Anodic oxidation of aromatic aldehydes at mercury electrodes

Electroanalytical Chemistry and lnterfacial Electrochemistry, 43 (1973) 365-375

365

© Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

ANODIC OXIDATION OF AROMATIC ALDEHYDES AT MERCURY ELECTRODES

O. MANOU~EK and J. VOLKE

J. Heyrovsk~ Institute of Physical Chemistry and Electrochemistry, Czechoslovak Academy of Sciences, Prague (Czechoslovakia) (Received 10th August 1972; in revised form 30th October 1972)

INTRODUCTION

Several years ago, one of the authors of the present paper found 1 that some aromatic aldehydes, e.g. those derived from benzaldehyde or from pyridinecarbaldehydes, give anodic waves in alkaline solutions. It was assumed that these waves corresponded to oxidation of the aldehydic group. However at that time no experimental proof of the course of the oxidation could be presented. An interpretation of the observed phenomenon is the aim of this communication. We have carried out experiments using classical polarography with a dropping mercury electrode, microcoulometric measurements with a dropping mercury electrode, experiments with a stationary hanging mercury drop and a vibrating platinum electrode and investigations with the Kalousek commutator and have presented spectrophotometric proof of the identity of the product from the electrode process. EXPERIMENTAL

Apparatus The polarographic curves were recorded with an LP 60 polarograph of the Czechoslovak firm Laboratorni p~istroje, Prague. Microcoulometric measurements were carried out with CMT 50 coulometer (A. Jaissle Elektronik, Neustadt). The characteristics of the dropping mercury electrode No. 1 are given in Table 1. The hanging mercury drop used in some experiments was the E 410 type produced by Metrohm, Herisau, Switzerland. A vibrating platinum anode was used in some of the investigations. The Kalousek commutator 2 was an unpublished modification by this Institute of the original construction. For spectral measurements in the u.v. a Unicam SP 800 spectrophotometer was used. Chemicals The aromatic aldehydes were products of: Fluka, Buchs, Switzerland; KochLight Lab. Ltd., Colnbrook, England; Aldrich, Milwaukee, Wis., USA; Dr. T. Schuchardt, Miinchen, Federal Republic of Germany. They were purified either by recrystallization or redistillation so that their physical constants corresponded to literature data. Potassium hydroxide used in all cases as supporting electrolyte was analytical

366

o . M A N O U ~ E K , J. V O L K E

TABLE 1 A N O D I C P O L A R O G R A P H I C BEHAVIOUR O F A R O M A T I C ALDEHYDES a (capillary No. 1 : m = 2 . 7 3 m g s - a ; t=3.55 s; concn.= 1 x 10 -3 M; 1 M K O H ; 10% ethanol)

Number

Substituents (!n addition to to 1-CHO)

Ek/V (vs. SCE)

iJ(itA)

ocn (from the log plot)

napp (microcoulometric)

I 1I lli IV V VI VII VIII IX X

4-NO 2 3-NO 2 4-CHO 3-C1 3-CHO 4-C1 2-CHO 4-COOH 4-H 2-OCH a

-0.355 -0.340 -0.305; -0.195 - 0.270 -0.260; -0.170 -0.255 - 0.245 -0.240 - 0.230 - 0.175

6.05 6.00 12.20 7.00 12.10 6.90 3.90 6.12 6.90 6.50

0.92 0.88 0.74/0.68 1.27 0.74/0.84 1.28 0.94 1.04 0.98 1.40

-1.42 2.75 -2.75 1.28 -1.58 ---

XI (4-OI-I); XII (2-OI-I); XIII (3-OCH3, 4-OH); XIV (2-COOH); XV (4-N(CH3)z) are inactive.

reagent grade (Lachema, Brno, Czechoslovakia). The non-aqueous solvents, such as ethanol and acetonitrile, were of spectral purity (E. Merck, Darmstadt). Procedures

