THE JOURNAL OF I N D UXTRIAL A N D ENGINEERING CHEMISTRY
910
Vol. 14. No. 10
Progress in Organic Electrochemistry By E. K. Strachan LABORATORY O F PHYSICAL C H E M I S T R Y ,
BROWNUNIVERSITY,
PROVIDENCE.
R . I.
HE ELECTROCHEMICAL manufacturers of this trolytic cell especially devised for the purpose. And finally country have not, as a rule, been interested in organic they were used as an alternative to continue the oxidation until chemicals, except as an outlet for some of their by- the aldehyde was converted to acetic acid.3 Other investiproducts, chief of which have been chlorine and hydrogen. gations have dealt with the electrolytic oxidation of aldehyde The manufacture of monochlorobenzene has reached the or paraldehyde to acide4 Apparently the formation of acid is accelerated by the presence of salts of proportions of a real industry, and the proh!h, Ce, Mo, and V, substances that are duction of p-dichlorobenzene has been stimknown to catalyze other oxidation reaculated by the discovery of its insecticidal tions. The acetic acid may be increased properties, while benzaldehyde and bento 40 per cent before it is removed from zoic acid have cut no small figure. Apthe electrolyte by distillation. Or, if equivparently the chlorination of acetylene has alent dcohol is added before distillation, never been carried out on any very large ethyl acetate is produced.6 The conversion scale. The manufacturers of organic of acetic acid to acetone is essentially a chemicals in several instances have turned thermal process, but the reduction of aceto electrochemical methods for the retone to pinacone may be performed eleccovery of spent reagents, particularly the tr~lytically,~ and has received more or chromate used for oxidation processes. less attention. It has been found that More than one fair-sized factory has the ratio of pinacone to isopropyl alcohol been established on this basis, and promis determined by the electrode material; ises to survive considerable competition. with electrodes of 90 P b and 10 Sn, the However, it is the purpose of this review ratio pinacone: isopropyl alcohol is 10 : 1, to set forth recent progress in the study of while with graphite electrodes the ratio is those processes which consist of reaction 6 or 7: 1. Furthermore, the presence of salts on an organic material in the electrolytic of Bi, Hg, Mn, Nil Sb, Ag, and Fe tend to cell. Very few such are in operation on a reduce the proportion of isopropyl alcohol. factory scale, but those that are give The production of ethyl alcohol from promise of considerable commercial sucacetaldehyde has received considerable cess both for materials requiring tonnage E. K.STRACHAN attention.' It appears that the conaroduction. and for the manufacture of ;mall quantities of fine chemicals of high purity. But on the centration of aldehyde must be keptlow during the process, whole, the investigation of organic electrochemical methods never exceeding 10 per cent, and preferably less, to avoid the has proceeded but little beyond the laboratory research stage. formation of aldehyde condensation products. By altering During the period when acetic acid had to be obtained a t the conditions of electrolysis, e. g., raising the temperature, any cost whatever, thousands of tons of it were made from or increasing the aldehyde concentration, polymers and their calcium carbide through acetylene and acetaldehyde. This reduction products, such as crotonic aldehyde, butane -1, 3-, method cannot compete with wood distillation under present diol, butyl alcohol, etc., may be formed which can serve as economic conditions, particularly as the makers of pyro- denaturants, and thus a denatured alcohol is produced diligneous acid can throw the burden of competition on their rectly from aldehyde.* The reduction of mixtures of aliphatic aldehydes and methanol.' Nevertheless, this war-time development called attention to the great variety of organic chemicals that can ketones to diols is described in Brit. Patent 156,145 (1920). It is interesting to note that in the complete absence of be produced from acetylene.2 These include aldehyde, paraldehyde, alcohol, acetic acid, ethyl acetate, acetone, ethylene, air, aldehyde may be catalytically reduced to ether, although acetonitrile, pyridine, pyrrole, thiophene, mercaptan, di- apparently no one has ever done this ele~trolytically.