently, and increases the first wave
Polarography of organic compounds in aprotic solvents
1231
at the expense of the second wave. Protonation of the semiquinone must occur to give an intermediate, which is reduced more readily than the semiquinone Q-t- H+-+QH QH+e-+QHThis behaviour parallels that observed with the anion radicals from aromatic hydrocarbons. Wydroxyl groups, hydrogen-bonded intramolecularly, in 1-hydroxyanthraquinones are found to facilitate the reduction more in dimethylfo~a~de than in aqueous ethanoi.24 o-Quinones in dimethylformamide show a similar stepwise reduction to that observed with p-quinones.la Carbon blacks, which have complex quinone structures, are reducible polarographically if present as slurries in dimethylfo~amide.~ Similar studies can only be made in an aqueous medium if adequate stirring is used.% CARBONYL
COMPOUNDS
In the carbonyl compound series, aliphatic aldehydes only give polarograpbic waves in dimethylfo~a~de~ in the presence of tetraethyla~o~um salt~.~ Phenol
(C,H&C=Q (C,H&CO-
+ e * (C&H&CO+ e --f (C,H&@-6
to form benzilic acid and diphenylethyl~rbinol, respectively. Electron spin resonance studies indicate that the anion radical generated from benzophenone is stable,17 that the intermediate from acetophenone has a half-life of about twenty seconds, but that the product from benzaldehyde is much less stable.= In agreement with these stabilities, pinacols are obtained from large-scale electrolysis of the last two compoundsW Addition of phenol as a proton donor causes the frrst wave of benzophenone to grow at the expense of the second wave. Benzaldehyde in the presence of phenol gives a new wave, intermediate between the two original waves. Benzoic acid as proton donor generates a third wave for both benzophenone and benzaldehyde, and this wave is more positive than the original first wave; this behaviour is ascribed to the reduction of the protonated form.= a,&Unsaturated carbonyl compounds in the aromatic series give two oneelectron waves,!@ and large-scale electrolytic reduction studies indicate that the intermediates from benzalacetophenone are more stable than those from benzalacetone and ~innam~dehyde. Reduction of the diketones benzil, p-diacetylbenzene and odibenzoylbenzene gives two waves, m-dibenzoylbenzene gives one and trans-dibenzoylethylene three.s5
S. WAWZONEK
1232
In all cases the Kalousek techniqueSP has indicated that the initial step is a reversible one involving the addition of one electron C6H6COCOC,H, + e G C,H,COCC,H,
C,H,COCC,H, I O-
+ e + C,H,C==CC,H, A- A-
Substitution of lithium chloride for tetrabutylammonium iodide in the reduction of benzil gives a reversible two-electron reduction wave. These mechanisms have been verified by electron spin resonance spectroscopy.% NITRILES
AND
OTHER
ACID
DERIVATIVES
In the aromatic acid series, nitriles have been found to yield a one-electron reduction wave in dimethylformamide. 36 Electron spin resonance studies are in agreement with a mechanism such as C,H,CN + e ti C,H,CN-. A similar reduction is observed for pyromellitonitrile (1,2,4,5-tetracyanobenzene) in acetonitriles7 and occurs at approximately the same point as that for pyromellitic anhydride.s8 Tetracyanoethane and tetracyanopropane in dimethylformamide give reduction waves, and electron spin resonance studies indicate that the tetracyanoethylene radical anion is formed as a product.39 Studies with a,@nsaturated acid derivatives have been numerous, and stepwise reductions are reported for tetracyanoethylene, 39 7,7,8,8-tetracyanoquinodimethan,4Q dicyanomethylene-2,2,4,4-tetramethylcyclobutanone, bis-dicyanomethylene-2,2,4,4,tetramethylcyclobutane,” diethyl azodicarboxylate,42 substituted acrylonitriles and ethyl acry1ates.m The addition of phenol and benzoic acid to the last class gives behaviours which vary with the substituents.44 NITRO