Current-voltage curves were measured by polarising the dropping mercury electrode or stationary hanging mercury electrode anodically from a starting potential of - 1 . 0 V vs. saturated MSE at a scan rate of 200 mV min-1. The experiments were carried out with a volume of 10 rnl in a cell d.esigned by one of the authors (O.M.). The half-wave potentials tabulated were measured with a three-electrode system. Microcoulometry was done with a dropping mercury electrode using 1 ml of a 1 × 10 -3 M solution. The electrode potential was kept constant during the electrolysis by means of a polarograph and corresponded to the limiting current of the anodic wave. The polarographic wave-height was measured at regular intervals. Its value as a percentage of the starting value was plotted versus the electric charge consumed. Preparative microelectrolysis, followed by spectrophotometric examination, was carried out similarly. RESULTS

Classical polarooraphy

The results of polarographic measurements, i.e. the half-wave potentials E~, the limiting currents i~, the values of an and napp obtained by microcoulometry at a dropping electrode, are summarized in Table 1 for substances (I)-(XV). The anodic waves observed are diffusion-controlled and irreversible as follows from the value of 0r. The dependence of the wave-height on depolariser concentration is linear over the concentration range 10 .4 M - 2 x 10 -3 M. The temperature coefficient o~=(2.3/AT) log i2fi 1 of the aldehydes studied is on average +l.6%/deg. The substances (XI)-(XV) are inactive and no anodic wave could be observed. Except

OXIDATION OF AROMATICALDEHYDES AT Hg

367

L~;

for substance (VII), aldehydes containing one aldehyde group, viz. (I), (II), (IV), (VI), (VIII), (IX) and (X) are oxidized in a 2-electron wave, whereas in (III) and (V) the total anodic limiting current corresponds to a loss of 4 electrons; the latter two compounds contain two aldehydic groups. The conclusion regarding the number of electrons participating in the polarographic electrode process was arrived at by comparing the height of the cathodic 4-electron wave of the nitro group in (II) with the height of the anodic wave in the same compound. The ratio is exactly 2:1 (cf. Fig. 1).

J

i, 0~/,.---

B3~A

p,..ff 200 rr~

-0.45

Fig. 1. Polarographic reduction of 3-nitrobenzaldehyde(II). 1 x 10-a M (II), 1 M KOH, 10% ethanol; (A) supporting electrolyte,(B) 2-electron anodic oxidation wave of aldehydic group, (C) 4-electron cathodic reduction wave of nitro group. CapillaryNo. 2, vs. std. MSE. The half-wave potentials of all these aldehydes are a linear function of hydroxyl ion concentration. The following slopes were obtained: AE½/A log [ O H - ] = -0.136 V for 10 -a M solutions of (VI), -0.104 V for.10 -4 M solutions of (II) and -0.112 V for 10-3 M solutions of (II). The half-wave potential of (VI) varies from -0.093 V (0.04 M K O H ) to -0.298 V (1.8 M KOH), that of 10 -a M solution o f ( I I ) from -0.211 V (0.04 M K O H ) to -0.396 V (1.8 M KOH). The half-wave potentials at lower depolariser concentrations are more negative: 10 -4 M (II), -0.258 V (0.04 M K O H ) and -0.423 V (1.8 M KOH). All these values were obtained with solutions containing 10~ acetonitrile.

Constant potential coulometry with a dropping mercury electrode To obtain values of n under conditions as close as possible to classical polarography, microcoulometric measurements were carried out as described above.

