ELECTROCHEMICAL METHODS in ORGANIC CHEMISTRY J. CARL BELTZ State Teachen' College, Saint Cloud, Minnesota INTRODUCTION
T
N TEACHING organic chemistry, the use of electro-
chemical methods has, for the most part, been -somewhat neglected. As a means of preparing a
nascent hydrogen. At the anode (a) the primary proceSS involves discharge of hydroxyl ions, 20H- = H20
+ 0 + 2.
and ( b ) the interaction of the anodic depolarizer and nascent oxygen. When halides are used, the anodic process liberates nascent halogen which reacts as quickly as produced. The nature of the reaction is influenced and determined by three factors. They are: (1) electrode a, t the electrodes, and potential, ~ ~ (2)fvelocity ~ of~ reaction ~ (3) The potential may be shown to be Propo*ional to the co~centrationof active hydrogen or Oxygen. The magnitude of the electrode ~otentialis a measure of its reducing (4) or oxidizing power, being greater for higher potentials. These potentials may he regulated by controlling the current density, the temperature, the kind of electrode materials and the conditions of electrolysis. This regulation results in various intermediate products, depending upon the amount of reaction energy necessary, and those substances which react a t a potential above.that'required for liberation of nascent hydrogen or oxygen are not acted upon to any great extent. Materials of high hydrogen overvoltage THEORETICAL or high oxygen overvoltage permit high electrode poFrom the time of Faraday ( I ) , who has given us the tentials without liberation of gaseous hydrogen or laws of electrolysis, and of KoJbe (2) who, by electroly- oxyge" and are necessary in reactions requiring greater sis, attempted to isolate free methyl, much fruitful amounts of energy. Electrode materials differ among research has been done on both in8rganic and organic themselves with respect to overvoltage and frequently determine the resulting products of an electrolytic compounds and their electrochemical preparation. About the electrodes, many organic compounds react Process. If the velocities of the primary reactions (a), above, with the products formed by the discharge of ions. These compounds may take up hydrogen or yield are faster than those of (b), above, the concentrations oxygen, simultaneously, a t the cathode or be oxidized of nascent hydrogen and'oxygen increase, electrode a t the anode (3). By varying the conditions of electroly- potentials rise, and the gases are liberated. Any factor, sis, the same compound may he made to yield a series then, which makes for a lower current density is desirof products without the use of different reducing or able. This condition may be accomplished by emoxidizing agents which, if used, would be present as ploying either a large cathode surface in reducing reacimpurities. Yields generally show an increased per- tions or a large anode surface in oxidation reactions, centage and the products a higher degree of purity and and by providing for an increase in concentration of finer crystalline form. Since organic compounds, them- depolarizing material around the appropriate electrode. selves, are poor conductors, an electrolyte is added and Stimng is, of course, advantageous. the electrode products combine with the organic comCatalysts are used to accelerate slow reactions hepound which is suspended or dissolved in the electrolyte. tween active hydrogen, or active oxygen, and reacting Electrolytic preparation depends upon two processes materials (5, 6, 7). The catalyst should be one which, a t each of the electrodes. At the cathode (a) the pri- itself, is capable of being either reduced or oxidized, mary process involves discharge of hydrogen ions, acting as a hydrogen or oxygen carrier, and being regeneH++r=H rated a t the corresponding electrode for further duty. and (b) the interaction of the cathodic depolarizer and The electrode potential, on the other hand, need not be 516 number of organic compounds such methods possess many advantages over purely chemical processes and their use should be emphasized. As a matter of fact, industrial organizations employ electrolytic processes, to whenever practical, to avoid losses of gain better control, with simplification of method; and to obtain subsequent purer products. ~ h ~ i t would seem that electro-organic practices not only have a legitimate place in the organic classroom and lahoratorv but also afford an excellent 0*~. ~ o r t u n i to tv teach th; principles involved and to give the stu&nt wider understandmg by breaking down the barriers built up by individual subjects or courses. Such integration is in line with the most modem procedures in education and, in planning a beginning organi~course, the idea merits consideration. ~h~ following plan for incorporating electrochemical methods in the organic course presentsfirst, some important points of the theory involved, and second, some applications which the student may cany out in the laboratory.