AND
NITROSO
COMPOUNDS
Anion radicals are formed in the reduction of aliphati@ and aromatic46 nitro compounds in acetonitrile46 and dimethylformamide.47 (C&.)&NO2 + e + (CH&,CNO,-. In an aqueous or alcoholic medium a similar behaviour is observed for nitrobenzene in the presence of inhibitors such as camphoP and triphenylphosphine.49 Nitrodurene at pH 12 is reported to give a one-electron wave while nitrobenzene gives only an inflection.60 The first wave for the reduction of an aromatic nitro compound is reversible, and is followed by an irreversible second wave, corresponding to a reduction to Addition of benzoic acid to nitrobenzene in dimethylformphenylhydroxylamine. amide causes the first wave to increase to a height equal to the sum of the original waves. 47 Water, when added, shifts the second wave to more positive potentials,61 and alkali metal salts shift the first wave to more positive potentials.51 The formation of the anion radical has been verified by electron spin resonance
Polarography of organic compounds in aprotic solvents
1233
studies for both the aliphatic46 and aromatica series. The aliphatic radical was found to be unstable. The reversibility observed for the reduction of aromatic nitro compounds in these solvents allows meaningful correlation of half-wave potentials for m- and p-substituted nitrobenzenes with various substituent constants.‘j2 Nitrosobenzene in dimethylformamide gives two waves and forms an unstable anion radical; the instability of the intermediate makes the first wave slightly irreversible.6s Addition of benzoic acid causes the formation of a new more positive reduction wave, and this is ascribed to the formation of a complex with benzoic acid.47 HETEROCYCLIC COMPOUNDS In the heterocyclic series, pyridinium salts are reduced in the same way in both acetonitrile and water, but the intermediate radical dimerises much more slowly in acetonitrile.m HALOGENATED COMPOUNDS Dimethylformamide is a useful solvent for the reduction of halogenated compounds. Reductions are observed for chlorobenzene68s6Q and p-chloracetanilide70 in the presence of quatemary salts; these compounds show no observable reduction wave in an aqueous medium. Trifluoromethyl groups attached to benzene rings are reducible in an aqueous medium only if activating groups such as sulphonyl are present,‘l but the reduction in dimethylformamide occurs even for trifluoromethylbenzene.28 In addition, the reduction occurs in one, two or three steps, depending upon the number and type of substituents on the aryl ring, in contrast to an aqueous medium where all fluorisz atoms are removed simultaneously. The polarographic reduction of polyhalogenated hydrocarbons in acetonitrile and dimethylformamide is unique. Carbon tetrachloride and bromide are reduced through a dihalocarbene72 in one step to the methylene compound Ccl, + 2e -+ Ccl,- + Cl. CC&- -+ ccl,
+ Cl-
Ccl, + 2e + solvent + H,CCl,. o-Dibromobenzene and o-chlorobromobenzene form benzyne as an intermediates9 and are reduced to benzene. The polarographic data for the reduction of o-bromonitrobenzene and o, m and p-iodonitrobenzene at a hanging mercury drop are at variance with the electron spin resonance data. Consideration of half-peak potentials indicates that the reduction of the nitro group occurs first, whereas electron spin resonance seems to show that elimination of halogen and formation of the nitrobenzene anion-radical are involved in the first step.73 OXIDATIONS The use of aprotic solvents in electrolytic oxidations at solid electrodes was first applied to aryl carbinolsss in acetonitrile