368

O. MANOU~EK,J. VOLKE

%/e ×\

so

\\

Fig. 2. Microcoulometric measurements for benzaldehyde derivatives. 1 x 10-3 M aldehyde, 1 M KOH, 10~ ethanol, DME, total volume 1 ml. Change of wave-height ~ during electrolysis vs. electric charge consumed in inC. Dashed lines calcd, for n=2, n=4. Anodic waves:

( - o - - c ~ ) or), (--e-~e-) (~), c-cr--~-) (~zD, (--~--m-) (g), (--t~--l~) ('~rr);(-o.~O-) (~)

The functioning of the apparatus and the reliability of the method were checked by measuring n both for the cathodic wave and the anodic wave of 3-nitrobenzaldehyde (II). As shown in Fig. 2 experimental points for the 4-electron reduction, i.e. the cathodic wave of the nitro group, lie exactly on the computed straight line of the ~i~ = f ( Q ) plot, corresponding to a consumption of 4 electrons per molecule. The electrolysis was carried out at a potential corresponding to the limiting current of the wave, E = -0.77 V (vs. SCE). On the other hand, the slope of the experimental straight line (~oiz=f(Q)) for the anodic process is higher than that computed for n=2; consequently, as can be seen in the last column of Table 1, n = 1.42. Similar values have been obtained with the other monoaldehydes, such as (VI), (VIII) etc. These values are given in Table 1 and the experimental points in Fig. 2. If two aldehydic groups are present in the molecule, e.g. in terephthalaldehyde (III) or isophthalaldehyde (V), the value of n obtained is approximately double the value found with monoaldehydes, i.e. in both cases 2.75.

Stationary mercury drop electrode Current-voltage curves with a hanging mercury drop electrode were recorded to get additional information about the possibility of carrying out preparative oxidations for the elucidation of the oxidation mechanism, and coulometric measurements. All substances that give anodic waves with the dropping mercury electrode also yield anodic peaks at a stationary mercury drop. However, there is a very significant difference: if the recording is repeated in the same solution and, in particular, without renewing the electrode surface, i.e. with the same mercury drop, the height of the peak is lowered after each recording and, simultaneously, the

• 369

OXIDATION OF AROMATIC ALDEHYDES AT Hg

potential Ep (corresponding to the maximum current) is shifted towards more positive values so that the anodic oxidation peak gradually becomes ill-defined. In the case of benzaldehyde (IX) this effect is even more pronounced (cf. Fig. 3). Here, after several recordings the anodic wave almost disappears since it coalesces with the final increase of current due to the anodic dissolution of mercury. The shift of Ept from the value for the first recording to the value Ep, after n = 4 repetitions amounts to 80 mV. Ep, does not change with further recordings. The oxidation in repeated recordings is inhibited most probably by adsorption of the oxidation product. However, an addition of for example, benzoic acid to the solution does not cause such a shift to positive potentials of the anodic wave as repeated recordings. The behaviour of dialdehydes ((III) and (V)) differs from that observed with the dropping mercury electrode. Whereas the normal polarographic curve scarcely reveals that the electrode process takes place in two steps, a clear-cut double wave results with the hanging-mercury drop electrode (cf. curve 2 in Fig. 4 for (V)) when the first curve is registered. Evidently the product of the oxidation of the first aldehydic group is adsorbed on the surface of the mercury drop and the oxidation of the other group proceeds on the covered surface, being thus inhibited; consequently Ep of the second step is shifted to more positive potentials. Measurements with the hanging mercury drop resulted in the finding that preparative oxidations of benzal-

4

2pA

7p

rnV

Fig. 3. Repeated recordings of current-voltage curves of benzaldehyde with hanging mercury-drop electrode. Mercury drop is not renewed after each recording. 1 × 10 -3 M benzaldehyde, 1 M KOH, 10~o ethanol; satd. MSE. Each curve recorded starting from - 1.0 V ( i = 0 ) to less negative potentials. The gradually increasing inhibition follows from (1)-(4). Scan rate 200 mV min-1. Fig. 4. Polarographic curves of 1 x 10 -3 M V, 1 M KOH, 10~o ethanol; (1) Dropping mercury electrode, (2-6) stationary hanging mercury drop electrode v s . satd. MSE; recorded from - 1.0 V ( i = 0 ) towards more positive potentials. Curves (2)-(6) recorded on the same drop without renewing the surface. Scan rate 200 mV min-½

370

o. MANOUgEK,J. VOLKE

dehyde cannot be performed even with stirred mercury pool electrodes; this is because of the difficulty described above.