~ There has been interesting progress in the reduction of chloroethane, tetrachloroethane, many other chloro derivatives, chloroacetic acid, crotonic aldehyde, pinacone, isobutyl w,w-dibasic acids.IO The type reaction + alcohol, and many other compounds. The shifting of economic conditions and further research may make these reac- 2NaOOC - X - COOCzHh = 2Na f 2C02 + X - COOC2H6 I tions of considerable commercial importance: power may be X - COOCzHs cheaper, motor fuel will undoubtedly increase in price, wood for distillation is apt to be very much scarcer, substitutes or in which X = CH2 has been repeated until a CISacid has been new manufacturing methods may eliminate the strategic im- prepared from sodium ethyl malonate. The next step, which portance of methanol. The whole group of materials pre- would result in the formation of a C34acid, was unsuccessful, probably on account of the soapy and colloidal character of sents interesting electrochemical possibilities. a Brit. Patents 143,891 and 156,147 (1920). Electrochemical methods were applied to this group first 4 Brit. Patent 124,195 (1919). for the recovery of the spent mercury catalyst used in the 6 Brit. Patent 131,600 (1919). For ethyl acetate see also Brit. Patent hydration of acetylene. Then it was proposed to make the 140,116 (1918). 6 D. R. P. 324,920; 306,304; 324,919 (1917). process continuous by conducting the hydration of the acetyl7 D. R. P. 328,342; Brit. Patent 140,115 (1918). ene and recovery of the mercury simultaneously in an elec-
T
Brit. Patents 140,527 (1919); 140,115 (1918). Brit. Patent 156,145 (1920); D. R . P. 317,589 (1918). 10 J . SOC. Chem. Ind., 40 (1920), 169R. 8
1 2
J . SOC.Chem. Ind., 40 (19211, 345R. Chimie et industvie, 6 (1921), 239; J . SOG.Chem. Ind., 41 (1921), 18R.
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THE JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
Oct., 1922
the products. This method has not been applied to aromatic compounds, but work is promised in the near future. The same type of reaction has been proposed as a starting point for the synthesis of tropine derivatives, the basic substances of the atropine, cocaine, and scopolamine groups of alka1oids.l' It affords an excellent example of the .use of electrochemical methods for the production of fine chemicals, a field of manufacture for which they are particularly well adapted. The potassium ethyl ester of acetone dicarboxylic acid, which can be prepared from citric acid, is reduced electrolytically and condenses with itself to succinyl diacetic diethyl ester. The latter substance can be condensed with methyl amine to form N-methyl pyrrole diacetic diethyl ester, which in turp can be transformed to tropinone. HzC-C!OOCzHa I
HaC -COOCzHs
70. ..........*
HzCyC!OOK HzC-C!OOK
I
:
I
co I
:
HzC :+1 : HZC
..........:
FO H~C-COOC~H~
I
HC
*
HzNCHa
I
C-CHz-COOCzHs
I
7CHa
H C = C-CHpCOOCzHr
$.o I
H~C-COOC~H~
By the same method, the potassium ethyl ester of azelaic acid KOOC-(CHp)7-COOC~Ha has been converted to CaH600C-(CH2)isCOOCaH, the acid of which proved to be identical with the thapsic acid from juniper berries.12 The work of Fichter and his associates deserves especial atttention. They have investigated the oxidation of toluene in considerable detail, and find that it proceeds in accordance with the following diagram:13 OH
OH CsHaCRs + CeH6CHzOH + CsH6CHO-CsHsCH-O~CHCs" OH
COOH
COOH
c
CsHsCOOH
COOH
OH
(TggH
COOH f- / \ O H