0--e_ CH,& =0 CH,O ’
-
-
’ CHaOH--B_ CH;O =
=
~H,OH
+ -
ZH+
0 0-
CH,O ’
-
I_ CH,OH \ CHO
1234
S. WAWZONEK
Oxidation occurs by a two-electron process to the aldehyde which, sometimes, may be further oxidised. Extension of this work to aromatic hydrocarbons indicated that they lose two electrons and form dipositive ions which, depending on the structure, may be stable or may lose a proton to form a carbonium ion. sB Pyridine when added as a proton acceptor shifts the waves to more negative potentials. The mechanism of oxidation has been verified by the large-scale oxidation of anthracene in the presence of pyridine and a perchlorate salt to 9, lo-dihydroanthranyldipyridinium diperchlorate. Oxidations have been carried out in dimethylsulphoxides7 but not in dimethylformamide, because this solvent gives a high residual current.6g Data are given for the oxidation of tropilidine,60 aromatic hydrocarbons,61 amines,57*ss*61*62 tetraphenylborate ion,6g tetrakisdimethylaminoethylenea3 and sulphur compounds.67~s4,s8 The oxidation potentials obtained for various aromatic hydrocarbons and amines have been correlated with various calculations!a*e7 MISCELLANEOUS
APPLICATIONS
The inertness of these solvents makes them ideal for the study of easily hydrolysed compounds. Data are reported for the reduction of the triphenylcyclopropenyl and triphenylcarbonium ions in acetonitrile74 and trialkylchlorosilanes in dimethylsulphoxide.76 The latter reduction is ascribed to the product from the following reaction : RaSiCl + (CH$,SO -+ RsSiOS(CH&.+ + Cl-. Carbonium ions have also been studied in methanesulphonic acid.76 The greater solvent properties of these solvents over aqueous mixtures have been demonstrated in the determination of styrene in polystyrene,” the analysis of solvent extracts of coa1,7gthe oxidation of 3,4-dimethoxypropenylbenzene7g and the reduction of methylene quinones.sO Zusamrneufassuug-Es wird eine I)bersicht iiber die. Polarographie organ&her Verbmdungen in protonenfreien L&umgsmitteln gegeben. R&sum6-On prbsente une revue sur la polarographie des compods organiques en solvants aprotiques. REFERENCES l A. Cisak and P. J. Elving, Rev. Pokzrog. (Kyoto), 1963, 11,21. * P. J. Elving and J. M. Markowitz, J. Phyk. Chem., 1961,65,686. * P. J. Elving, J. M. Markowitz and I. Rosenthal, ibid., 1961,65,680. 4 D. E. Sellers and G. W. Leonard, Jr., Analyt. Chem., 1961,33,334. b S. Wawzonek and R. C. Duty, Rev. Polarog. (Kyoto), 1963,11,1. 6 J. D. Voorhies and E. J. Schurdak, Andyt. Chem., 1962,34,939. ’ S. Wawzonek, E. W. Blaha, R. Berkey and M. E. Runner, J. Electrochem. s S. Wawzonek and D. Wearring, J. Amer. Chem. Sot., 1959,2067 81, @A. C. Aten, C. Buthker and G. J. Hoijtink, Trams. Faraaby Sot., 1959,55, lo A. C. Aten and G. J. Hoijtink, 2. Physik Chem., 1959,21,192. I1 P. H. Given, J. Chem. Sot., 1958,2684. I* P. H. Given and M. E. Peover, J. Chem. Sot., 1968,385. Is W. Kemula and J. Kornacki, Roczniki Chem., 1962,36,1835,1849,1852. I4 P. G. Grodzka and P. J. Elving, J. Electrochem. Sot., 1963, 110,225,231. I6 P. H. Given and M. E. Pecver, Coil. Czech. Chem. Comm., 1960,25,3195. 16 G. J. Hoijtink, J. Van Schooten, E. de Boer and W. Ij. Aalbersberg, Rec. 355. 1’ D. E. G. Austen, P. H. Given, D. J. E. Ingram and M. E. Peover, Nature,
Sot., 1955,102,235. 324.
Truu. chim., 1954, 73, 1958,182, 1784.