Spectrophotometry of the electrolysis product The formation of benzoic acid in the anodic oxidation of (IX) was proved as follows: The electrolysis was carried out in the same cell as the microcoulometric measurements with the dropping mercury electrode. 1 ml of a 2 x 10-3 M solution of(IX) in 1 M KOH was electrolysed at a potential corresponding to the beginning of the limiting current of the anodic wave. The amount of electricity consumed during the electrolysis was measured with an electronic integrator with simultaneous checking of the decrease in height of the anodic wave. For spectral measurements the solution was diluted with ethanol to a final concentration of 10 -4 M (IX) in 0.05 M KOH with 5~o water. A semi-quantitative method for the determination of benzoic acid and (IX) in the presence of each other from their u.v. spectra had been worked out and the necessary calibration graphs (absorbance as a function of concentration) were available. The wave-lengths of the absorption maxima are 2=222 nm and 2=247 nm for benzoic acid and (IX), respectively. The results showed unambiguously that benzoic acid is the main product. However, a difference between the electric charge consumed and the amount of carboxylic acid formed was found, the latter being higher than the theoretical quantity calculated for a 2-electron process. Obviously a parallel chemical oxidation occurs.

Periodically changed square wave voltage Experiments with discontinuously changed square-wave voltage 3 (Kalousek commutator) revealed that no cathodic current results with the auxiliary potential Eaux in the production period corresponding to the region of the limiting current of the anodic wave. In particular, no reversible cathodic wave could be observed nor any wave at relatively positive potentials. The only exceptions are the dialdehydes (III) and (V) which give a cathodic current 3 under the above conditions, the ratio (Tcomm)¢ath./(Td)anud"being 0'03 ; (Td)anoa" is the original anodic current without commutating, (1-eomm.)eath. is the cathodic current obtained by commutating.

Influence of a Nonaqueous solvent The anodic behaviour of aromatic aldehydes is affected by the presence of nonaqueous solvents. Virtually all measurements were performed in presence of 10~ ethanol because the solubility of some of the substances described in this paper is very low in purely aqueous solution. For this reason also the stock solutions were originally prepared in pure ethanol. This fact enables a final interpretation of our finding1 that an anodic peak is observed with the aldehyde solution also at a vibrating platinum electrode. This anodic peak is caused by the ethanol-containing stock solution and corresponds to an anodic process of ethanol the concentration of which increases simultaneously with the concentration of the dissolved aldehyde. It follows from this that the mercury surface of the electrode is a necessary condition of the anodic oxidation of an aromatic aldehyde over the potential region studied. On the other hand, the quality and concentration of the nonaqueous solvent in mixtures with water also influences the half-wave potential and the shape of the anodic

OXIDATION

371

OF AROMATIC ALDEHYDES AT Hg

TABLE 2 INFLUENCE OF ACETONITRILE ON THE ANODIC WAVES OF AROMATIC (10 -3 M A L D E H Y D E I N 1 M K O H ; E~ R E F E R R E D T O S C E )

III

E~/V 10%

ethanol

- 0.305

0.38

~/V

I

E~/V

~

~",/V

-0.260

0.37

-0.355

-0.170 -0.330

0.42 0.44

-0.435

-0.240 -0.265

0.65 --

-0.375

-0.185 --

---

-0.285

- 0.225 0.34 0.55

- 0.300

- 0.280 - 0.250

0.48

- 0.285

0.43

- 0.240

- 0.235 -0.195 -0.215

30~

acetonitrile

~

-0.195 - 0.360

20%

acetonitrile

~/V

- 0.240

10%

acetonitrile

V

ALDEHYDES

0.52 --

-0.165

not measurable

not measurable

E~ half-wave potential measured for the double wave as a whole. E~ half-wave potential measured for each wave separately.

