Anodic reactions of aromatic compounds

ELECTROANALYTICAL CHEMISTRY AND INTERFACIAL ELECTROCHEMISTRY Elsevier Sequoia S.A., L a u s a n n e - P r i n t e d in T h e N e t h e r l a n d s

137

REVIEW

ANODIC REACTIONS OF AROMATIC COMPOUNDS

K. S A S A K I * AND W. J. N E ~ V B Y

Department of Physical Chemistry, University of Newcastle upon Tyne (England) (Received D e c e m b e r 2 i s t , 1966; in revised f o r m April 26th, 1968)

CONTENTS

I. General introduction II. Anodic oxidation of aromatic compounds I. Oxidation potential Ia. Choice of solvent 2. Oxidation of hydroquinone 3. Phenols and aromatic amines I I I . Substitution and addition reaction i. Introduction 2. Reaction of anthracene 3. Reaction with solvents 4. Substitution of inorganic group 5. Methoxylation of alkylbenzene side chain and halogenation of aromatic ring IV. Electrolytic Kolbe reaction and its related problems I. Kolbe reaction 2. Acetoxylation of aromatic compounds 3. Reaction of benzoic acid. I. GENERALINTRODUCTION

Various types of organic electrode reactions reported before the early part of 194o are compiled in FICHTER'S book 12z. Some later work has been reviewed b y ALLEN122. In this brief review, the subjects treated have mainly been confined to the anodic reactions of aromatic compounds. Also, references have been restricted to those published in the last decade. An anodic reaction is an oxidation reaction as far as the charge transfer reaction is concerned. However, it is found experimentally that the stable products obtained by an anodic treatment are usually not those expected by ordinary chemical oxidation. As will be seen later, a solution of toluene in methanol gives benzylmethyl ether and benzaldehyde. Although there is no doubt that these compounds are not * P e r m a n e n t a d d r e s s : F a c u l t y of E n g i n e e r i n g , H i r o s h i m a U n i v e r s i t y , H i r o s h i m a , J a p a n .

j. Electroanal. Chem., 20 (1969) 137-165

138

K. SASAKI, W. J. NEWBY

formed cathodically, it is not a simple problem to know whether or not they are the result of the electrochemical oxidation of toluene. Nitrobenzene is reduced cathodically to form anilinel-a; the reverse, however, is not true. Usually, anodic treatment in aqueous solution detaches the amino group of aniline from the aromatic ring and replaces it b y a hydroxy group 4,5 but this is not always so 6. A sulphonic group on an aromatic ring is eliminated b y both cathodic 7 and anodic s,9 treatment. Many examples of this sort of complexity will be shown in the later sections. Consequently, it is of little value to talk about the products of anodic reaction without specifying the experimental conditions, i.e., salt, solvent, electrode potential, current density, electrode material, etc. This complexity is, in part, an intrinsic problem in the study of organic electrode reactions. I t must be noted, however, that not a small part of the present confusion has been caused by inadequate application of experimental technique. Quite frequently, for instance, no effort has been made to measure the electrode potential at which the reaction is carried out. The fact that the use of the dropping mercury electrode is largely restricted to the cathodic process greatly hinders the study of anodic reactions, since solid surfaces are usually less reproducible than mercury and often exhibit specific catalytic effects. Fortunately, recent development of electronic potentiostats 10 has made it easy to carry out a controlled-potential electrolysis. The application of E S R (Electron Spin Resonance absorption) measurement to the electrode system is receiving increasing attention 11. The introduction of various instrumental techniques has brought about great changes in this field of study. Mass spectrometers also provide very useful information. Thus GESKE12 confirmed the mechanism of the formation of diphenyl from the anodic oxidation of tetraphenylborate in acetonitrile solution, to be an intra-molecular dimerization.

2 B(C6Hs)4 -4e - -

i. 2 . i ~

B+ Ci

--

/~,-J

Upon electrolysis of two mixtures of deuterated and ordinary tetraphenylborates in which the ratio, B(C6Ds)4-/B(C6Hs)4- had values of o.91 and o.3o, he observed the respective diphenyl ratios to be I.O and 0.38. This makes the following coupled reaction unlikely,

2 B (C6FI5)4

-4e

C©

since it would lead to a mixed diphenyl.

j. Eleclroanal. Chem., 20 (1969) 137-165

139

ANODIC REACTIONS OF AROMATIC COMPOUNDS I1. ANODIC OXIDATION OF AROMATIC COMPOUNDS 11. I. Oxidation potential

I n this section, those c o m p o u n d s t h a t are k n o w n to be i n v o l v e d directly in a charge transfer reaction are considered. I n an actual e x p e r i m e n t , it is often difficult to know w h e t h e r t h e c o m p o u n d s concerned transfer electrons to th e electrode. One of t h e quickest ways to answer this question is b y the application of polarography. A l t h o u g h t h e devices an d techniques of c h r o n o p o t e n t i o m e t r y are simple and its principle is similar to polarography, it is n o t always to be r e c o m m e n d e d since the analysis of t h e r esu l t an t c h r o n o p o t e n t i o g r a m requires a d v a n c e d experience and knowledge. Th e d a t a of t h e polarographic half-wave p o t e n t i a l of various a r o m a t i c compounds are g i v en in Tables I - 3 . TABLE 1 THE HALF-WAVE POTENTIALS OF AROMATIC HYDROCARBONS (IN VOLTS)

Lund la (Ag/o.z N A g + in CH3CN)

Neikam et al.14, is (dg/o.; N A g + in CH3CN)

Pysh and Yang TM (SCE in CH3CN)

]3enzenenaphthalene Naphthalene Anthracene Tetracene Phenanthrene Chrysene Pyrene Coronene Perylene

2.00 1.31 o.84 0.54 (1.2o)** 1.23 I.I3 0.86 (1.12)** 0.93 o.55

2.08 1.34 o.84

2.30 1.54 I.O9 0-77 1.5° 1.35 1.16 1.23 o.85

Indene Acenaphthene Fluorene Fluoranthene Retene Picene Azulene

1.23 i.ii 1.25 1.18 1.18

1.21

I.OO

1.36 1.65

o.91

1.5o 1.89 1.48

1.9 I 1.52

1.43" 1.55

Triphenyl

I, 3,5 -Trinlethylbenzene I, 2,3-Trimethylbenzene 1,2,4-Trimethylbenzene

1.20

1.64

trans-Stilbene

Toluene p-Bromotoluene p-Chlorotoluene o-Xylene m-Xylene p-Xylene

1.72 1.2o

1.45 1.33 o.71

Styrene Cyclohexene Diphenyl i, i-Diphenylethylene

i. IO

Eberson 17 (SCE in CH3COOH)

1.93

1.98 1.72 1.76 1.57 1.58 1.56

1.98

1.55 1.58 1.41

1.8o

I.SI

1.89 1.91 1.77 1.9o

J. Electroanal. Chem., 20 (1969) 137 165

K. SASAKI, W. J. NEWBY

140 T A B L E 1 (Continued)

Lund 13 (Ag/o.z NAg+ in CHsCN)

Neikam et al. i4,1~ (Ag/o.z N A g ÷ in CH3CN)

i, 2 , 3 , 5 - T e t r a m e t h y l b e n z e n e I, 2,4, 5 - T e t r a m e t h y l b e n z e n e Pentamethylbenzene Hexamethylbenzene

1.43 1.29 1.28 1.16

Ethylbenzene iso-Propylbenzene tert-Butylbenzene n-Propylbenzene I-Methylnaphthalene 2-Methylnaphthalene 2,3-Dimethylnaphthalene 2,6-Dimethylnaphthalene 2, 7 - D i m e t h y l n a p h t h a l e n e 2-Nitronaphthalene 9-Nitroanthracene 9-Methylanthracene 9-Bromoanthracene 9, I o - D i m e t h y l a n t h r a c e n e 9, I o - D i b r o m o a n t h r a c e n e

1.96 1.87 1.87

0-99 (1.48)** 0.65 1.15 (1.47)**

1,2-Benzanthracene

0.92

Pysh and Yangl6 (SCE in CHsCN)

Eberson 17 (SCE in CH3COOH)

1.62

1.62 1.52

1.97

1.24

1.43

1.22

1.45 1.35 1.36

1.o8 i .08 1.12 1.62 1.25

o.96

I-Methyl-l,2-benzanthracene 2-Methyl-i,2-benzanthraeene 3-Methyl- I, 2 - b e n z a n t h r a c e n e 4-Methyl-I,2-benzanthracene 5 -Methyl- i, 2 - b e n z a l l t h r a c e n e 6-Methyl-i,2-benzanthracene 7-Methyl-I,2-benzanthracene 8-Methyl-i,2-benzanthracene 9-Methyl-I,2-benzanthracene I o-Methyl- i, 2 - b e n z a n t h r a c e n e I i-Methyl-i,2-benzanthracene x 2-Methyl- I, 2 - b e n z a n t h r a c e n e

0.65

0.87 1.18 1.14 1.14 1.14 1.14 1.15 I.I 5 1.o8 1.13 1.15 1.14 1.14 1.o 7

* t~ef. 18. ** E½ of second w a v e .