Polarography
of organic compounds
in aprotic solvents
1235
I* A. Streitwieser, Jr. and I. Schwager, J. Phys. Chem., 1962,66,2316. lo S. Wawzonek, R. Berkey, E. W. Blaba and M. E. Runner, J. Electrochem. Sot., 1956,103,456. *OP. H. Given, M. E. Peover and J. Schoen, J. Chem. Sot., 1%8,2674. I1 I. M. Kolthoffand T. B. Reddy, J. Efectrochem. Sot., 1961, 108,980. 14 R. L. Ed&erg, D. Eicblin and J. Gavis, Analyt. Chem., 1953,25,798. 1a M. E. Peover and J. D. Davies, J. Electroanalyt. Chem., 1963,6,46 U R. Jones and T. M. Spotswood, Austral. J. Chem., 1962,15,492. Is J. V. Halhun and H. V. Drushel, J. Phvs. Chem.. 195863,110. _ I6 K. Micka, Coil. Czech. Chem. &mm.,*l957,21,.647. I7 A. Kirrrnan. J. Saveant and N. Moe. Comot. rend.. 1961.253.1106. ** A. I. Cohen; B. T. Keeler, N. H. Coy and‘H. L. Yale, kmlyt: Chem., 1962,34,216. s9 S. Wawzonek and A. Gunderscn, J. Electrochem, Sot., 1964,111,324. a0A. I. Cohen, Analyt. Chem., 1963,35, 128. *I S. Wawzonek and A. Gundersen, J. Electrochem. Sot., 1960,107,237. 8p N. Steinberger and G. K. Frankel, J. Chem. Phys., 1964,40,723. a8 R. H. Philip, Jr., R. L. Flurry and R. A. Day, Jr., J. Electrochem. Sot., 1964,111, 328. 84M. Kalousek, Coil. Czech. Chem. Comm., 1948, 13, 105. 85 R. H. Philip, Jr., T. Layloff and R. N. Adams, J. Electrochem. Sot., 1964,111, 1189. a6P. H. Rieger, I. Bemal, W. H. Reinmutb and G. K. Fraenkel, J. Amer. Gem. Sot., 1963,85,683. 8’ A. Zweig, J. E. Lebnsen, W. G. Hodgson and W. H. Jura, ibid., 1963,85,3937. ** M. E. Peover, Nature, 1961, 191,702. 88P. H. Rieger, I. Bernal and G. K. Fraenkel, J. Amer. Chem. Sot., 1961,83,3918. 40D. S. Acker and W. R. Hertler, J. Amer. Chem. Sot., 1962,84,3370. *r M. T. Jones, E. A. LaLancette and R. E. Benson, J. Chem. Phys., 1964,41,401. 4a A. Zweig and A. K. Hoffman, J. Amer. Chem. Sot., 1962,86,273. U M. Bargain, Compt. rend., 1962,255, 1948. 44 Idem, ibid., 1963,256, 1990. 45A. K. Hoffmann, W. G. Hodgson and W. H. Jura, J. Amer. Chem. Sot., 1961,83,4675. 4ED. H. Geske and A. H. Maki, ibid., 1960,82,2671. 4TW. Kemula and R. Sioda, Bul. Acad. polon., 1962,10, 107. 4BL. Holleck and B. Kastening, Rev. Polarog. (Kyoto), 1963,11, 129. 49B. Kastening, Naturwiss., 1962,49, 130. 6o M. Field, C. Valle and M. Kane, J. Amer. Chem. Sot., 1949.71.421. sx L. Holleck and D. Becker, J. Electroanalyt. Chem., 1962, 4, 321: sa A. H. Maki and D. H. Geske, J. Amer. Chem. Sot., 1961,83,1852. 6a W. Kemula and R. Sioda, J. Electroanalyt. Chem., 1963, 6, 183. 6p W. M. Schwarz, E. M. Kosower and I. Shain, J. Amer. Chem. Sot., 1961,83,3164. 55 H. Lund. Acta Chem. Stand.. 1957. 11.491. . ’ 6e Zdem, ibid., 1957, 11, 1323. . 57R. F. Dapo and C. K. Mann, Analyt. Chem., 1963,35,677. KSC. K. Mann, ibid., 1964,36,2424. SDD. H. Geske, J. Phys. Chem., 1959,63,1062. (OIdem, J. Amer. Chem. Sot., 1959, 81,4145. oxJ. W. Loveland and G. R. Dimeler, AnuZyt. Chem., 1961,33, 1196. 6s C. D. Russell, ibid., 1963,35, 1291. 6*K. Kuwata and D. G. Geske, J. Amer. Chem. Sot., 1964,86,2101. +UA. Zweig, W. G. Hodgson, W. H. Jura and D. L. Maricle, Tetrahedron Letters, 1963, 26, 1821. 65J. Billon, Bull. Sot. chim. Prance, 1961, 1923. OeE. S. Pysh and N. C. Yang, J. Amer. Chem. Sot., 1963,85,2124. 67 W. C. Neikan, G. R. Dimeler and M. M. Desmond, J. Electrochem. Sot., 1964, 111,1190. esF. L. Lambert and K. Kobayashi, J. Org. Chem., 1958,23,773. ao S. Wawzonek and J. H. Wagenknecht, J. Electrochem. Sot., 1963.110,420. 7o R. Jones and B. C. Page, Analyt. Chem., 1964,36,35. ‘l H. Lund, Acta Chem. Scand., 1959,13,192. 78 S. Wawzonek and R. C. Duty, J. Efectrochem. Sot., 1961,108, 1135. ‘a T. Kitagowa, T. P. Layloff and R. H. Adams, Anafyt. Chem., 1963,35, 1086. ‘I4R. Breslow, W. Bahary and W. Reinmutb, J. Amer. Chem. Sot., 1961,83, 1763. 7s W. Gutmann, P. Heilmayer and G. Schober, Monatsh. 1961,92,240. ‘a S. Wawzonek, R. Berkey and D. Thomson, J. Electrochem. Sot., 1956,103,513. ‘17J. Pasciak, Chem. Analit., 1960, 5,477. ‘* P. H. Given and M. E. Peover, J. Chem. Sot., 1960, 394. 79J. J. 0’ Connor and I. A. Pearl, J. Electrochem. Sot., 1964,111, 335. 8oA. S. Lindsey, M. E. Peover and N. G. Savill, J. Chem. Sot., 1%2,4558.