higher than is necessary for the reduction or oxidation of the catalyst. It must be borne in mind that the electrode surface should be such as to allow the reaction to proceed readily and with a high velocity. This is attained, generally, by employing rough electrodes in preference to smooth surfaces. Catalytic action of the electrode material often plays an important role in the electrolytic process (8). In carrying out electrochemical reactions, it must be remembered that cathodic reduction products may be oxidized a t the anode and, also, that anodic oxidation substances may be reduced a t the cathode. To prevent this, in the former case, the anode is frequently separated from the cathode by a diaphragm, or porous cup; or a small anode surface is employed for the purpose of attaining a high current density. Many times it is desirable to use an anode material of low oxygen overvoltage. The same kind of precautions need to be taken to prevent reduction of anodic oxidation products a t the cathode (9). The number of electrode materials which may be employed in electrolytic oxidations is rather limited since oxide layers coat the electrodes. In acid or alkaline media, carbon and platinum are usually employed while iron and nickel are best suited to alkaline solutions. The presence of traces of foreign metals in
the electrode materials lowers the overvoltage considerably. Aside from purely oxidation and reduction reactions, the electrolysis of many organic compounds is possible owing to the large number of organic anions. The cations of organic electrolytes usually do not take part in any cathodic reaction except, possibly, in a relatively few cases of organic bases. With concentrated solutions of aliphatic monobasic acids, or their salts, union takes place between discharged anions with subsequent splitting off of carbon dioxide from the newly synthesized molecule. In these types of reactions higher current densities are necessary. As a rule these reactions may be classified as follows: 2CnH9, + I COO- = G.&. + z 2C.Han + COO- = C.Hnn
+ 2COn + 2
(1)
r
+ CnHsn+ r COOH + C02 + 2r
2CmHzn+ I COO- = C-Ha, + t COOGHm +
+ CO* + 2e
(2) (3)
By the use of suitable coulometers (lo),the student may determine the current eiliciency, which is an important factor in many electrochemical processes. He learns, also, to understand the occurrence of side reactions and how yields are, thereby, diminished. Tables suggesting some electrochemical reactions, which may be carried out in the organic laboratory, together with detailed references, follow.
SOME EXERCISES INVOLVING ELECTROCHEMICAL APPLICATIONS TABLE 1 RBDUCTIONS Yield. Cdholvls
lsopropyl alcohol and pinmooe (20)
Cothodc
Cone.
P t gauze
sulfuric add; few ce. water in paova cup 82% sulfuric acid ~olution in porous m p Sodium carbonate solution in pomus cup Sodium carbonate solution in porous cup Sodium sulfate solution, acidified with sulfuric seid Sodium carbonate cold aat"rated solution
20 g. nitrobenzene: 150 cc. alwhol: 125 cc. sulfuric add, sp. gr. 1.2 same as aniline 40 g, nitrobenzene; 150 g. sulfuric acid (fuming), rp. gr. 1.88
Sulfuric a d d , sp. gr., 1.l.i" porous cup
Pb perforated cylinder D, 3-6
Cone. sulfuric acid
P t gauze D, 2-6
30 g. bmzophenone: 6 g. crystallized sodium acetate; 500 cc. 96% alcohol; 100 cc. water 30 g. acetone; 6 g. crystallized .odium acetate: 500 cc. 98% alcohol: 100 ec. water 20 g 0-nitraniline: 5 g. Crystallized .odium acetate; 200 ee. 70% aleohol in porous cup 20 g. oleie acid; 300 e.nleohol; 2 cc. 20% ~ulfuricacid 20 9. pyridine: 200 ee. 1070 sul-
Sodium carbonate cold sat- Pb perforated cylurated solution in porous inder DI 0.4-0.8 CUP Sodium carbonate cold Pb vessel DC 4 saturated oolvtion in porous cup Sodium carbonate solution Ni gauze D. 2
%
C . E.*
Pt gauze DO 3-4 P t gauze D. 10 Pt gauze D, 10
Ni gavze DI 5-7 Pb or P t sheet
-
same ar arabenzene Pb sheet
P t sheet
our cup
5% sdfuric acid in clay eylinder No diaphragm
efficiency. current density. amperes per square decimeter.
t Aft" 1amperr hour.
in p a r
T m p . , 'C.