waves obtained with a dropping mercury electrode. The results of a change from 10% ethanol to varying concentrations of acetonitrile are illustrated by the data in Table 2. In (III), (V) and (I) a transition from 10% ethanol to 10% acetonitrile causes a shift of E½ to more negative values by about 50-80 inV. This results in a better development of the oxidation wave. However, a further increase in concentration of acetonitrile causes a shift of the anodic wave back to less negative potentials. In (V) this results in a coalescence of the anodic wave with the current corresponding to the anodic dissolution of mercury for the solution with 30% acetonitrile in 1 M KOH. In both dialdehydes (III) and (V) the slopes of the two waves (characterized by the value of an) increase in 10% acetonitrile as compared to solutions with 10% ethanol. The change of half-wave potentials, slopes of waves and the separation of waves of (III) are shown in log plots in Fig. 5. DISCUSSION

All experiments described in this paper, in particular the preparative reductions, point to an electro-oxidation of the aromatic aldehydic group to a carboxylate. However, we arrived at the conclusion that the anodic electrode process only occurs at the surface of a mercury electrode. The oxidation at a platinum electrode is a completely different process. For this reason a possible reaction with the mercury electrode must be taken into account. The assumption of such a reaction is strongly supported by two facts: (a) The anodic waves appear only in alkaline solution over the range of potentials in which the anodic dissolution of mercury ions starts. (b) The literature 4 states that an aldehydic group can be oxidised either by mercuric oxide HgO or by mercuric ions Hg z+. In accordance with this, we also

372

O. M A N O U ~ E K , J. V O L K E

"7

I

t~n

o

c

i I

2

-d.i

-~.2

-o'.3

E/v(SCE)

Fig. 5. Log plots of the polarographic waves of 1 x 10 -3 M (III) in 1 M K O H ; (A) 10~ ethanol, (B) 10yo acetonitrile, (C) 20~o acetonitrile. Values of ct are given in Table 2.

succeeded in oxidising the aldehydes chemically by means of the yellow modification of HgO in a test-tube. In this reaction the corresponding carboxylate was also formed. These results lead to postulating the following sequence of reactions giving rise to the formation of anodic wave of aromatic aldehydes.* Mg + 2 0 H - ~

FIg(OH)2+ OH- ~

Hg- 0(-) I OH

HgO.0H- + H20

-R "

R-CI - O - - N g - - O H M

(2)

lOI(-)

~C +

(1)

H9(Ot.-I)2 + 2e-

~

R-&-O-Hg-OH

(3)

I~

~

Hg + W20 + R - C\~I(_ I

(4).

The indirect oxidation of the aldehyde by reaction with the material of the mercury electrode could occur either via HgO or the corresponding hydroxide. According to Armstrong and co-workers 7 the formation of a solution-soluble Hg(OH)2 must be considered possible at the electrode at potentials less positive * In strongly alkaline media only the cations of divalent mercury Hg 2+ (Hg 2 + does not exist there) should be considered s. It is interesting that the only anodic oxidation of benzaldehyde described in the literature 6 was on a copper oxide electrode.