If the various reactions indicated in this diagram can be controlled it suggests interesting possibilities in the production of fine organic chemicals. The oxidation of dimethylaniline yields tetramethylbenzidine, which is interesting in view of the fact that the analogous reaction with aniline, in which the amino group is unprotected, has never been observed to yield benzidine.14 The yield is poor, owing to the formation of Cot, N, CO, and CH20, the last of which condenses with the amine. Diethylaniline on the contrary yields almost exclusively tetraethylbenzidine. A series of exploratory studies'; is in progress in the same laboratory, one of which16 shows the formation of p-aminophenol from azobenzene. The electrolytic oxidation of azobenzene yields p,p-dihydroxyazobenzene, and about twice D.R. P.300,672 (1917). Biochem. Z . , 108 (1920),75. 1 3 Fichter and Uhl, HeZvelica Chim. Acta, 3 (1920),22. 14 Fichter and Rothenberger, Ibid., 5 (1922),166. 15 Fichter and BonfiBte, Ibzd., 3 (1920), 395; Fichter, Brandlin and Hallauer, Ibid., 3 (1920), 410; Fichter and Schmidt, Ibid., 8 (1920), 704; Fichter iind Grissard, I b i d . , 4 (1921),928. 16 Fichter and Jaeck, Ibid., 4 (1921) 1000. 11
911
as much tetrahydroxyazobenzene. This is another illustration of the tendency of organic compounds to oxidize to a greater degree than is desired. Dihydroxyazohenzene of course reduces readily to p-aminophenol. The electrolytic oxidation of naphthalene apparently is similar to that of bensene.17 Just as the oxidation of benzene does not stop with the formation of phenol or hydroquinone, but proceeds to quinone, so the oxidation of naphthalene yields naphthoquinone as the principal product with traces of a-naphthol, some 1,4-dihydroxynaphthalene1a compound of the quinhydrone type, and phthalic acid. It was observed sometime ago that, when formal dehyde solution is electrolyzed, equal amounts of hydrogen appear a t the anode and cathode for a while, and that then the evolution a t the anode gradually ceases. The explanation of this phenomenon constitutes a valuable contribution to the theory of oxidation of aldehydes.'* The investigator found that by use of a suitable anode constructed of copper coil covered vr;ith molten cuprous chloride and reduced electrolytically in alkaline solution the evolution of hydrogen a t the anode could be made continuous. The same result is obtained with a similarly constructed silver anode. Moreover, the electrolysis of both acetaldehyde and benzaldehyde exhibits the same phenomenon. The course of the reaction is explained by the following scheme: R
OH
R
0-0
R
H
' 0
//
2 R - < \
+ 0
0
Hz+2H+
\ H
The catalytic effect of the electrode causes the intermediate peroxide to decompose by Reaction I1 with the evolution of hydrogen gas a t the anode, while the electrolytic oxidation proceeds by Reaction I. It is quite interesting to compare this with the electrolytic oxidation of methyl and ethyl alcohols which yield, respectively, hydrogen and methane.19 Some int,eresting facts have come to light in a study of the effect of adding various salts to the electrolyte to decrease the cathodic reduction during the electrolysis of chloride to chlorate.20 I n general it appears that. the effect of the electrode depends on the system (electrode material-hydrogenmetal deposited from the added salt). Since only small amounts of salt are added-and indeed small amounts of salt appear to be more effective than larger ones-it is probable that the deposited metal does not entirely cover the cathode but merely forms a network on it. Copper deposited on an iron cathode was found to increase reduction] while iron deposited on a copper cathode decreased the reduction to a very small figure. This is strange, since a plain iron cathode reduces much more chlorate than a copper one. The explanation perhaps lies in the capacity of electrolytic iron to occlude hydrogen. These facts should be suggestive to experimenters who are attempting to control the electrochemical oxidation or reduction of an organic material by selection of a suitable electrode. A great deal more work of this sort is needed. The electrolytic reduction of a number of azo dyes led the investigators to conclude that the rupture always took place at the azo double bond.21 A suitable small electrolytic cell
12
17 J . Chem. SOC. J a p a n , 42 (1921), 38; J . Chem. Soc., 120,I (l92l),334; J . Chem. SOC. J a p a n , 42 (1921), 559; J . Chem. Soc., 120,I (1921),726. 18 Erich Muller, A n n , 420 (1920),241. 1s Erich Muller and A Miro, Z . Electrochem., 27 (1921). 54. 20 Schlbtter, Ibid., 27 (1921),394. 21 Gam chim. i t d , 50 (1920),149
