January, 1923
INDUSTRIAL A N D ENGINEERING CHEMISTRY
15
T h e Electrochemistry of Organic Compounds’ By Alexander Lowy DEPARTMENT OF CHEMISTRY, UNIVERSITY
OF
PITTSBURGH, PITTSBURGH, P A .
HE APPLICATION of electricity to organic chemistry is not new but comparatively undeveloped. Experimental work dates back to the beginning of the 19th century. As early as 1807, Grotthus electrolyzed an
the catholyte it is known as a “cathodic depolarizer;” if to the anolyte, as an “anodic depolarizer.” Depolarizers may be dissolved or suspended in the electrolyte. In .the electrolytic oxidation of anthracene to anthraquinone, sulfuric alkaline solution of indigo-white and obtained the character- acid is the electrolyte and anthracene is the anodic depolaristic blue precipitate on the surface of the izer. In the electrolytic preparation of p anode. Although many papers have been chloroaniline, hydrochloric acid is the elecpublished on the electrochemistry of ortrolyte and nitrobenzene is the cathodic ganic compounds, up to date very few depolarizer. For the preparation of organic commercial processes have been worked compounds direct current is generally out. Patent literature reveals the fact used. The utilization of alternating curthat up to 1910 approximately 100 patents rent has hardly been touched upon. were granted in this field, 89 of which were Many organic compounds can be pretaken out in Germany. Only a limited pared electrolytically by the following amount of research work has been done type reactions: oxidation, reduction, haloin this particular line, especially in the genation, synthesis, etc. Since a t the United States. Within the last few years anode, oxygen or halogens may be evolved, the Swiss, German, and Japanese journals by using certain inorganic electrolytes, have contained many articles on this oxidation or halogenation can be effected; subject. while a t the cathode, hydrogen is evolved, In comparison with inorganic chemistry, thereby effecting reduction. Since oxidathis phase of organic chemistry is practition, reduction, halogenation, substitucally undeveloped. Provided the same tion, etc., reactions are used extensively impetus is followed in the electro-organic in organic chemistry, oxygen, hydrogen, and halogens generated electrolytically field as in the electro-inorganic field, within the next decade we will undoubtedly could be used to produce the same resee new methods for the preparation of ALEXANDER LOWY actions during the course of the elecorganic compounds, perhaps new industrolysis in place of more expensive tries will be developed and thereby substitute electrical chemical compounds. energy for more expensive chemical reagents to effect certain Reduction of organic compounds by means of electrolysis organic reactions. has been more thoroughly investigated than oxidation and In conducting experiments for the preparation of organic substitution reactions. Reduction of an organic compound compounds by the electrolytic process, the organic com- generally takes place at certain reducible groups in the molepound can act as an electrolyte, as in the case of organic cule without further destroying the latter. On the other acids, salts of these acids, etc., but in most cases, the organic hand, oxidation of organic compounds is far less selective compound is a nonelectrolyte. If the organic compound is and takes place by degrees. Possibilities exist between a nonelectrolyte or one of very low conductivity, an electro- a slight attack of oxygen and the complete destruction of a lyte must be added in order to produce electrolytic effect. compound. Therefore, in conducting experimental work for I n order to accomplish this, inorganic acids, bases or salts are the preparation of an organic compound, it is absolutely generally utilized. Upon electrolyzing the mixture, the ions necessary to note all possible details of the variable factors migrate to their respective electrodes and are discharged. affecting the course of the reaction. At the moment of their discharge to the atomic or molecular The following are the important variable factors which state, a reaction takes place and the carbon compound is affect the course of reactions, yields and the selective deacted upon by the primary or secondary products of the elec- polarizations of organic compounds: current density; COD trolysis. The carbon compound acted upon by this means is centration and conductivity of electrolytes; concentration called a “depolarizer.” If the organic compound is added to of the depolarizer (organic compound); temperature of bath; diaphragm; dimension of electrodes; composition of electrodes 1 Extracts from lectures presented before the Cincinnati, Cleveland, and Pittsburgh Sections of the American Chemical Society, 1921-22. (overvoltage effect);e. m. f. of cell; structure and shape of
T
CATHODE COMPARTMENT
NITROBEXZENE IN THE CATHODE COMPARTMENT CURRENT DENSITY TEMPERANODECOMPARTMENT ELECTRODES X N AMPERES VOLTAGE ATURE
] Lead Platinum
4-5
3.7-4
6-8
7-8
65’-80’
TIMEIN AMPERE HOURS
P-Aminophenol
30
6-8
22
Azoxybenzene
8-9
30
Azobenzene
8-9
35
fr6.5
5o
First 6-8 Platinum
1.5-2
PRODUCT Aniline
35
{ {
Hydrazobenzene o-
and p-chloroanilines
INDUBTRIAL A N D ENGINEERING CHEMISTRY
16
--
electrodes; relative position of electrodes; rate of addition of organic compounds; time of electrolysis; overvoltage effect; use of catalysts, and stirring. A few examples given below show the effects of the abovementioned variable factors in producing certain organic compounds under specific conditions.