O X I D A T I O N O F A R O M A T I C A L D E H Y D E S AT Hg

373

than the oxidation-reduction potential Er of the Hg/HgO system (cf. eqn. (1)). At high solution pH dissociation of the hydroxide giving a particle, such as e.g. HgO. O H - must be taken into account s (cf. eqn. (2)). The first, rate-determining step of the oxidation reaction proper proceeds according to eqn. (3) and a mercurycontaining intermediate (analogous to that in CrO3 oxidations 9) results which rapidly decomposes to the anion of the corresponding carboxylic acid and mercury (eqn. (4)). This intermediate is so unstable that its presence was not proved e.g. by the Kalousek commutator. It follows from the polarographic measurements that the oxidation current is controlled by diffusion of the aldehyde to the dropping mercury electrode. As regards eqn. (3), even if the aromatic aldehydes were hydrated (this being uncertain in alkaline media) only the non-hydrated form would be the reactive particle towards a nucleophilic agent, such as HgO. O H - . The mechanism proposed is in accordance with observed values for AE~/A pH, AE+/A log c aldehyde, the substituent effects, AE~/A log [ O H - ] etc. Chemical reactions play an essential role in interpreting the difference between the number of electron n participating per molecule in classical polarography (n = 2) and that from microcoulometric measurements (n = 1.4)*. It can be assumed that there is competition between the electrochemical oxidation of the aldehyde as described by eqns. (1), (2), (3) and (4) and two simultaneous chemical processes. One of them is an oxidation of the aldehyde in alkaline solution by the mercuric oxide film present on the mercury surface on the bottom of the electrolysis cell, the other the Cannizzaro reaction 1° which certainly occurs in alkaline media. The rate of this reaction should be highest with 3nitrobenzaldehyde. The actual ratio between the rates of these assumed heterogeneous and homogeneous chemical reactions is being studied. The validity of the mechanism suggested in eqns. (1)-(4) is strongly supported by a significant relationship between the logarithms of the homogeneous rateconstant k2 of the oxidation of the aldehyde with chromic acid 9 and the corresponding half-wave potential. The results are summarized in Table 3. If the ratio log (k/ko) (ko is the rate constant for the oxidation of unsubstituted benzaldehyde) is plotted versus the half-wave potential a straight line is TABLE 3 H A L F - W A V E P O T E N T I A L S O F A N O D I C WAVES A N D RATE C O N S T A N T S O F H O M O G E N E O U S O X I D A T I O N 9 F O R A R O M A T I C A L D E H Y D E S BY C H R O M I U M ( V I )

Substance

Substituent E~/V (vs. SCE)

k2/imol~1 s-1

IX VI IV II I

4-H 4-C1 3-C1 3-NO2 4-NO 2

6.17 8.50 11.80 29.60 46.80

- 0.230 -0.255 - 0.270 - 0.340 - 0.355

* This value is only valid with our experimental conditions and a higher or a lower value may be obtained, depending on the speed at which the electrochemical oxidation is carried out.

374

O. M A N O U ~ E K , J. VOLKE

to 4-NO2 0,~

~

3-N

O.4 3

0 (]

0.2.5

0.30

03.5

-Ev,/V~SCE)

Fig. 6. Correlation between the logarithm of the rate constant of chemical oxidation by Cr(V1) and half-wave potential of substituted benzaldehydes. Logarithm of relative rate constant k / k o (ko=rate constant for non-substituted benzaldehyde) for oxidation with chromic acid v e r s u s half-wave potential of anodic wave.

obtained (el. Fig. 6). We also plotted the half-wave potentials of the anodic waves against the o- values according to Hammett 1t, 12. In this case the accordance between the experimental points and their position as required by the value of the o- constant was not so satisfactory as in the above case*. Nevertheless, ifa straight line was drawn its slope gave a value of the reaction constant P,~,R= -0.178 V; its sign and the fact that an oxidation is studied here points to the fact that a nucleophilic attack such as that described in eqn. (2) is the potential-determining step. Consequently substituents such as O H (in (XI), (XII), (XIII)), 2-COOH (in (XIV)) or 4-N(CH3)2 (in (XV))-4DH or - C O O H are dissociated in strongly alkaline media and cause such an increase of electron density in the reacting aldehydic group that reaction (3) cannot occur within the range of attainable potentials and the expected anodic wave is overlapped by the current of the anodic dissolution of mercury. Moreover, the formation by dissociation of a negatively charged phenolate or carboxylate group on the benzene nucleus may affect the orientation of the aldehydic group toward the surface of the mercury electrode and thus render the reaction with HgO. O H - more difficult. The anodic waves described in this paper are different from those found t3 with ketones such as acetone. The latter waves were only observed with depolariser concentrations approximately 10-50 times higher than in our work (in the present paper normal polarographic concentrations could be studied) and a stable mercury* This may mean a2 that an effect is involved which is characteristic for the given reaction series, but is not included in the effect of the particular substituent on benzoic acid dissociation, and which has the same relative effect on the polarography and the rate of the chemical reaction.