I I . za. Choice of solvent. Most organic compounds are insoluble in aqueous solution. Consequently, the use of an appropriate non-aqueous solvent, usually an organic solvent, is often necessary. The solvent suitable for the electrode reaction must be able to dissolve a sufficient quantity of supporting electrolyte as well as the organic substrate of interest. I t is also required that the solvent be stable in the presence of the electrode reaction. Depending upon the purpose of the experiment, the dielectric constant or viscosity of the solvent may be an important criterion of selection. Non-aqueous solvents are frequently employed for the purpose of a comparison or confirmation of the reaction mechanism observed in aqueous solution. In such cases, the proton availability of the solvent will become important. A general review of organic solvents is available 123. Solvents for electrochemical use have been discussed, for instance, by GIVEN j . Electroanal. Chem., 20 (1969) 137-165

I4I

ANODIC REACTIONS OF AROMATIC COMPOUNDS TABLE 2 THE

HALF-WAVE

POTENTIALS

OF PHENOLS

AND AROMATIC

A M I N E S 19

Volts vs. SCE in isopropanol Phenol

0.633

Aniline

0.625

p-Cresol

o.543

p-Toluidine

o. 537

o-Cresol m-Cresol

o.556 o.6o 7

o-Toluidine m-Toluidine

0.595 o. 6o6

p-Ethylphenol

o.567

p-Ethylaniline

o. 568

o -Ethylphenol m-Ethylphenol

o. 55 I o.616

0-Ethylaniline

o. 6o2

p-Anisidine

o. 393

p-Methoxyphenol

o.4o6

0-Anisidine

o.498

o-Methoxyphenol

o.456

m-Anisidine

o. 615

m-Methoxyphenol

o. 619

p-Phenetidine

o. 388

p-Ethoxyphenol

o.4i 3

o-Ethoxyphenol

o.45 I

o-Phenetidine m-Phenetidine

o.499 o. 6o8

m-Ethoxyphenol

o.62o

p-Nitrophenol

o. 924

p-Nitroaniline o-Nitroaniline

o.935 o. 989

o-Nitrophenol

o.846

m-Nitro aniline

o. 854

m-Nitrophenol

o. 855

p-Aminoacetophenone

o.82o

p-Hydroxyacetophenone

o. 79 I

o-Aminoacetophenone

o.847

o-Hydroxyacetophenone

o. 8Ol

m-Aminoacetophenone

o.758

m-Hydroxyacetophenone p-Hydroxybenzoic acid

o. 754 o.716

p-Aminobenzoic acid

o.714

Anthranilic acid

o.676

Salicylic acid

o.845

m-Aminobenzoic acid

o.668

p-Chlorophenol o-Chlorophenol

o.653 o.625

p-Chloraniline

o.675

0-Chloraniline

o. 742

m-Chlorophenol

o. 734

p-tert- B u t y l p h e n o l

o.578

m-Chloraniline 2,4-Dimethylaniline

o. 5oo

o-tert-Butylphenol

o. 552

3,5-Dimethylaniline

o.587

p-Phenylphenol o-Phenylphenol

0.534 0.563

2,5-Dimethylaniline

o. 578

2,6-Dimethylaniline

o. 576

2,5 -Diphenylphenol

o. 47 i

2,5-Dichloraniline

o. 798

3,4-Dimethylphenol

o.513

4-Methoxy-2 -nitraniline

o. 744

3,5-Dimethylphenol

o.587

2 -Methoxy- 5 -nitraniline

o. 822

2,4-Dimethylphenol 2,6-Dimethylphenol

o.459 o. 427

2,6-Diethylaniline

o. 578

2-Aminoanthracene

o.44"*

2,5 -Dichlorophenol

o. 695

2-Fluoreneamine

o.53"*

2-Methyl-4-tert-butylphenol

o. 5 o I

2-tert-Butyl-4-methylphenol

o.465

i-Naphthylamine 2- N a p h t h y l a m i n e

o.54"* o. 64 **

2-Methoxy-4-methylphenol

o. 37 i

2,4-Di-tert-butylphenol 2,6-Dimethoxyphenol

o.487 o. 317

2,6-Di-tevt-butylphenol

o.378

2-Methyl-6-tert-butylphenol

o.429

0.65*

0.774

* vs. Ag/o.I N A g + in CH~CN 13. ** vs. SCE in CH~CN 16. j . Electroanal. C h e m . , 20 (1969) 137-165

142

K. SASAKI, W. J. NEWBY

TABLE 3 THE HALF-WAV]~ POT]gNTI&LS OF CARBINOLS 20 AND Mt~THOJ~Y BI~NZENES 21 (IN VOLTS)

Carbinols~O (vs. Ag/o.x N Ag+ in CH3CN)

Methoxybenzenes2l (vs. SCE in CH3CN) Anisole

1.76

p-Methoxybenzyl alcohol

1.22

1,2-Dimethoxybenzene

1.45

Anisaldehyde

1.63

1,4-Dimethoxybenzene

1.34

m-Methoxybenzyl alcohol

1.28

i, 2,3-Trimethoxybenzene

1.42

o-Methoxybenzyl alcohol

1.25

i, 2,4-Trimethoxybenzene

1.12

o-Naphthylcarbinol

1.25

1,3,5-Trimethoxybenzene

1.49

1,2,3,4-Tetramethoxybenzene

1.25

p-Chlorobenzyl alcohol

1.79

1,2, 3,5-Tetramethoxybenzene

1.o9

m-Chlorobenzyl alcohol

1.85

i, 2,4,5-Tetramethoxybenzene

o.81

o-Chlorobenzyl alcohol

i. 84

Pentamethoxybenzene

i.o7

p-13romobenzyI alcohol

I. 75

Hexamethoxybenzene

1.24

p-Iodobenzyl alcohol

1.59

Furfuryl alcohol

1.33

Cinnamyl alcohol

1.36

p-Nitrocinnamyl alcohol

1.72

Methyl-p-methoxyphenylcarbinol

i. 24

Methyl-p-chiorophenylcarbinol

I. 8o

Methyl-p-iodophenylcarbinol

1.5 8

2 -Naphthylcarbinol

I. 29

Fluorenol

i. 3 i

2,7-Dichlorofluorenol

1.32

4-Methoxybenzhydrol

1.23

4,4 -Dimethoxybenzhydrol

1.22

Phenylnaphthylcarbinol

1.27

4,4-Dichl°robenzhydrol

1.77

p-Bromophenylethylglycol

1.62

Benzhydrol, methylphenylcarbinol, benzyl alcohol, o-nitrobenzyl alcohol, p-bromophenacyl alcohol, cyclohexanol and allyl alcohol

* vs. SCL in acetic acid 17. jr. Etectroanal. Chem., 20 (1969) i37-I65

2.0

1.67"

143

ANODIC REACTIONS OF AROMATIC COMPOUNDS

AND PEOVER124, and CISAK AI~D ELVING 22. Among various solvents, acetonitri]e and N,N'-dimethylformamide are recommended by many authors2a-~5, les as suitable for various electrochemical purposes. The properties of acetonitrile as a solvent have been described by KOLTHOFF AND COETZEE24 and BILLON26. The standard electrode potential in acetonitrile has been discussed by KOLTHOFF AND THOMAS27. Dimethylformamide was discussed by HALE AND PARSONS125. Pyridine is described in ref. 22. Electrochemistry in dimethyl sulphoxide has been reviewed extensively by BUTLER2s. I I . 2. H y d r o q u i n o u e

The benzoquinone hydroqninone couple is the best known example of a reversible organic redox system, but the detailed mechanism of the reaction is still a subject of dispute. The mechanism of polarographic reduction of several quinones in both N,N'dimethylformamide and acetonitrile has been established by WAWZONEKet al. 23 and by t?~DSBERGeta[. 29. The reduction takes place in two steps: o in the first wave,

(~

/

+

e

~

I~l

h

0

TI01

tSi

J6,

in thesecond wave, ~

+ e

--

~

IOI

IOI

The product, hydroquinone dianion, reacts with the solvent. i~i

OH

+ 2 CH2CN- (or 2 CON(CH3)2- )

+ 2 CH3CN(or 2 HCON(CH3)2) EC)I

OH

The oxidation of hydroquinone on a mercury electrode in these non-aqueous solvents, however, does not give rise to a wave. On addition of water, an oxidation wave appears. The quantity of water added affects only the half-wave potential, which shifts in a negative direction. TURNER AND ELVINGa0 have reported that the overall oxidation of hydroquinone at a pyrolytic graphite electrode in pyridine solution is an irreversible twoelectron reaction probably involving coupling with pyridine. They provide no evidence concerning the reversibility of the first step, however. The use of the mercury electrode to study the oxidation of hydroquinone is of course difficult in aqueous solution. Therefore, the work so far reported has been done using platinum electrodes. VETTER31 extensively examined the Tafel region for both the cathodic reduction of benzoquinone and the anodic oxidation of hydroquinone. He concluded that the number of electrons involved in hydroquinone oxidation is unity and hence the whole reaction is consistent with two one-electron steps. HALE AND PARSONS32 confirmed VETTER'S view by studying the polarographic j . Electroanal. Chem., 2o (1969) 137-165

144

K. SASAKI, W. J. NEWBY

reduction of/%quinones at a dropping mercury electrode. In this paper, the energies of various possible intermediates were estimated, and it was suggested that two one-electron transfers involve almost equal activation energies. LOSKKAREV AND TOMILOVaa challenged this view and stated that the whole reaction proceeds in one step. Quite recently, however, another group of Russian workers, BAGOTZKII et al. 34, published reports which support the reaction scheme of WETTERfor Pt electrodes. According to them, although the rate-determining step is still undetermined, the number of electrons involved in the rate-determining step is unity regardless of the acidity of the environment. The magnitude of the exchange current is, in contrast to the observation of other authors, stable if purified chemicals are used. The reaction scheme proposed by BAGOTZKIIet al. for acidic media is: -e

e

QH2 ~- (QH2)~a ~--~ (QH2)%,, ~ e

(QHz)2%a ~- QH~ 2+ # Q + 2 H + e

In alkaline media, the proposed scheme is: -e

QH, ~- Q 2 - + 2 H + ~- (Q)2-~a ~

--e

(Q) ~ d ~ - ~ (Q)~a ~ Q e

e

The conclnsion that n = I for the hydroquinone oxidation on both carbon and platinum electrodes was also reached from chronopotentiometric experiments in aqueous solutions aS. Three isomers of dihydroxybenzene were studied b y ELVlNG AND KI~IVlS36 using a graphite electrode. They reported that the meta-compound, resorcinol, behaves differently from the other two isomers. In the case of the reversible redox couple where electrolysis is carried out at a low overpotential, a complicated group of products cannot be expected. However, particularly for a synthetic purpose, it is usually impractical to carry out electrolysis at a low overpotential since the current is usually insufficient to obtain an adequate yield in reasonable time. SANTKANAM AND KRISHNAN 37 reported a potentiostatic electrolysis of some substituted hydroquinones in aqueous sulphuric acid. The products were the quinones of the respective compounds. The coulombic efficiency was affected markedly b y the potential at which the experiment had been done. Only at a particular potential was the coulombic efficiency almost IOO% (they called this the optimum potential) and the current decay an exponential function of time, indicating the reaction to be reversible. In Table 4, the value of the optimum potential is shown, together with that TABLE 4

Hydroquinone Monomethyl HQ 2,3-Dichloro HQ Bromo HQ Chloro HQ 2,5-Dichloro HQ 2,6-Dichloro HQ

Eo~,. (v, SCE)

E~ (v, SCE)

1.3oo 1.25 o.9o 0.80 0.55

o.537 o.547 o.498

j . Eleetroanal. Chem., 2o (1969) 137-165

o.558 0.502 0.482 (ist wave) 0.579 (2nd wave)

145

ANODIC REACTIONS OF AROMATIC COMPOUNDS

of the ordinary half-wave potential which has been determined b y SASAKI AND ]~RABI3s using a carbon paste electrode. I t can be seen from the Table that the optimum potentials are different from the half-wave potential, not only in their values but also in their order. Thus, the question arises of wily the current efficiency is lower at the half-wave potential than at the optimum potential. In other words, what is the physical significance of the " o p t i m u m potential"? They concluded that the optimum potentials in a series of substituted hydroquinone decrease in order of increasing electron density at the hydroxy group.