Anode
DI 4 4 t P t sheet
20 g. nitrobenzene; 150 g. eaoe. sulfuric acid: few drops water 20g. nitrobeoleneinsl% sulfuric acid solution 20 g. 0-nitrophenol: 6 g. sodium hydroxide in 300 cc. p t e r 20 g o-nitrophenol; 6 g. nadium hydroxide in 300 ee. water 30 g. nitrobenzene: 240 g. 2.5% sodium hydroxide solution .in porous cup 20 g. nitrobenzene: 5 g. crystallized sodium acetate; 200 ce. 70% alcohol in porous cup same ss ambenzene
* C. E.--eurreot t D-thode
Anolyrc
Pb plate
Heated during experiment
90
80-90
Ph
Kept cool
-
-
Ph or P t rh~et
Ni gauze D, 1.5
C
Pb sheet D. I M 2
Pb (small)
TABLE 2 OXID~T~NB
p-Nitrobenryl hol (24)
alm- Sulfuric stid, sp. xr. 1.6-1.7
15 g. p-nitrotoluene; 80 g. gla- Pb cial seetie acid; 15 g. eonc. sulfuric a d d ; 7 g. water in
Yield,
%
Anoda
Temp., T .
Pt f d l m g a v a (large) DS
Heated by bailins water bath
40
30
Below 20
10
-
50-70, COapased
-
80
Calkodr
IS*
porous cup Bcnraldehyde ( 2 5 )
No diaphragm
59 9. toluene; 200 ec. 10% sul- Pt spiral furic acid; 250300 ec. see-
Pt foil D. 1.52
tone Iodofmm (26)
Same as aoolyte; diaphragm of parchment paper
~ r o m o f o r m(271 Chlomform (28)
20 g. anhydrous Jodium carbonate; 20 s. pota&um iodide; 50 ce. 96% aleohol; 200 ee. water Same as analyte; diaphragm 60 g pot-um bromide: 0.3 g. of parchment paper potassium chromate; 150 ce. water: 20 ce. aeetone Same as anolyte: diaphragm 120 g. sodium chloride; 10 g. of pnrrhment paper sodium carbonate; 500 ce. water; 30 g. sodium biear-
C. E .
Pt foil (small) P t gauze in parchment (large) D. paper 1 3 spiral in parchment paper Pt spiral in Parchment
P t sheet D. 3
Pt
into solution
Ia20. CO8pasred 60
-
into solution P t foil D. 65
Below 30
m 3 0
-
bonafe Methyl aleohol (29)
No diaphragm
225 g. potassiumaeetate; 52 g. potassium carbonate; 55 g.
Aothraquinane (30)
No diaphragm
io mare I liter 20 g. antbraeene; 900 ee. water; 100 ee. conc. sulfvrie acid; 1.5 g. cede sulfate in Pb ves-
~otaeriumbicarbonate: water
sel
* D.-aoode
current density. ampere. per square decimeter.
TABLE 3
Colhalyrr sodium hydroxide SOIIIUO~ in 2 porous
10%
Cup3
Anolrlc
Colhadr
Anode
Tamp.. ' C .
19.5 g. sodium salt of sulfanilie acid; 14.43. Eoaphtho~; 8.9 g. ptwc SOdium nitrite; 150 cc. water
Ni or Pt aim
P t sheet D.