OXIDATION OF AROMATIC ALDEHYDES AT Hg

375

containing acetone derivative was proved to be the reaction product. It follows, in particular from Table 2, that an increase in polarity of the solvent, e.g. when passing from ethanol to acetonitrile or when lowering the concentration of acetonitrile in a water-acetonitrile mixture, renders the oxidation of aromatic aldehydes at mercury electrodes easier. This results in a shift of halfwave potentials of the anodic wave to more negative potentials. An analogous phenomenon was observed in the alkaline hydrolysis of some esters 14. Here a similar intermediate is formed with hydroxyl ions by a nucleophilic attack and in wateracetone and water-ethanol mixtures the rate constant decreases with decreasing polarity of the solvent mixture. An interpretation 14 based on the role of solvation effects was presented. In our opinion it does not explain all aspects of the problem. For this reason the dependence of E~ of the anodic wave of aromatic aldehydes on the dielectric constant of the solution will be investigated in more detail. ACKNOWLEDGEMENT

We thank Dr. Petrosjan, Moscow, for carrying out some of our experiments in this Institute. SUMMARY

An interpretation of the anodic oxidation waves of aromatic aldehydes in alkaline solution obtained with a dropping mercury electrode is presented. The corresponding carboxylate is formed in the electrode process which proceeds via an unstable mercury-containing intermediate. The material of the electrode plays a decisive role in this mechanism since the oxidizing species proper is most probably a HgO" OH ~-) type particle. A linear relationship has been found between the halfwave potentials of the anodic waves of substituted benzaldehydes and the logarithm of the corresponding relative rate constants of chromic acid oxidation. A nucleophilic attack is the potential-determining step. REFERENCES 1 J. Volke, d. Electroanal. Chem., 10 (1965) 344. 2 M. Kalousek and M. Rfilek, Collect. Chem. Commun., 19 (1954) 1099. 3 0 . Manou~ek and J. Volke, Disc. Faraday Soc., 1973, in press. 4 M. Hudlick~, and J. Trojfinek, Preparativn[ Readkce v Oroanick~ Chemii I., Praha, Nakl. (2SAV, 1953. 5 A. F. Cotton and G. Wilkinson, Advanced lnoroanic Chemistry, Interscience Publishers, John Wiley and Sons, New York, 1962, p. 482. 6 R. McKee and J. Heard, Trans. Electroehem. Soc., 65 (1934) 301. 7 R. D. Armstrong, W. P. Race and H. R. Thirsk, J. Electroanal. Chem., 19 (1968) 233. 8 A, B. Garret and A. E. Hirschler, J. Amer. Chem. Soc., 60 (1938) 299. 9 K. B. Wiberg and J. Mill, J. Amer. Chem. Sot., 80 (1958) 3022. 10 C. K. Ingold, Structure and Mechanism in Organic Chemistry, G. Bell and Sons Ltd., London, 1953, p. 706. 11 L. P. Hammett, Physical Organic Chemistry, McGraw-Hill Book Co. Inc., New York, 1940, p. 188. 12 P. Zuman, Substituent Effect in Organic Polarooraphy, Plenum Press, New York, 1967, pp. 46, 112. 13 M. Heyrovsk~, Collect. Czech. Chem. Commun., 28 (1963) 26. 14 F. Tommila, Ann. Acad. Sci. Fenni., Ser. A 11, 139 (1967) 1.