II. 3. Phenols and aromatic amines Anodic oxidations of phenolic compounds are much more complex than that of hydroquinone. VERMILLION AND PEARL39 studied the reaction of some substituted phenols in acetonitrile. They state that there are two different reaction paths depending upon the nature of substituents and the alkalinity of solution. The first is an irreversible two-electron oxidation which has been exemplified in the oxidation of 2,6-di-tertbutyl-p-cresol. The product was 2,6-di-tert-butyl-4-methyl-4-methoxycyclohexadienone.

- 2e

~

CH3

CH

etc

L

+ CH30 H ~-~

CH3

3 R = tePt

-

butyL

The second mechanism, which is favoured in alkaline solution, is a one-electron reaction producing a phenoxy radical as an intermediate. OH

O-

()

A similar scheme complicated b y subsequent rapid chemical reactions has been postulated b y CHAMBERS AND CHAMBERS4° for the oxidation of a substituted quinol phosphate in aqueous solution. This type of reaction has been found in the oxidation of vanilinate anion. The product was dihydrodivanillen. The half-wave potential of both types of reactions has a linear relationship with the H a m m e t t constant. HAWLEY AND ADAMS41 studied the oxidation of fl-methoxyphenol in aqueous sulphuric acid. The reaction is an irreversible two-electron oxidation.

The anodic oxidation of aromatic amines also takes place rather easily. Here "easily" means that the oxidation can be observed at a fairly low potential. As will be shown later, products vary widely depending upon the conditions of the experiment, and thus it is not an easy matter to determine the reaction path. j . Electroanal. Chem., 2o (1969) 137-165

I46

K. SASAKI, W. J. NEWBY

TABLE 5 Solution

pH

E~ (cathodic)

E~ (anodic)

iclia

I M NH3, I M NH,tC1 I M NaC104 I M HAc, I M NaAc

9.2 7.0 4.3

--o.79 --0.79 --o.68

0.78 0.95 o.97

2.94 2.98 3.44

The electro-oxidation of aromatic amines in aqueous solution has been studied extensively by the ADAMS'3,6,41-45 school during the last few years. OLSON et al. a, using polarography, determined the number of electrons required in the oxidation of the amino group of fl-nitroaniline. The result is shown in Table 5. It can be seen that the ratio of anodic limiting current (oxidation of amino group) to cathodic limiting current (reduction of nitro group) is roughly three. Since the reduction of a nitro group is a six-electron reaction, it was concluded that n = 2 for the oxidation of the amino group of this compound. ~/~IZOGUCHI AND ADAMS 42, and GALUS AND ADAMS 43,44 studied the reaction mechanism of the oxidation of N,N'-dimethylaniline (DMA) using both carbon and platinum electrodes. In each case, the reaction order determined from the effect of the concentration of the oxidation current, din i/d In C, was 1.1-1.3 and the kinetic parameter was nD= 1.23. Both these results, i.e., (i) the reaction order is unity with respect to DMA and (if) nfl is more than unity, lead to the conclusion that n = 2 . The reaction product was identified to be tetramethylbenzidine (TlV[B). Since n was found to be two, the reaction path proposed was not

~

~

TMB

+ 2H +

H

but ~'+/ N DMA

~+./ N

='-

+ H

"-'

~--- T M B

H DMA

This conclusion, however, has been changed by the authors 45 quite recently. Before describing that work we shall first deal with the work of some other authors. This series of works by the Adams' school seems to be self-consistent. However, there is another group of authors 4,5 who state that the number of electrons in the oxidation of an amino group is unity instead of two. SNEAD AND REMICK4 studied the oxidation of p-aminophenol. The reaction, as a whole, is a two-electron oxidation and forms benzoquinoneimine which decomposes rapidly to give fl-benzoquinone. OH

0

0

NH2

NH

0

dr. Electroanal. Chem., 2o (1969) 137-165

147

ANODIC REACTIONS OF AROMATIC COMPOUNDS

This, of course, means that the oxidation of the amino group is a one-electron reaction. VOORKIES AND DAVlESa studied the same compound using a somewhat specialized technique. They employed compressed carbon black as the working electrode on which the organic compound of interest adsorbs. This electrode was subjected to current reversal chronopotentiometry. Since the organic compound is adsorbed on the electrode, there is no complication b y diffusion, and hence the chronopotentiometric transition waves can substitute for a coulometer. The resultant chronopotentiogram is shown in Fig. i. During process A, p-aminophenol is oxidized completely to p-benzoquinoneimine, which decomposes partly to p-benzoquinone. On reversing the current flow, p-benzoquinoneimine is reduced at wave B and p-benzoquinone at wave C forming p-aminophenol and hydroquinone, respectively. The number of electrons thus observed was two per molecule of p-aminophenol. These authors also studied p-toluidine and aniline using the same technique. The transition wave of p-toluidine was poorly defined; however, there is no doubt that the reaction involves one electron. Thus, upon reversal of current, toluidine radicals return to toluidine. This work obviously contradicts that of ADAMS cited above. We have been interested in this discrepancy. However, the discrepancy seems to be resolved b y a recent article published b y the ADAMS'45 school. After the completion of the original manuscript of this review, SEO et al. reported work 45 covering the reaction of triphenylamine (TPA) in acetonitrile. An ESR study has been employed to identify the intermediate species of the reaction. They report that TPA in acetonitrile is oxidized at o.98 V SCE by a one-electron step to form a TPA monovalent cation. The resultant cation dimerizes quickly to form tetraphenylbenzidine (TPB). T P B is oxidized more easily than the parent molecule, TPA, via two one-electron steps; the product is a quinoidal di-cation (TPB2+).

¢\../¢ N

¢\÷.1¢ N

¢ \ ,N/ ¢

-e

..x2 "

I

I

N

N

+ 2H +

e H

- e TPB

~

TPB+

-e ~ e ~

TPB2+

It is interesting to note that the existence of a methoxy group on the aromatic ring stabilizes the radical cation, thus giving rise to no dimerization in this case. This was demonstrated b y N,N'-methylphenyl-p-anisidine.

OCH3

OCH3

~-~

dimer

dr. E l e c t r o a n a l .

Chem.,

20 (1969) 137-165

148

K. SASAKI, W. J. NEWBY

In any case, the paper of SEO et al. 45 obviously disagrees with their previous papers, cited above. Although they have tried and failed to detect the DMA cation radical, predicted by the one-electron mechanism that they previously accepted, they now believe that the oxidation of DMA is probably a one-electron reaction.

i.o 'o15

I

0

- 0 i,5

VscE

Fig. I .

The reaction of aniline itself seems very complicated. On anodic treatment, aniline in aqueous sulphuric acid changes to a coloured insoluble substance. According to MOHILNER et al. 6, this substance is an emeraldine-like octamer, which is only soluble in concentrated sulphuric acid. Using the same procedure that was used to study the reaction of DMA, the authors obtained the following values of the parameters; n~ = 1.2-1.3 and d in i/d In C = 1.6 1.7. Since the latter figure is nearer to two than unity, they concluded that the primary electrode reaction is a second-order reaction with respect to aniline. Consequently, it was concluded that n = I per molecule of aniline. The study of VOORIilES AND DAVIES 5 w h o used a compressed carbon electrode, was also complicated. In contrast to p-aminophenol or toluidine, the transition wave of aniline oxidation showed a non-integral value of n having a value of between two and three. A recent voltammetric study (WAwzONEK AND MCINTYRE46) of the oxidation of aniline, p-nitroaniline and other substituted anilines in acetonitrile in the presence of pyridine, found simpler behaviour. They suggest that aniline undergoes an initial one-electron step followed by a series of steps leading to coupling with pyridine to form the appropriate diazo compound. BILLON47,4s and co-workers had earlier found similar behaviour in the oxidation of some substituted anthranylamines in anhydrous acetonitrile. PARKER AND ADAMS 49 reported the appearance of two one-electron waves when o- and p-phenylendiamine were oxidized chronopotentiometrically in molar hydrochloric acid at a platinum rotating disk. Only a single two-electron wave was found in solution buffered in the range, pH = 1.7-1o. ELVlNG AND KRIVlS36 obtained similar results at a graphite electrode. They found that the meta-isomer showed only a one-electron wave. MELCHIOR AND ~V~AKI5° and ADAMS et al.51, 52 established the existence of tile j . Electroanal. Chem., 20 (1969) 137-165

149

ANODIC REACTIONS OF AROMATIC COMPOUNDS

stable mono-positive radical ion of p-phenylenediamine in acetonitrile and aqueous perchlorate solutions as a result of this two-electron oxidation. ADAMS postulated that this was the result of the reactions:

~

+

~

~4-

+

NH 2-

2e

+

2H +

NH

+

0_ _ 1

NH

=

~ 2

NH 2

The ortho- and meta-isomers do not give rise to the equivalent radical cation but will react with the para-radical cation if present. MARK AND ANSON5a re-examined the oxidation of p-phenylenediamine in the region, p H = o - I o , b y chronopotentiom e t r y and concluded that the second wave was due to the oxidation of the doublyprotonated amine. The tertiary diamine, N-dimethyl-p-phenylenediamine, oxidizes 54 in a similar manner. In solutions of high pH, the initial two-electron step is followed b y deamination in aqueous solutions, leading to the quinoneimine, or b y coupling with solvent in non-aqueous solvents. A study of the application of the rotating disk electrode to this reaction has recently been published aS. The rate constants of the subsequent chemical reactions have been determined. As has been shown, an aromatic amino group is oxidized to an imino group and is substituted finally b y a hydroxy group. This seems to be the case also in the sulphonic group, which has not so far received much attention. According to FlCttTERS, 9, benzene sulphonic acid is oxidized anodieally in the following scheme :

[~

°H t

clecomposes fumatic acid

0

OH

0

OH

)

i.e., a sulphonic group is detached from the aromatic ring. This is, however, the result of non-controlled potential electrolysis. T h a t the sulphonic group is more stable to anodic oxidation than the hydroxy group has been demonstrated b y MARl( AND ATKIN 56. They have shown that 2,5-dihydroxybenzene mono-sulphonic acid is oxidized to the corresponding quinone, losing two electrons.

od'o: S%H j . EIectroanal. Chem., 2 0 (1969) 1 3 7 - 1 6 5

15o

K. SASAKI, W. J. NEWBY

III. SUBSTITUTIONAND ADDITIONON THE AROMATICRING II[.

5. Introduction The most exciting topic regarding mechanisms to attract the interest of many workers during the last decade is the dispute concerning the reaction intermediate. The argument has been particularly concentrated on the substitution reaction. Strictly speaking, there are two different viewpoints on the reaction intermediate, i.e., the free radical theory, and the carbonium ion theory. Up to ten years ago, a free radical mechanism for the anodic reaction of organic compounds had been accepted without serious criticism. However, during the past decade there has been a growing tendency for authors to insist upon the carbonium ion rather than the free radical mechanism. Undoubtedly, this is the result of progress in the field of organic chemistry with the growing instrumentation of electrochemistry promoting this tendency. As a consequence, the classical free radical mechanism has been replaced in many cases by the earbonium ion mechanism but the reverse case is very rare. Of course, this does not mean that the possibility of the free radical mechanism has been totally denied; the fact that work supporting the carbonium ion mechanism so far reported usually lacks kinetic information is, however, a great disadvantage. In connection with this problem, we generally suppose that a substitution on an aromatic side-chain should be subject to a free radical mechanism, and a substitution into an aromatic ring should be explained by a carbonium ion mechanism. This is, however, purely a speculation. The reality of the speculation is the matter of future study. The electrode reaction of an organic compound gives rise to various products depending upon the combination of various factors such as substrate, solvent, supporting electrolyte, etc. Various examples of this sort of complexity can be found in the reactions described in this section. Electrolytic means of introducing various groups into an aromatic ring or a side chain is interesting from the standpoint of organic synthesis. Many papers have been published on this account121,122. On the other hand, the substitution reaction is also interesting from the kinetic viewpoint, since knowledge of the position on which substitution takes place will give some additional information with which it will be possible to determine the reaction mechanism. In our view, in addition to the usual electrochemical approach, there must be two alternative methods of approach in the systematic study of electrolytic substitution. The questions to be answered are: i. How the substituted group or groups that exist in an aromatic ring already affect the electrolytic substitution reaction of a particular substituent group. 2. What sort of atoms or atomic groups substitute and what sort do not on a particular aromatic ring. These are obviously of more physical organic than electrochemical interest. Unfortunately, it seems that no report has been published along this line, while there are a fairly large number of papers13,1s,1% 51-~9 which have studied the inter-relationship between the half-wave potential and various physico-chemical characters of certain series of compounds (refs. 58 and 59 refer to the cathodic halfwave potential). J. Electroanal. Chem., 2o (1969) 137-165

151

A N O D I C R E A C T I O N S OF A R O M A T I C C O M P O U N D S

I I I . 2. Reaction of anthracene Anthracene is oxidized at a fairly low potential. I n order to show t h a t the 9- and Io-positions of the anthracene ring are attacked, LUND la demonstrated the formation of a pyridinium salt. I n the presence of pyridine, an acetonitrile solution of anthracene was electrolysed using sodium perchlorate as the supporting electrolyte. The product was 9,Io-dihydro-9,Io-dipyridiniumanthracene perchlorate

H

-2e

@

~

2CLO 4-

H

The same sort of reaction was demonstrated for 3,4-dimethoxy-propenylbenzene (DMPB) b y O'CONNOR AND PEARL6°. cH3o

cH - 2e

_

CH3~C--C,~CH3

f

This reaction proceeds with lOO% coulombic efficiency. Oxidation of anthracene was also studied chronopotentiometrically b y V O O R H I E S AND FURMAN61. I n his case, n was assumed to be two, according to LUND, and the value of the diffusion coefficient of anthracene in acetonitrile was estimated as ten times greater t h a n t h a t in aqueous solution. If, therefore, n is assumed to be unity, the diffusion coefficient will become forty times larger. I n the case of phenolic compounds, VERMILLION AND PEARLa9 found t h a t n changes from two to one according to the nature of the substituent group. EBERSON AND NYBERG 17 pointed out t h a t the presence of a nucleophile such as acetate will favour the two-electron transfer from an aromatic substrate. These studies indicate t h a t n should be two for electrolytic oxidation of anthracene. The work of FRIEND AND OHNESORGE~2, who studied the potentiostatic oxidation of anthracene in acetonitrile, disagrees with this. According to the latter authors, anthracene releases one electron from its ~-system to form a radical cation (I) which dimerizes to give bianthrone (II). When naphthalene was examined, polymerization was observed and the anode was coated. 0

x2 H I

©

An interesting observation was made b y O'CONNOR AND PEARL6° in their s t u d y oi D M P B oxidation. W h e n DIV[PB was oxidized in the absence of pyridine, j . Electroanal. Chem., 2 0 (1969) 1 3 7 - I 6 5

152

K. SASAKI, W. J. NEWBY

the charge transfer reaction was found to be a reversible one-electron step. This product dimerizes and undergoes ring closure.

~ / I e CH3

CH30~

+ + ~C CH--CH --CH --CH H30. ~ .CH ~+ / I I / I. II CH--CH3I ~ I II I It ~.H30~"~-'/ I ] CH30-~ J I~OCH3 OCH3

4CH30-.v,/'~,/- C,~.

c-

L. ~.:

CH--CH3 CH30~'/L"~H +/.CI H--CH3 CH

QCH3

OCH3

~~

~

cH3

OCH3

Subsequent work by PEOVER63,64 and BARDe5 has provided evidence favouring an initial one-electron step for the oxidation of this type of compound, on the basis of cyclic voltammetric oxidation of a number of condensed ring hydrocarbons in acetonitrile and methylene chloride. BARD was able to stabilize some of these cationic radicals at the temperature of liquid nitrogen. ADAMS66 obtained similar results using nitrobenzenes as solvents. For some compounds such as 9,io-diphenylanthracene, a one-electron reaction resulted in stable cation radicals, while for others, such as anthracene, the initial step appeared to involve two electrons, or more. Since the limiting current increased with decreased rate of rotation of the rotating disk electrode, he has suggested that the oxidation of these compounds also involves an initial one-electron step, but is followed by very rapid reactions that give it the appearance of a two- (or more) electron step. This is consistent with the earlier work which indicates a change to a two-electron initial step in the presence of pyridine, but clearly more evidence is required before a firm conclusion can be reached on this point. The oxidation of 5,Io-dihydro-5,io-dimethylphenazine in acetonitrile adheres rigidly 67 to the theoretical criterion of cyclic voltammetry at a rotating disk electrode for two reversible one-electron steps. It has been used as a model for this work. I I I . 3. Reaction with the solvent

Many cases are known in which primary products of an electrode reaction react with solvent molecules to form stable compounds. Ross et ala s obtained an acetate of dimethylformamide from the electrolysis of a solution of potassium acetate in acetonitrile containing dimethylformamide. CHa /"

[ -2e ---+

(CHO)N

CH + 2] J (CHO)N

\

\ CH~

j . Electroanal. Chem., 2o (1969) 137-165

CH2-OCOCH~ c~IaCOO/ -~ (CHO)N \

CH3

CH~

153

ANODIC REACTIONS OF AROMATIC COMPOUNDS

A similar compound was obtained from the electrolysis of a solution of lithium benzoate in dimethylformamide 69. Tile authors of the latter paper explained their result b y the formation of a benzoyloxy free radical. However, the free radical mechanism seems unlikely in this sort of reaction. According to the reviewer's 70 experience, dimethylformamide is a relatively unstable solvent to anodic oxidation and decomposes at an appreciable rate at 1. 5 V SCE. Ritter's reaction, which is utilized to prepare N-alkyl acetamides, is explained b y a mechanism in which carbonium ion attacks the electron pair of the CN-group. Thus, EBERSON AND NYBERG attempted to produce the carbonium ion of alkylcarbonate b y means of an electrolytic method. Their first experiment 71,7' was carried out in a solution of tert-butyl acetate in acetonitrile, which is much more stable to anodic oxidation than dimethylformamide. A 40% yield of N-tert-butylacetamide was observed. In their recent paper 7a, the formation of N-benzylacetamide has been demonstrated in the same way.

oH3

C ~

CH3

~

CH3CN

cH3

2 H20 _

coc 3

CH3~ "CH3 CH3 This work is particularly interesting, since the intermediate formation of benzyl cation is proved (See section IV-2).