Effect o f Elecfrodes
Vol. 15, No. 1
The reduction of nitro compounds gives the following types :
\
COOH
CHZ OH
Effecf o f Llecfrodes
(&OH
of Bath
Effecf of Concenfration of Elecfroiy +e
iofmzed pf,
BO Percent yield of HCHO Less HCHO- More CO, A11 to CO'
HCOOH
Effec f of Alkalinity
Ac%&
Be
czH2
C4CHD-C C&COOY BOPercenfyie/dof HCHO
CH, OH
No
It has been reported that a number of these compounds have been made in Germany on a commercial scale by means of electrolytic methods. During the war several tons of p-amidophenol were made in this country by the electrolytic reduction of nitrobenzene. As t o future possibilities, the field for the utilization of electricity for the preparation of certain organic compounds is unlimited. A few of the topics listed below will be investigated in the near future a t the University of Pittsburgh. 1-The oxidation of toluene to produce a good yield of benzaldehyde. 2-The oxidation of nitrotoluenes to nitrobenzaldehydes. 3-The oxidation of o-cresol to salicylic aldehyde and acid. 4-Oxidation of leuco bases of dyestuffs to their color bases and dyes. &Oxidation of hydrocarbons to aldehydes and acids. 6-Halogenation of aromatic hydrocarbons. 7-Use of catalysts in electro-oxidation processes. 8-Nitration and sulfonation with dilute nitric and sulfuric acids, respectively. During the electrolysis of dilute solutions of these acids concentrated acids are produced on the surface of the anode; therefore nitration or sulfonation of the organic compound can take place. 9-Utilization of both the anode and cathode compartments of an electrolytic cell for simultaneous oxidation and reduction reactions. 10-Use of alternating current. The following advantages may be given in connection with the utilization of electricity for the preparation of organic compounds:
The study of these variable effects, although complex to a certain extent, shows them to be within range of experimental manipulation, and foreshadows great possibilities for the utilization of electricity for the preparation of certain organic compounds. The following are some of the aliphatic compounds that have been prepared by electrolysis: formaldehyde from methanol with an 80 per cent yield; chloral; propionic acid from n-propyl alcohol with a 90 per cent yield; aldehydes from primary alcohols; ketones from secondary alcohols; halogenated acetone; isopropyl alcohol from acetone; chloroform] bromoform, and iodoform (industrial process) ; hydrocarbons; esters, etc. In the aromatic series there is a more fruitful field for research for the preparation of intermediates, dyes, drugs, developers, etc. Among the aromatic products prepared by electrolysis, the following are listed: o-Toluidine from o-nitrotoluene; benzaldehyde from toluene; o-nitrobenzyl alcohol from o-nitrotoluene; anthraquinone from anthracene; vanillin from isoeugenol; benzoquinone from benzene; o-amidophenol from o-nitrophenol; malachite green color base from its leuco base; eosin from fluorescein; saccharin from o-toluenesulfonamide; terephthalic acid from p-toluic acid; piperidine from pyridine; p-amidophenol from nitrobenzene, etc.
1-Electric current produces changes in place of expensive oxidizing and reducing agents, such as PbOz, CrOa, SnC12,etc. 2-Only electrical energy is used. 3--p\To by-products of inorganic chemicals to remove. &It is easy to manipulate, provided conditions have been worked out. &T -he use of both the anode and cathode compartments simultaneously in order to obtain full current efficiency.
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