OF ORGANIC COMPOSTS IN APROTIC SOLVENTS
(I&T&IJ~~25 Jonumy 1%S, Accepted 5 April 196% S---A review of the polarogmphy of or@c aprotic solvents is presented.
compounds in
INTRODUCTION STUDIES of the polarographic
behaviour of organic compounds in aprotic solvents have been numerous in the past decade. The majority of the work had been carried out in ~methy~o~~de~ acetonitrile and dimethyl sulphoxide; pyridine91 liquid sulpbur dioxide,=+ me~yla~~ide,~ te~~e~yl~~~ and ~~orne~~~ have been examined briefly aud seem, with the exception of nitromethane, to offer no advantage. Nitromethane is relatively unaffected by Nchloro-compounds, which chlorinate many other common solvents, and has been used successfully in studies of these compounds. The three commonest solvents allow electron additions to be observed with many compounds, without accompanying complications due to protonation, and give more insight into the mechanism of the electrode process. This property leads to stepwise reductions of various functional groups with the formation of stable anion radicals, and in many systems, which are irreversible in solvents containing water or alcohols, become reversible under aprotic conditions. These solvents in oxidations at solid electrodes, offer a wider potential range for the study of anodic reactions than aqueous media. In aqueous media the range is limited to +l*O V, whereas in acetonitrile the range is+20 V (ZW.A@>. Additional advantages for these solvents are their synergistic elect on the reduction of halogen compounds, better solvent properties than aqueous systems and inertness towards easily hydrolysed compounds. This review will stress the investigations in which the results were either uot possible, or were better than those obtained in solvents containing water or alcohols. HYDROCARBONS
,
The first work7 dealt with the reduction of aromatic hydrocarbons in dimethylformamide and acetonitrile and it was found that it was possible ta form anion radicals from stilbene, anthracene and ~a~isyl~uo~ne
Studies of this type were extended to naphthalene, 1229
phenanthreue,
biphenyl,
1230
s.