I I I . 4. Substitution reactions of inorganic groups Electro-oxidation of aniline in hydrochloric acid results in chlorination 74 of the aromatic ring, while the same compound in sulphuric acid gives an emeraldinelike octamer 6. According to ERDELYI TM, electrolysis of aniline in hydrochloric acid gives triehloraniline, trichloroquinone and tetrachloroquinone. The formation of trichloraniline indicates that, in aqueous solution, the chlorination of an aromatic ring precedes the oxidation of the amino group. I t was also shown that a high concentration of hydrochloric acid favours chlorination. If ethylene is bubbled into an aqueous chloride solution during electrolysis, both ethylenechlorohydrin and ethyleneglycol are obtained 75. Competition between chlorination and oxidation can be governed solely by the experimental conditions. Many instances are known in which the supporting electrolyte substitutes or adds on both the aromatic ring and the side chain. These are acetoxylation of various aromatic compounds17, 7G-Ts, cyanation of several compounds 79,s°, rhodanation of phenol sl and aniline s2 derivatives and sulphonation of hydroquinone derivatives s3. Halogenation and nitration are the typical reactions which have been studied b y m a n y workers121,122. We shall give particular attention to acetoxylation later in section IV-2. Halogenation will be discussed in the section on methoxylation Rhodanation was studied for phenol and aniline derivatives b y MELINIKOV et al. sl and FICI-ITER AND SCHONMANN 82, respectively. A fact which is common to the several compounds examined is that the thiocyanide group substitutes invariably j . Electroanal. Chem., 20 (1969) 137-165

154

K. S A S A K I , W. J . N E W B Y

at the para-position to the amino or hydroxy group

HO~..~ CH 3

HO HO . ~L..sCN ; ~..CH3 CH 3

HO .~-.~C, .~..SCN

OH

OH

"SeN SCN

No attention, however, has been given to this reaction in recent years Reports on electrolytic sulphonation have attracted little interest. However, sulphonation of hydroquinone is a well known reaction in the field of photographic processes (the homogeneous phase reaction of hydroquinone with sulphite was discussed by Lu VALLES4). It is commonly accepted that, in a photographic developing process, the reaction is 2n A g + + H 2 Q + n SOs 2- -+ H~Q(SOa ) n + 2 n Ag

n~< 4

This reaction has been demonstrated electrolytically b y SASAKI* et al. sa. The equilibrium potential of the sulphite sulphate couple is lower than that of H Q - B Q so that sulphite discharges first on a platinum electrode. However, on a carbon electrode, the overpotential of the sulphite-sulphate couple is very high and the oxidation of hydroquinone precedes the sulphite oxidation. Potentiostatic electrolysis confirmed that the sulphonation of hydroquinone occurs under the condition at which hydroquinone is discharged. TSUTSUMI and co-workers 79 first reported the electrolytic cyanation of tetraline, toluene and anisole. They first explained their results b y a free radical reaction and later by an electrophilic substitution s0 of CN + formed on the anode --2e

CN- - - - ~ CN + R H + CN + --> RCN + H+ This explanation was, however, immediately challenged b y PARKER AND BURGET85, who showed that no eyanoanisole was found at an anode potential of 1.2 V, in spite of the fact that cyanide ions were actually discharged at this potential. At 2.0 V, cyanoanisoles were formed, showing that the primary electroactive species is anisole. Thus, according to them, the reaction should be expressed as:

-2e

"CN-~

+H

+

o, p-CN

I t is worthwhile to note that there is another possibility which has not, so far, been suggested, i.e., * To whom correspondence and requests for reprints should be directed. J. Electroanal. Chem., 2o (1969) 137-165

ANODIC REACTIONS OF AROMATIC COMPOUNDS

+ 0

I55

+ OCH3

I ~ ~ CN-

- - - ,-,e-

c~

+H+ 0,p-CN

The E S R study of the oxidation of anisole 21 seems to be positive support for this mechanism (see section 111-5). I I I . 5. Methoxylation of side chain and halogenation of ring In this section, we shall be concerned mainly with the methoxylation of alkyl benzene which has been studied b y SASAKI et al.S< When methanolic solutions of certain aromatic compounds are electrolysed, the methyl ether of the respective compounds is obtained. CLAUSON-KAAs et al. s7 obtained 2,5-dihydro-2,5-dimethoxyfuran from the methanolic solution of furan. BELLEAU AND WEINBERGs8 studied the methoxylation of some methoxysubstituted benzenes. In both cases, quite high current efficiencies (86% for furan and 88% for dimethoxybenzene) were reported.

CH30 ~ _

<< << OCH3

-h'-C~OC CH30 OCH3

H

OC 3

H3

~

OCH3 CH30 OCH3

CH3 CH3

OCH3

OCH3

OCH3

< CH3~]OC H3 OCH3 CH3O~

CH30 OCH3

CH3O OCH3

OCH 3

~C/~OCH3 [" ~- QCH3 ~OCH3

Although BELLEAU proposed a free radical mechanism for this reaction, this mechanism is not completely acceptable. The reason for this is that the oxidation potential of methoxy-benzenes is not particularly high, indicating the possibility of direct charge transfer to methoxy-benzenes. ZWEIG et al. 21 observed the anodic formation of the cation radical of anisole in both acetonitrile and concentrated sulphuric acid solution. The life of this radical was estimated by cyclic v o l t a m m e t r y to be of the order of I sec. When methanolic solutions of toluene, ethylbenzene or cumene are electrolysed, a common product is the side-chain substituted methyl ether of the correspond]. Electroanal. Chem., 20 (1969) 137-165

150

K. SASAKI, W. J. NEWBY

ing hydrocarbonS6, 39. Several products are obtained simultaneously besides the laethyl ether, but these are affected b y the nature of the supporting electrolyte used. The etherification on the side chain seems to be a free radical reaction which pi 'Jceeds in the following way, x - - . 5:

R H + ~ 2 -+ ! ~ + X H

(2)

CHaOH -~ CH36 + H + + e

(3)

+cm6

CH30R

(4)

Considering the circumstances of the experiment, there is no doubt that the primary electrochemical reaction is the discharge of the supporting electrolyte itself, (I). A difficulty arises in step (3). There is no direct proof that a neutral methanol molecule is oxidised to form a methoxy free radical 9°. Stable products from the electrolysis of absolute methanol are invariably carbon monoxide and formaldehyde 91, a fact which gives no useful information about the mechanism. In contrast to this, step (2) seems more probable for several reasons. The first is that the oxidation potentials of alkylbenzenes are very high (nearly 2 V in acetonitrile la,s6) and it can only be observed in a stable solvent such as acetonitrile. Methanol, on the other hand, is a rather unstable solvent against anodic oxidation and is decomposed at an appreciable rate at 1.5 V SCES6. Therefore, it is unlikely that alkylbenzene in methanol contributes appreciably to the charge transfer. Further, both dimerized and disproportionated compounds, known to be typical products of a free radical reaction, are found s9 among the products obtained b y electrolysis. TSUTSUMI and co-workers 89 studied the same reaction and found that the relative reactivity of several hydrocarbons was in agreement with that found by SZWARC92 for the removal of hydrogen from various hydrocarbons. I l l . 5a. Aromatic haIogenation. When halide is used as supporting electrolyte, tiara- and ortho-substituted halogen derivatives are found, besides the methyl ether. In contrast to the methoxylation on a side chain, there is no doubt that the halogenation of an aromatic ring is an electrophilic substitution reaction, i.e., the reaction between hydrocarbon and molecular halogen which is produced anodically from halide anion. Generally, except for fluoride, the oxidation potentials of halides are low. Therefore, in a solution containing halide, it can be expected that the primary electroactive species is the halide ion which will form molecular halogen upon electrolysis. TSUTSUMI et al. found that an aliphatic double bond of stilbene and styrene s9 Call be substituted b y both methoxy and bromine upon electrolysis of a methanolic bromide solution. Both methoxylation and bromination of an olefinic double bond, however, has been known to occur b y a homogeneous reaction when stilbene is dissolved in a methanolic solution of bromine 93. Speculating that the aromatic halogenation should be caused b y molecular halogen, SASAKI et al. s6 performed an analysis of the products of the homogeneous reaction of ethylbenzene with bromine in methanolic media. The j . Electroanal. Chem., 2o (1969) 137-165

ANODIC REACTIONSOF AROMATICCOMPOUNDS

157

result is shown in Table 6. Four main products of the homogeneous reaction detected by gas chromatography were identical with those obtained from electrolysis. The problem is, however, to determine the possible contribution of atomic halogen to the whole reaction. Various types of free radical halogenation of organic compounds have been studied extensively by RUSSELL and co-workers 94. There is no reason to ignore the reaction of atomic halogen in the electrode system.

TABLE 6 C

o

n

d

Dark Light Electrolysis

Product ~

Styrene (%)

Methyl etcher

o-Br compound

p-Br compound

3. i 5.4 2o

trace 14.5 I9

12 trace 2.2

22 trace 6.6

In connection with this, an interesting fact can be seen in Table 6. The product distribution of the homogeneous reaction is affected appreciably by light. Of the four products detected, styrene and c~-methoxyethylbenzene were produced predominantly in light, and ring-substituted bromine compounds predominated in the dark. The formation of the latter compounds may be explained in terms of electrophilic substitution. On the other hand, the reaction which occurs in the side chain may be considered to be the removal of hydrogen by a bromine atom produced photochemically. The products obtained by electrolysis using ammonium bromide as the supporting electrolyte were not affected significantly by light. A free radical halogenation was also reported by RUEHLEN et al. 95 for the reaction of n-dodecane in hydrochloric acid solution. Products were mono- and dichloro-n-dodecane. Assuming that the relative yield of chlorinated compounds depends on the number of hydrogen atoms to be substituted by chlorine atoms, they calculated the relative yield of two chlorinated compounds to be: R1/R2 = - (52 + i n x)/25 In x where R1 and Re are the mole fractions of mono- and di-substituted compounds in the product, and x is the mole fraction of unreacted n-dodecane. The prediction of this equation agreed with observation when the current density was high (7o-18o mA/cmS). Iv. ELECTROLYTICKOLBE REACTIONAND ITS RELATEDPROBLEMS IV. ±. Electrolytic Kolbe reaction Free radical formation in the course of organic electrode process has been frequently proposed for various types of reaction. In fact, if the stable products were the only information that could be obtained, it would be possible to explain almost all electrode reactions in terms of a free radical mechanism. For instance, as far as we are concerned only with the stable products, it is j. Electroanal. Chem., 20 (1969) 137-165