Cwled by i e bath
a12
TABLE 4 ANIONRBACTIONS
P"ep0""lion Ethane (32)
Diethyl ester of adipic a d d (35)
Colholrlc
Anolrrc
Colkoda
.. Pt
Awodc
Temp., OC. Below 20
Saturated sodium acetate Same as catholyte solution in porous+ Ni or Cu gauze spiral or .elution in cold: few cup around porsheet D. 50ec. arrtie a d d 0"s eup 100 30 g. sodium pmpionate; Same as eatholyte solution in porovs Pt foil Pt wire or spi- Up t o 40 25 8. propionic add; 80 cup (bromine added converts ethylral Da 100 g. water ene to the dibromide) W i u m chloride d u t i o n a0 g. trichloroaeetie acid; 100 8. water P t m Pb eglin- Pt sheet D. ~ e p cool f 40-50 saturated with equal weights aohyder arovnd drous sodium carbonate and dne carporous cup banate in porous cup No diaphragm E 1.5 parts potassium ethyl ester of suc- P t sheet Pt spiral D n Kept coal cinic a d d t o 1 part water 50-100
LITERATURE CITED (1) MOORE."A history of chemistry," 3rd ed., McGrsw-Hill Book Co., Inc., New York City, 1939, p. 230. (2) Ibid.. D . 152. AND KOEHLER,"Principles and applications of (3) CRE&ON electrochemistry," 3rd ed., John Wiley and Sons, Inc.. New York City, 1935, Vol. I, p. 284. Ibid.. Vol. 11, p. 311. B R O C F ~ A"Electro-organic N, ehernistrg." John Wiley and Sons, Inc., New York City. 1926, pp. 1-381. LOB. ''Electrochemistry of organic compounds," 1st ed., John Wiley and Sons, Inc., New York City, 1906. KNOBEL, BRocKMAN AND RESEARCH INPORMILTION SERVICE, "Bibliography of electro-organic chemistry," National Research Council. Washington. D. C.. 1926, pp. 1-100. ATEN, "Elektrdytische Reductie." Chew. Weckblad, 19, 349-52 (1922). (4) HAEER."Uber stufenweise Reduktian des Nitrobenzols mit hegrenzten Kathodenpotential," Z. Elektrochnn.. 4, 506-15 (1898). (5) TAFEL, "her den Verlauf der elektrolytischen Reduktion schwer reduzierhar Suhstanzen in schwefelsaurer Losiingen," 2.physik. C h . ,34, 220 (1900).
FICHTER, "Biachernische und elektrachemische Oxydation organisher Verbindungen." Z. Elektrochnn., 27, 487-94 (1921). M~~LER "Die , elektroehemische Oxydation organkcher Verbindungen," did., 28, 101-6 (1922). LOB. "Uher den E d n s s des Kathodenrnaterials bei der elektrolytischen Reduktion arornatischer Nitrokijrper," ibid., 8, 778-9 (1902). MULLER, "Ein Nachtrag zur 'Stamng der katbodiihen Depolarisation durch Kaliurnchramat,"' ihid., 8, 90914 (1902). CREIGHTON AND KOEHLER, oP. it., Vol. I , p. 18. ELBS,"Electrolytic preparations," Edward Amold, London, England, 1903, p. 70. MCDANIEL,SCHNEIDER AND BALLARD, "The electrolytic manufacture of paraminophenol." Tram. Am. Elcclrochem. Soc., 39,441-50 (1921). ELBS,oP. tit., P. 88. ELBS,op. cil., p. 89. ELBS,op. cit., p. 77; HABBR,op. cit. ELBS.op. cil.. p. 78; HABER.op. tit. ELBS,op. ~ 3 .p.. 68; HAWER, op. it.
ELBS,op. cit., p. 88. Ems. op. cit.. P. 92. PE-N. "Practical methods of electrochemistry," Longmans Green and Co., New Yark City. 1905, p. 254.
PERKIN, op. cil., P. 267. FEYER,"Die elektrolytische Chloroformdarstellung," Elehtrochem., 25, 11545 (1919). P E ~ I Nop. , cit., P. 232. op. cit., p. 269. PERKIN, PERKIN, op. Cit., P. 274. Ems, op. cit., p. 51. Ems, op. cit., P. 53. Ems, op. it.. P. 54. Ems, op, cit.. P. 56.