WAWZONJX
terphenyls, phenylacetylenes8 and more complex aromatic hydrocarbons.Q-15 For a number of these examples only the first step was observed; the second reduction occurred at potentials beyond the usable range of the supporting electrolyte. Similar two-step reductions have been observed for anthracene and non-alternant hydrocarbons, in dioxan containing 4% water, but not for stilbene, which showed only a two-electron step under these conditions. l* Behaviour resembling that in solvents containing water can be obtained by adding proton donors, such as water,T phenolls and benzoic acid,7 to the aprotic solvent; the last two are better proton donors than water. The nature of the one-electron reduction step in aprotic solvents has been confhmed by large-scale electrolysis ,’ electron spin resonance studiesl’ and a.c_ polarography. lo The last of these has demonstrated that the first wave of stilbene in dimethylformamide is completely diffusion controlled and reversible. The polarographic results for these compounds parallel results obtained by the addition of alkali metals to aromatic hydrocarbons and other compounds. They suggest the possib~ity of a rapid method for dete~ng those compounds that will form anion radicals. The half-wave potentials obtained have been found to correlate with molecular orbital calculations.18 QUINONES
Quinones in these solvents are reduced first to the semiquinone anion and then to the dianonr6*1s-21 Q + e8 Q1st wave 2nd wave
Q- + err: Q”
The hydroquinones are not oxidised at the dropping mercury electrode, probably because of low ionisation; the dianion, if preformed with base, gives two anodic waves at the same potentials as those observed for the reduction of the quinone.22 The reduction of the semiquinone is not reversible at potentials more negative than approximately -1.4 V us. S.C.E and is shifted to more positive potentials by lithium ions. This phenomenon is ascribed to ion association and varies with the structure of the quinone.as The formation of the semiquinone anion has been demonstrated for anthraquinone by electron spin resonance studies,1’ and that of the dianion by the fo~ation of 9,Wdiethoxyanthracene in a large-scale electrolytic reduction in the presence of ethyl bromide.rQ The dianion can be seen around the dropping mercury electrode as a red colour. Addition of water and phenol as proton donors has no effect upon the first wave, but shifts the second wave to more positive potentials. In the presence of a suflkient amount of the proton donor, the two waves merge and give a single wave approximately equal in height to the sum of the two waves obtained in anhydrous media. This behaviour can be explained by the protonation of the quinone dianion, with the potential being determined by the rate of the reaction Q-+HA7+QII--l-A-. Benzoic acid as a proton donor behaves differently, and increases the first wave
Polarography of organic compounds in aprotic solvents
1231
at the expense of the second wave. Protonation of the semiquinone must occur to give an intermediate, which is reduced more readily than the semiquinone Q-t- H+-+QH QH+e-+QHThis behaviour parallels that observed with the anion radicals from aromatic hydrocarbons. Wydroxyl groups, hydrogen-bonded intramolecularly, in 1-hydroxyanthraquinones are found to facilitate the reduction more in dimethylfo~a~de than in aqueous ethanoi.24 o-Quinones in dimethylformamide show a similar stepwise reduction to that observed with p-quinones.la Carbon blacks, which have complex quinone structures, are reducible polarographically if present as slurries in dimethylfo~amide.~ Similar studies can only be made in an aqueous medium if adequate stirring is used.% CARBONYL
COMPOUNDS
In the carbonyl compound series, aliphatic aldehydes only give polarograpbic waves in dimethylfo~a~de~ in the presence of tetraethyla~o~um salt~.~ Phenol
(C,H&C=Q (C,H&CO-
+ e * (C&H&CO+ e --f (C,H&@-6
to form benzilic acid and diphenylethyl~rbinol, respectively. Electron spin resonance studies indicate that the anion radical generated from benzophenone is stable,17 that the intermediate from acetophenone has a half-life of about twenty seconds, but that the product from benzaldehyde is much less stable.= In agreement with these stabilities, pinacols are obtained from large-scale electrolysis of the last two compoundsW Addition of phenol as a proton donor causes the frrst wave of benzophenone to grow at the expense of the second wave. Benzaldehyde in the presence of phenol gives a new wave, intermediate between the two original waves. Benzoic acid as proton donor generates a third wave for both benzophenone and benzaldehyde, and this wave is more positive than the original first wave; this behaviour is ascribed to the reduction of the protonated form.= a,&Unsaturated carbonyl compounds in the aromatic series give two oneelectron waves,!@ and large-scale electrolytic reduction studies indicate that the intermediates from benzalacetophenone are more stable than those from benzalacetone and ~innam~dehyde. Reduction of the diketones benzil, p-diacetylbenzene and odibenzoylbenzene gives two waves, m-dibenzoylbenzene gives one and trans-dibenzoylethylene three.s5