158

K. SASAKI, W. J. NEWBY

impossible to distinguish the following two different processes: RH --> R ' + H + + e -+ R~ R H + ~2 -+ R ' + H X -+ R2 With the progress in modern techniques such as ESR or potentiostatic methods, the carbonium ion intermediate has drawn attention ~7,9o,96,97, in many cases, instead of the classic free radical mechanism. The electrolytic Kolbe reaction is one of the most representative reactions that is accepted as proceeding via free radical intermediates. It must be noted, however, that, even in this reaction, there are some authors 96,9s who criticize the free radical mechanism. The free radical mechanism of the Kolbe reaction, although there are several different viewpoints on the matter, can be expressed by a sequence of steps such as the following: RCO2- -+ RC02 + e RC02' --> R" +CO~

(I) (II)

RCO~' + R" --> RCO2R

(III)

2 R" -> R2

(IV)

EBERSON17,71,98 is Suspicious of the formation of acyloxy free radicals. According to his calculation 9s, the primary product of the Kolbe reaction should be a carbonium ion produced by a two-electron oxidation, --2e

RCO2- ~+ RCO~+ However, experimental evidence so far reported seems to favour the acyloxy radical formation, since three groups of workers have shown independently that the number of electrons involved in the Kolbe reaction (acetate discharge in acetonitrile) is unity. Because the Kolbe reaction takes place at a very high electrode potential at which most ordinary solvents decompose, little is known about the kinetic characteristics of the reaction. GESKE99 first determined the number of electrons involved in acetate discharge. His method was based on the comparison of the diffusion-limiting currents of acetate and iodide ion in acetonitrile. ANSON100 studied the same problem employing potentiostatic coulometry. Recently, SASAKI'Scolleagues l°x carried out a similar experiment in order to study the effect of the nature of the electrode material on the Kolbe reaction. All this work confirmed that n = I for acetate discharge, indicating that the primary product should be the acetoxy free radical rather than the carbonium ion. Since the formation of acyloxy free radical has been confirmed, the general scheme shown in eqns. (I)-(IV) becomes more acceptable. SASAKIeta/. 102 examined this view from the thermodynamic standpoint. In their paper, Polanyi's rule t2G, which states that a linear relationship exists between the activation energy and enthalpy change in a free radical reaction, was assumed to apply to each elementary process of the Kolbe reaction. Although the assumption they made was rather arbitrary, the result agreed with the experimental findings. According to them, the J. Electroanal. Chem., 20 (I969) 137-165

ANODIC REACTIONS OF AROMATIC COMPOUNDS

159

rate of the decarboxylation process decreases with decreasing number of carbon atoms in a molecule of the homologous series of normal aliphatic acids. This treatment has been strongly criticized by CONWAY AND VIJH l°a. The prediction, however, accords with the work of WILSON AND LIPPINCOTT76 who studied the discharge step of acetate and propionate in both aqueous and acetic acid solutions. In order to determine the rate-determining step of acetate discharge, they employed a pulsating current electrolysis. If the second-order reaction step is rate-determining, then the total rate of ethane formation will decrease with increasing frequency of current interruption, since the steady concentration of radical intermediate will decrease because of frequent interruption of current. On the other hand, the current interruption will not affect the rate of ethane formation if the rate-determining step is of first-order. The interruption of current did not affect the rate of the reaction of acetate. Therefore, they concluded that the rate-determining step is a first-order reaction, indicating that the decarboxylation step is the most probable. On the other hand, in the case of propionate, the rate of ethylene formation was greatly affected b y the frequency. Accordingly, they concluded that a second-order step is rate-determining for the reaction of propionate. 10-11

10-2i "7 m o* J

E u _

E

10-2

10-4

~ 10 -3

1 10-2 sec

_

I 10-1

_

O

I 1.0

Fig. 2.

Although their treatment was somewhat ambiguous, the idea of pulsating current electrolysis is unique and interesting. FLEISCHMANN et al. 1°4 developed this technique in a more rigorous form. They applied potentiostatic pulse electrolysis to a solution of aqueous acetate in order to determine the mechanism. A plot of the rate of ethane formation against the pulse width applied is shown in Fig. 2. I t is obvious that the appreciable change in the rate of ethane formation appears in a very short pulse time, i.e., less than IO -a sec. For longer pulses, the rate becomes virtually constant. Unfortunately, this very interesting experiment cannot be compared with a rigorous mathematical analysis, since the evolution of oxygen as well as the formation of the surface oxide of the platinum electrode took place simultaneously during this short period of time. The transient current during a pulse had a discontinuous break which was, no doubt, due to these complications. A similar study in non-aqueous media might give interesting information, though the experiment will be difficult. j . Electroanal. Chem., 2o (1969) 137-165

I00

K. SASAKI, W. J. NEWBY

All the experiments described above were done using a platinum electrode. When the electrode material is changed to some other metal, a substantial change is frequently observed. For instance, it is well known that on a gold electrode in aqueous solution no Kolbe reaction takes place 105. In methanolic solution, however, the Kolbe reaction takes place on a gold electrode l°a as well as on a platinum electrode. Lead dioxide behaves differently from either platinum or gold electrodes even in methanolic solution76,77,1°6-109. KOEItL°6 compared the reaction of some aliphatic acids using platinum and carbon electrodes. Different products were obtained depending upon the electrode material. In the case of platinum, the products agreed with those expected from the free radical mechanism; however, on the carbon electrode there was no agreement. For example, the products from butylate were cyclopropane and propene in the ratio of 2 to I. The formation of these compounds indicates the reaction to be a carbonium ion mechanism. Side products were iso-propyl-n-butylate and n-propyln-butylate. This also confirms the above view. KOEHL'S work suggests a possibility that the free radical mechanism is favoured on the platinum electrode while carbonium ion mechanism is favoured on the carbon electrode. The experiment of URATA et al. 1°~, which was described previously, was carried out on this account and they found no difference in n for platinum, carbon and lead dioxide electrodes. Ironically, EBERSON11° repeated KOEHL'S experiment and found that carbon electrode behaves in the same manner as the platinum electrode. It has been found that some substituents on the ring suppress the Kolbe reaction. Thus WLADISLAWAND GIOI~A~11 found that nitro-substituted mono- and di-phenylacetic acids were methoxylated instead of undergoing the expected Kolbe reaction. CONWAY AND VIjH have recently studied ~19,1z°,127 the Kolbe reaction on platinum using potentiostatic and potentiodynamic methods in trifluoracetic acid systems where the water concentration varies from anhydrous to high concentrations. They accept the radical mechanism of the reaction but stress the adsorbed nature of these radicals. They believe that these adsorbed radicals are responsible for the anomalous kinetic measurement they have obtained ~9 for the evolution of oxygen on gold from solutions containing a carboxylic acid. They have found that the presence of adsorbed oxide is inhibiting since the rate of reaction was a maximum in the anhydrous system where oxide could not be detected. These and other aspects of Kolbe reaction have been discussed 12s in their recent review of the Kolbe reaction.

IV. 2. Acetoxylation of aromatic compounds In the past, the reaction process of I - I V has not been seriously questioned. As a matter of course, several authors attempted to trap the intermediate free radical using certain hydrocarbons as the radical scavenger. Polymerisation initiated by methyl or acetoxy radicals have also been reportedl°S, 1°9. SMITH AND GILDE1°6 studied the addition reaction of both methyl and acetoxy groups to butadiene and isoprene in methanol. They studied the addition reaction of butadiene in methanol containing potassium propionate and found the formation of 4-0ctene 107. When cyclohexene was present in a solution of methanolic acetate the products were p- and m-dimethylcyclohexene 107. j . Eleclroanal. Chem., 2o (1969) 137-165

ANODIC REACTIONS OF AROMATIC COMOUPNDS

I(3I

Since this experiment was done without controlling the electrode potential the actual mechanism seems to be complex. The mechanism of the anodic reaction of the unsaturated double bond has not been so well studied. STANIENDA18 proposed the oxidation of diphenylpolyene as follows :

~[j (CH=CH)n-CH=CH~

1st wave

2

nd

~

@/CCH=CH)o--~H--dH.,~ ~

fiCH=CH)~--~H--*CH ~

W I L S O N AND H A Y A S H 1 7 7 , and W I L S O N AND L I P P I N C O T T 76, studied the reaction of anisole in acetic acid solution. In both cases, the authors proposed the formation of methyl and acetoxy free radicals. Acetoxylation of an aromatic ring does not seem, however, to be reasonably explained b y a free radical mechanism. There are increasing numbers of paperslT,71,Ts,97,112,113 which criticize the free radical mechanism of this sort of reaction. Among several papers, a recent paper of EBERSON AND NYBERG17 seems to be most representative. Since they have included an extensive discussion of not only acetoxylation but also various sorts of anodic substitution reactions, no additional description seems to be necessary. Their argument appears to be based on the following points: (I) Before discussing anything about the reaction mechanism, it is necessary to know the oxidation potential of each reactant. This contains nothing unusual. However, since in m a n y papers published, this simplest requirement has been ignored, it is worth noting at this stage. (2) In the case of acetoxylation, the discharge potential of acetate is much higher than that of anisole or several other compounds they studied. Consequently, the primary electroactive species cannot be acetate but the hydrocarbon itself. Thus, in general, the acetoxylation of an aromatic ring can be expressed as: -2 e

Ac -

R H ---+ R H 2+ ----> AcR + H+

In order to show this, they carried out the electrolysisat an anode potential lower than the half-wave potential of the compound concerned. Thus, mesitylene, the half-wave potential of which is 1.9o V, was subjected to electrolysis at 1.65 V resulting in 1,3,5-trimethylphenyl acetate and 3,5-dimethylbenzyl acetate in the ratio of 9 to I. Hexamethylbenzene (E~, 1.52 V) was electrolysed at 1.5o V to give the corresponding benzyl acetate.