S. WAWZONEK
1232
In all cases the Kalousek techniqueSP has indicated that the initial step is a reversible one involving the addition of one electron C6H6COCOC,H, + e G C,H,COCC,H,
C,H,COCC,H, I O-
+ e + C,H,C==CC,H, A- A-
Substitution of lithium chloride for tetrabutylammonium iodide in the reduction of benzil gives a reversible two-electron reduction wave. These mechanisms have been verified by electron spin resonance spectroscopy.% NITRILES
AND
OTHER
ACID
DERIVATIVES
In the aromatic acid series, nitriles have been found to yield a one-electron reduction wave in dimethylformamide. 36 Electron spin resonance studies are in agreement with a mechanism such as C,H,CN + e ti C,H,CN-. A similar reduction is observed for pyromellitonitrile (1,2,4,5-tetracyanobenzene) in acetonitriles7 and occurs at approximately the same point as that for pyromellitic anhydride.s8 Tetracyanoethane and tetracyanopropane in dimethylformamide give reduction waves, and electron spin resonance studies indicate that the tetracyanoethylene radical anion is formed as a product.39 Studies with a,@nsaturated acid derivatives have been numerous, and stepwise reductions are reported for tetracyanoethylene, 39 7,7,8,8-tetracyanoquinodimethan,4Q dicyanomethylene-2,2,4,4-tetramethylcyclobutanone, bis-dicyanomethylene-2,2,4,4,tetramethylcyclobutane,” diethyl azodicarboxylate,42 substituted acrylonitriles and ethyl acry1ates.m The addition of phenol and benzoic acid to the last class gives behaviours which vary with the substituents.44 NITRO
AND
NITROSO
COMPOUNDS
Anion radicals are formed in the reduction of aliphati@ and aromatic46 nitro compounds in acetonitrile46 and dimethylformamide.47 (C&.)&NO2 + e + (CH&,CNO,-. In an aqueous or alcoholic medium a similar behaviour is observed for nitrobenzene in the presence of inhibitors such as camphoP and triphenylphosphine.49 Nitrodurene at pH 12 is reported to give a one-electron wave while nitrobenzene gives only an inflection.60 The first wave for the reduction of an aromatic nitro compound is reversible, and is followed by an irreversible second wave, corresponding to a reduction to Addition of benzoic acid to nitrobenzene in dimethylformphenylhydroxylamine. amide causes the first wave to increase to a height equal to the sum of the original waves. 47 Water, when added, shifts the second wave to more positive potentials,61 and alkali metal salts shift the first wave to more positive potentials.51 The formation of the anion radical has been verified by electron spin resonance
Polarography of organic compounds in aprotic solvents
1233
studies for both the aliphatic46 and aromatica series. The aliphatic radical was found to be unstable. The reversibility observed for the reduction of aromatic nitro compounds in these solvents allows meaningful correlation of half-wave potentials for m- and p-substituted nitrobenzenes with various substituent constants.‘j2 Nitrosobenzene in dimethylformamide gives two waves and forms an unstable anion radical; the instability of the intermediate makes the first wave slightly irreversible.6s Addition of benzoic acid causes the formation of a new more positive reduction wave, and this is ascribed to the formation of a complex with benzoic acid.47 HETEROCYCLIC COMPOUNDS In the heterocyclic series, pyridinium salts are reduced in the same way in both acetonitrile and water, but the intermediate radical dimerises much more slowly in acetonitrile.m HALOGENATED COMPOUNDS Dimethylformamide is a useful solvent for the reduction of halogenated compounds. Reductions are observed for chlorobenzene68s6Q and p-chloracetanilide70 in the presence of quatemary salts; these compounds show no observable reduction wave in an aqueous medium. Trifluoromethyl groups attached to benzene rings are reducible in an aqueous medium only if activating groups such as sulphonyl are present,‘l but the reduction in dimethylformamide occurs even for trifluoromethylbenzene.28 In addition, the reduction occurs in one, two or three steps, depending upon the number and type of substituents on the aryl ring, in contrast to an aqueous medium where all fluorisz atoms are removed simultaneously. The polarographic reduction of polyhalogenated hydrocarbons in acetonitrile and dimethylformamide is unique. Carbon tetrachloride and bromide are reduced through a dihalocarbene72 in one step to the methylene compound Ccl, + 2e -+ Ccl,- + Cl. CC&- -+ ccl,
+ Cl-
Ccl, + 2e + solvent + H,CCl,. o-Dibromobenzene and o-chlorobromobenzene form benzyne as an intermediates9 and are reduced to benzene. The polarographic data for the reduction of o-bromonitrobenzene and o, m and p-iodonitrobenzene at a hanging mercury drop are at variance with the electron spin resonance data. Consideration of half-peak potentials indicates that the reduction of the nitro group occurs first, whereas electron spin resonance seems to show that elimination of halogen and formation of the nitrobenzene anion-radical are involved in the first step.73 OXIDATIONS The use of aprotic solvents in electrolytic oxidations at solid electrodes was first applied to aryl carbinolsss in acetonitrile
0--e_ CH,& =0 CH,O ’
-
-
’ CHaOH--B_ CH;O =
=
~H,OH
+ -
ZH+
0 0-
CH,O ’
-
I_ CH,OH \ CHO
1234
S. WAWZONEK
Oxidation occurs by a two-electron process to the aldehyde which, sometimes, may be further oxidised. Extension of this work to aromatic hydrocarbons indicated that they lose two electrons and form dipositive ions which, depending on the structure, may be stable or may lose a proton to form a carbonium ion. sB Pyridine when added as a proton acceptor shifts the waves to more negative potentials. The mechanism of oxidation has been verified by the large-scale oxidation of anthracene in the presence of pyridine and a perchlorate salt to 9, lo-dihydroanthranyldipyridinium diperchlorate. Oxidations have been carried out in dimethylsulphoxides7 but not in dimethylformamide, because this solvent gives a high residual current.6g Data are given for the oxidation of tropilidine,60 aromatic hydrocarbons,61 amines,57*ss*61*62 tetraphenylborate ion,6g tetrakisdimethylaminoethylenea3 and sulphur compounds.67~s4,s8 The oxidation potentials obtained for various aromatic hydrocarbons and amines have been correlated with various calculations!a*e7 MISCELLANEOUS
APPLICATIONS
The inertness of these solvents makes them ideal for the study of easily hydrolysed compounds. Data are reported for the reduction of the triphenylcyclopropenyl and triphenylcarbonium ions in acetonitrile74 and trialkylchlorosilanes in dimethylsulphoxide.76 The latter reduction is ascribed to the product from the following reaction : RaSiCl + (CH$,SO -+ RsSiOS(CH&.+ + Cl-. Carbonium ions have also been studied in methanesulphonic acid.76 The greater solvent properties of these solvents over aqueous mixtures have been demonstrated in the determination of styrene in polystyrene,” the analysis of solvent extracts of coa1,7gthe oxidation of 3,4-dimethoxypropenylbenzene7g and the reduction of methylene quinones.sO Zusamrneufassuug-Es wird eine I)bersicht iiber die. Polarographie organ&her Verbmdungen in protonenfreien L&umgsmitteln gegeben. R&sum6-On prbsente une revue sur la polarographie des compods organiques en solvants aprotiques. REFERENCES l A. Cisak and P. J. Elving, Rev. Pokzrog. (Kyoto), 1963, 11,21. * P. J. Elving and J. M. Markowitz, J. Phyk. Chem., 1961,65,686. * P. J. Elving, J. M. Markowitz and I. Rosenthal, ibid., 1961,65,680. 4 D. E. Sellers and G. W. Leonard, Jr., Analyt. Chem., 1961,33,334. b S. Wawzonek and R. C. Duty, Rev. Polarog. (Kyoto), 1963,11,1. 6 J. D. Voorhies and E. J. Schurdak, Andyt. Chem., 1962,34,939. ’ S. Wawzonek, E. W. Blaha, R. Berkey and M. E. Runner, J. Electrochem. s S. Wawzonek and D. Wearring, J. Amer. Chem. Sot., 1959,2067 81, @A. C. Aten, C. Buthker and G. J. Hoijtink, Trams. Faraaby Sot., 1959,55, lo A. C. Aten and G. J. Hoijtink, 2. Physik Chem., 1959,21,192. I1 P. H. Given, J. Chem. Sot., 1958,2684. I* P. H. Given and M. E. Peover, J. Chem. Sot., 1968,385. Is W. Kemula and J. Kornacki, Roczniki Chem., 1962,36,1835,1849,1852. I4 P. G. Grodzka and P. J. Elving, J. Electrochem. Sot., 1963, 110,225,231. I6 P. H. Given and M. E. Pecver, Coil. Czech. Chem. Comm., 1960,25,3195. 16 G. J. Hoijtink, J. Van Schooten, E. de Boer and W. Ij. Aalbersberg, Rec. 355. 1’ D. E. G. Austen, P. H. Given, D. J. E. Ingram and M. E. Peover, Nature,
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Polarography
of organic compounds
in aprotic solvents
1235
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