~

CH2OCOCH3

j . Eleetroanal. Chem., 20 (1969) 137 165

102

K. SASAKI, W. J. NEWBY

Supporting evidence has been provided by Ross et al. 114. They showed that if tetramethylammonium nitrate was substituted for the potassium acetate, a different set of products was obtained. One of these, 3,3',5,5'-tetramethylbibenzyl is undoubtedly the result of a free radical mechanism. This difference is attributed to the fact that nitrate ion is discharged at a lower potential than the hydrocarbon and thus initiates the reaction. We do not doubt their conclusion. At least at the present stage, this paper is the most reliable and convenient. At the same time, however, we cannot say that there is nothing questionable. First of all, the fact that no kinetic study has been made in this paper is a great disadvantage. Their experiment was carried out in acetic acid media in which the half-wave potentials seem to be higher by about IOO to 2o0 mV than in acetonitrile. Although they did not observe the oxidation wave of acetate in acetic acid, it can be observed in acetonitrile at a half-wave potential of about 1.6 V 99-101, Since the physical significance of the half-wave potential in nonaqueous media is not straightforward, it is necessary to employ several alternative experimental methods, say, for example,-direct confirmation of the intermediate species by ESR studies in order to establish a definite proof of the reaction mechanism. I V . 3. Reaction of benzoic acid Benzoic acid (more probably benzoate) does not form the usual Kolbe dimer, diphenyl, upon electrolysis 11a. However, benzoyloxy free radical has been assumed as the primary product of anodic reaction by analogy with the reaction of other carboxylic acids. FICHTER AND STENZL obtained the 2- and 4-isomers of phenylpyridine 116 from the electrolysis of a solution of benzoic acid in pyridine containing diethylamine as the supporting electrolyte. BUNYAN AND HEY 117 repeated this experiment recently and obtained 2-, 3- and 4-phenylpyridine which has been considered as evidence of formation of the phenyl free radical. From the electrolysis of benzoic acid in acetonitrile containing naphthalene, WlLSI~IRE115 obtained benzoates of I-naphthol and 4-hydroxy-I,I'-dinaphthyl which was also considered as proof of the benzoyloxy free radical. The anodic reaction of benzoic acid in dimethylformamide has been studied b y TAKESHITA and co-workers6% 11s. It is interesting to note, however, that when benzene is present in the solution, the formation of diphenyl was observed. This suggested the possibility of a free radical mechanism, e.g.,

~/COO"

~. --~

+ CO2

This seems unlikely, since when toluene was examined in place of benzene, the products were the ortho- andpara-isomers of toluyl benzoate, but neither diphenylmethane nor dibenzyl, which could be expected from a free radical mechanism, was found. Although the authors of this work consider the reaction to be a free radical mechanism, there is some doubt that this is so. dr. E l e c t r o a n a l . C h e m . ,

2o (1969) 137-165

ANODIC REACTIONS OF AROMATIC COMPOUNDS

163

EBERSON, who is strongly sceptical of the free radical acetoxylation, seems sympathetic 17 to free radical benzoyloxylation. The reason is that, in his estimation, the rate constant of decarboxylation from benzoyloxy radical is of the order of I sec -1, while that of the acetoxy radical is lO 9 sec-L Consequently, benzoyloxy free radical will have a much greater chance to react with its environment. There is no doubt that decarboxylation in the Kolbe reaction takes place in the adsorbed state on the electrode surface. Benzyloxy free radical, having a longer life than acetoxy, will also have more change to undergo further oxidation. In fact, the oxidation potential of benzoate/benzoyloxy was predicted by EBERSON 98 to be lower than the acetate/acetoxy couple. If this is the case, the formation of the toluyl ester observed by TAKESHITA et al. m a y also be explained by a carbonium ion mechanism. Whatever the actual case m a y be, a series of controlled experiments is required. ACKNOWLEDGEMENT

The authors express their gratitude for helpful criticism and suggestions in the preparation of this review to Professor W. F. K. WYNNE-JONES, Professor H. t~. THIRSK and Dr. M. FLEISCttMANN of the University of Newcastle upon Tyne, England. SUMMARY

The field of anodic reactions of aromatic compounds has been critically reviewed with emphasis on the work of the last decade. A number of suggestions for future work have been made. The half-wave potentials of a large number of aromatic compounds have been tabulated.

REFERENCES i 2 3 4 5 6 7 8 9 IO II 12 13 14 15 16 17 18 19 20 21 22

R. KOOPMAN AND H. GERISCHER, Bet. Bunsenges. Physik. Chem., 7° (1966) 127. C. L. WILSON AZCDI t . V. tZ. UDUPA, J. Electrochem. Soc., 99 (1952) 289. C. OLSOSr, H. Y. LEE AND R. N. ADAMS, dr. Electroanal. Chem., 2 (1961) 396. W. K. StCEAD AND A. E. REMICK, dr. Am. Chem. Soe., 79 (1957) 6121. J. P. VOORHIES AND S. M. DAVIES, J. Phys. Chem., 67 (1963) 332. D. M. MOrlILNER, R. N. ADAMS AND W. J, ARGa~RSINGER, J. Am. Chem. Sot., 84 (1962) 3618. P- A. BROOK AND J. A. CROSSLEY, Electrochim. Acta, i i (1966) 1189. F. FICI-ITER, Helv. Chim. Aeta, 3 (192o) 421. F. EICHTER, Helv. Chim. Acta, 7 (1924) lO65Operational Amplifiers Symposium, Anal. Chem., 35 (1963) I77O. R. N. ADAMS, J. Electroanal. Chem., 8 (1964) 151. D. H. GESKE, J. Phys. Chem., 66 (1962) 1743. H. LUND, dcta Chem. Scan&, i i (1957) 1323. W. C. ~_NTEIKAMAND M. M. DESMOIgD, J. Am. Chem. Soe., 86 (1964) 4811. W. C. NEIKAM, G. R. DIMELER ASrD M. M. DESMOND, J. Electrochem. Soc., i i i (1964) 119o. ~. S. PYSH AND N. C. YANG, J. Am. Chem. Soc., 85 (1963) 2124. L. EBERSON AND K. NYBERO, J. Am. Chem. Soc., 88 (1966) 1686. A. STANIENDA, Z. Physik. Chem., N.F. (Frankfurt), 33 (1962) 17o. J. C. SUATONI, R. E. SNYDER AND IR. O. CLARK, Anal. Chem., 33 (1961) 1894. H. LuNI), dcta Chem. Scan&, I I (1957) 491. A. ZWEIG, W. O. HODGSON AND W. H. JURA, J. Am. Chem. Soc., 86 (1964) 4124 . A. C. CISAK AI-rD P. J. ELVING, dr. Electrochem. Soc., i i o (1963) 16o,

j . Electroanal. Chem., 2o (1969) 137-165

16 4

K. SASAKI, W. J. NEWBY

23 S. ~VVAwzoNEK, R. BERKEY, E. W. BLAHA AND M. E. RUNNER, J. Eleetrochem. Soe., lO 3 (1956) 456 . 24 I. M. KOLTHOrF AND J. F. COErZEE, J. Am. Chem. Sou., 79 (1957) 1852. 25 R. T. IWAMOTO, Anal. Chem., 31 (1959) 955. 26 J. P. ]31LLON, J. Eleetroanal. Chem., I (1959/6o) 486. 27 I. M. KOLTHOFF AND F. G. THOMAS, J. Phys. Chem., 69 (1965) 3049. 28 J. N. BUTLER, J. Electroanal. Chem., I4 (1967) 89. 29 R. L. EDSBERG, D. EICHLIN AND J. J. GARIS, Anal. Chem., 25 (1953) 798. 3 ° W. R. TURNER AND J. FLYING, J. Eleetroehem. Sou., 112 (1965) 1215. 31 K. J. VETTER, Z. Elektrochem., 56 (1952) 797. 32 J. M. HALE AND R. PARSONS, Trans. Faraday Sou., 59 (1963) 1429. 33 M. A. LOSHKAREV AND ]3. J. TOMILOV, Zh. Fiz. Khim,, 34 (196o) 1753; 36 (1962) 132, 19o2. 34 L. YAO, V. S. BAGOTSKII AND YU. B. VASIL'EV, Flektrokhimiya, I (1965) 17o. 35 K. SASAKI AND ~I. t{IMURA, tO be published. 36 J. P. FLYING AND A. P- KRIVlS, Anal. Chem., 3 ° (1958) 1645. 37 K. S. V. SANTHANAMAND V. R. KRISHNAN, Z. Physik. Chem., N.F. (Frankfurt), 39 (1963) 137. 38 ](. SASAKI AND T. IERABI, unpublished work. 39 1~'. J. VERMILLION AND I. A. PEARL, dr. Electrochem. Soe., I I I (1964) 1392. 4 ° C. A. CHAMBERS AND J. A. CHAMBERS, f . Am. Chem. Sou., 88 (1966) 2922. 41 M[. D. HAWLEY AND R. N. ADAMS, J. Eleetroanal. Chem., 8 (1964) 163. 42 T. MIZOGUCHI AND R. N. ADAMS, dr. Am. Chem. Sou., 84 (1963) 2058. 43 Z. C-ALUS AND R. N. ADAMS, J. Am. Chem. Sou., 84 (1962) 2o61. 44 Z. GALUS, R. M. WHITE, F. S. ROWLAND AND R. N. ADAMS, J. Am. Chem. Soe,, 84 (1962) 2065. 45 E . T . SEo, R. F. NELSON, J. M. FRITSH, L. S. MARCOUX, D. W. LEEDV AND R. N. ADAMS, J . Am. Chem. Sou., 88 (1966) 3498. 46 S. WAWZONEK AND T. ~r. MclNTYRE, J. Electrochem. Sou., 114 (1967) Io25. 47 G. CAuQuis AND J. P. BILLON, Cornpt. Rend., 255 (1962) 2128. 48 G. CAuQUIS, J. 1:~. BILLOSr, J. RAISON AND Y. THIBAULD, Compt. Rend., 257 (1963) 2128. 49 IR. 2 . PARKER AND R. N. ADAMS, Anal. Chem., 28 (1956) 828. 5 ° M. T. MELCHIOR AND A. H. MAKI, J. Chem. Phys., 34 (1961) 471. 51 L. H. PIETTE, P. LUDWIG AND R. N. ADAMS, Anal. Chem., 34 (1962) 916. 52 H. Y. LEE AND R. N. ADAMS, Anal. Chem., 34 (1962) 1587. 53 H. ]3. MARK AND F. C. ANSON, Anal. Chem., 35 (1963) 722. 54 W. JAENICKE AND H. HOFFMAN, Z. Elektrochem., 66 (1962) 803. 55 L. K. J. TONG, K. LIANG AND W. R. RUBY, J. Electroanal. Chem., 13 (1967) 245. 56 H. ]3. MARK AND C. L. ATKIN, Anal. Chem., 36 (1964) 514 . 57 J- PERICHON AND R. BUVET, Eleetrochim. Aura, 9 (1964) 587 • 58 P. ZUMAN, Collection Czech. Chem. Commun., 27 (1962) 630, 648. 59 C. ]~. ]3ENNETT AND P. J. ELVING, Collection Czech. Chem. Commun., 25 (196o) 3213. 60 J. J. O'CoNNoR AND I. A. 1)EARL, J. Electrochem. Sou., I i i (1964) 335. 61 J. 1). VOORHIES AND N. H. FURMAN, Anal. Chem., 31 (1959) 381. 62 K. 2. FRIEND AND W, E. OHNESORGE, J. Of~. Chem., 28 (1963) 2435. 63 T. A. COUGH AND M. ]~. PEOVER, Polarography, z964, Proceedings of the 3rd International Polarography Congress, Macmillan, London, 1965. 64 M. ]~. PEOVER AND B. S. WHITE, dr. Electroanal. Chem., 13 (1967) 9365 J. ])H1LIPS, •. S. V. SANTHAIgAMAND A. J. ]3ARD, J. Am. Chem. Sou., 89 (1967) 1952. 66 L. S. MARCOUX, J. M. FRITSCH AND R. N. ADAMS, ft. Am. Chem. Sou., 89 (1967) 5766. 67 R. F. NELSON, D. W. LEEDY, 2 . T. SEO AND R. N. ADAMS, Fresenius" Z. Anal. Chem., 13 (1967) 245. 68 S. D. R o s s , M. FINKELSa:EIN AND R. C. PETERSEN, J. Am. Chem. Soe., 86 (1966) 2745. 69 K. FUJIMOTO, Thesis, I/:yushyu University, 1963. 7° ~ . SASAKI AND R. UEDA, u n p u b l i s h e d work. 71 L. EBERSON AND K. NYBERG, Acta Chem. Scan&, 18 (1964) 1567. 72 L. [EBERSON AND K. NYBERG, Aeta Chem. Scan&, 18 (1964) 1568. 73 L. EBF,RSON AND I~. NYBERG, Tetrahedron Letters, (1966) 2389. 74 J. ERDEL'ZI, Bet., 63 (193o) 12oo. 75 See ref. 122, p. 146. 76 C. L. WILSON AND W. T. L1PPINCOTT, J. Am. Chem. Soe., 78 (1956) 429 o. 77 C. L. WILSON AND T. HAYASHI, p r i v a t e c o m m u n i c a t i o n . 78 M, LEUNG, J. H]~RZ AND It. W. SALZBXRG, J. Org. Chem., 30 (1965) 31o. 79 K. KOYAMA, T. SUZUKI AND S. T. TSUTSUMI, Tetrahedron Letters, (1965) 625. 80 I~. I~OYAMA, private communication. 81 N, N. MEL'NIKOV, S. I. SKLYARENKO AND ]~. M. CHERKASOVA, Gen. Chem. (U.S.S.R.), 9 (1939) 1819,

J. Electroanal. Chem., 2o (1969) 137-165

ANODIC REACTIONS OF AROMATIC COMPOUNDS

165

82 F. I~'ICHTER AND P. SCHONM&NN, Helv. Chim. Aeta, 19 (1936) 1411. 83 K. SASAKI, H. KIMURA AiN-DH. SHIBA, J . Chem. Soc. Japan, in p r e s s ; / £ . SASAKI, I£. TAKEHIRA AND H. SHIBA, Electrochim. Acta, in press. 84 J. ]~. L u VALLE, J. Am. Chem. Soc., 74 (1952) 297 ° 85 V. D. PARKER AND /3. ]~. /3URGET, Tetrahedron Letters, (1965) 4065 . 86 K. SASAKI, K. UNEYAMA, H. VRATA AND S. NAGAURA, Electroehim. Acta, 12 (1967) 137. 87 N. CLAUSON-KAAS, •. LIMBORG AND K. GLENS, Acta Chem. Scand., 6 (1952) 53I. 88 /3. BELLEAU AND N. L. WEINBERG, J. Am. Chem. Soe., 85 (1963) 2525 . 89 T. INOUE, K. ~4~OYAMA,T. MA.TSUOKA, K. MATSUOKA AND S. TSUTSUMI, Kogyo Kagaku Zasshi, 66 (1963) 1659. 9 ° I~. SASAIKIAND S. 1NTAGAURA,Bull. Chem. Soc., Japan, 38 (1965) 64991 K. SASAKI, C. HAM&GUCHI AND S. NAGAURA, Bull. Chem. Soc., Japan, 37 (1964) lO86. 92 M. SZWARC, Chem. Rev., 47 (195 °) 75; J. Am. Chem. Soe., 83 (1961) 25. 93 P- D. /3&RTLETT AND D. S. TARBELL, J. Am. Chem. Soe., 58 (1936) 466. 94 G. A. RUSSELL et al., J. Am. Chem. Soc., 77 (1955) 4o3 x, 4578; 8o (1958) 4987, 4997. 95 F. !x~. RUEHLEN, G. /3. WILLS AND H. M. F o x , J. Eleetrochem. Soc., i i i (1964) 11o 7. 96 W. J. IKOEHL, J. Am. Chem. Soe., 86 (1964) 4686. 97 S. D. R o s s , M. FINKELSTXlN AND R. C. PETERSEN, J. Am. Chem. Soc., 86 (1964) 4139. 98 L. EBERSON, Acta Chem. Seand., 17 (1963) 2004. 99 D. H. GXSKE, J. Electroanal. Chem., i (i959/6o) 502. ioo C. D. RUSSELL AND F. C. ANsoN, Anal. Chem., 33 (1961) 1282. i o i H. URATA, A. I(UNUGI AND S. NAGAURA, C.I.T.C.E. Meeting, Tokyo, 1966. lO2 IK. SASAKI, i~. U1WEYAMAAND S. NAGAURA, Eleetrochim. Acta, I I (1966) 891. lO 3 t3. E. CoNwA¥ AND A. IK. VIJH, Electrochim. Aeta, 12 (1967) 137. 104 M. FLEISCHMANN, J. MANSFIELD AND W. F. K. WYNNE-JONES, J. Electroanal. Chem., io (1965) 5 I I , 522105 K. SUGINO, u n p u b l i s h e d work. lO6 W. ]3. SMITH AND H. GILDE, J. Am. Chem. Sot., 81 (1959) 5325. lO 7 W. ]3. SMITH AND H. GILl)E, J. Am. Chem. Soe., 83 (1961) 1355. lO8 W. /3. SMITH AND H. GILDE, J . Am. Chem. Soc., 82 (196o) 659. lO9 S. T. GOLDSCHMIDT, Angew. Chem., 69 (1957) 132. IiO L. EBERSON, p r i v a t e c o m m u n i c a t i o n . i i i /3. WLADISL&W AND A. GIORA, J. Chem. Soc., (1964) lO37. 112 M. LEUNG AND H. W. SALTZBERG, J. Org. Chem., 30 (1965) 2873. 113 F. D. MANGO AND W. A. /3ONNER, J . Org. Chem., 29 (1964) 1367. 114 S. D. R o s s , M. FINKELSTEIN AND R. C. PETERSEN, J. Am. Chem. Soc., 89 (1967) 4088. 115 J. F. K. WILSHIRE, Australian J. Chem., 16 (1963) 432. 116 F. FICHTER AND H. STENZL, Helv. Chim. Acta, 22 (1939) 97 o. 117 P. J. /3UNYAN AND D. H. HEY, J. Org. Chem., 25 (196o) 3787 . 118 I~. FUJIMOTO, S. ARITA AND I(. TAKESHITA, Rept. Inst. Ind. Sei. Res. (Kyushyu Univ.), 35 (1963) 8 ; 39 (1965) i. 119 ]3. E. CONWAY AND A. K. VIJH, Fresenius' Z. Anal. Chem., 224 (1967) 149, 16o. 12o ]3. E. CONWAY AND A. I~. VIJH, J. Phys. Chem., 71 (1967) 3637, 3655. 121 F. FICHTER, Die chemisehe Reacktion, Bd. VI, Organisehe Elektrochemie, Theodor Steinkopf, Dresden, 1942, p. 359. 122 M. J. ALLEN, Organic Electrode Processes, N e w York, Reinhold, 1958, p. 174. 123 J. A. RIDDICK AND E. E. TOOPS, Techniques of Organic Chemistry, Vol. V I I , Organic Solvents, Interscience Publishers, N e w York, 2nd ed., 1954, p. 544124 P. H. GIVEN AND M. E. PEOVER, Advanees in Polarography, Vol. 3, P e r g a m o n Press, N e w York, 1966, p. 948. 125 J. M. HALE AND R. PARSONS, Advances in Polarography, Vol. 3, P e r g a m o n Press, N e w York, 1966, p. 829. 126 N. N. SEM~NOV, Some Problems of Chemical Kinetics and Reactivity, P e r g a m o n Press, N e w York, 1958, p. 305. 127 A. K. VUH AND ]3. E. CONWAY, Z. Anal. Chem., 230 (1967) 8i. 128 /3. ]~. CONVc'AYAND J~. 1~. VlJn, Chem. Rev., 67 (1967) 623.

j . Electroanal. Chem., 20 (1969) 137-165