Reduction of Trichloroacetic Acid 'to Dichloroacetic Acid
Palle E. lversen University of Aarhus DK-8000 Aarhus C, Denmark
An organic electrosynthesis experiment Recently there has been a tremendous increase in the amount of research on preparative organic electrochemistry (for most recent reviews, see ref. (1-6)). The present experiment has been designed to demonstrate the method of controlled potential electrolysis (CPE) by selectively reducing trichloroacetic acid to dichloroacetic acid on a mercury cathode in a divided cell at room temperature in an aqueous ammonia buffer
The trichloroacetic acid was chosen as a substrate not because of any specific synthetic interest of the product, but rather to give a simple example. The experiment is most conveniently done overnight by automatically controlling the potential of the working electrode to a value of -1.15 V versus the Ag/ AgCl reference electrode corresponding to the first polarographic wave of trichloroacetic acid (7). This is done by means of an electronic potentiostat, hut can also be done with manual control of the output from a conventional direct current source (8); the latter method, however, requires the presence of an operatorespecially towards the end of the reduction where the concentration of trichloroacetic acid is low and the cell voltage has to be correspondingly lowered to avoid further reduction of the product. The work-up is straightforward involving continuous extraction with ether and fractionation in vacua to give the dichloroacetic acid in 75-84% yield. This compares favorably with the standard chemical procedure (9) of 87% yield, but we have never been able to reach the yield claimed in the original work (7). It must be firmly stressed that this is not the only way of doing CPE (1). Actually in most cases the same results could be obtained indirectly by controlling the current density and the concentration of electro-
Mognef
Mercury-Cathode
Grophlle-Anode
Semimacro-scale electrolytic cell.
136
/
Journal o f Chemical Education
active substance. However, with the development of commercially available electronic potentiostats, their use in preparative organic chemistry on a laboratory scale seems to he rather attractive. Experimental Apparatus. The threeelectrode divided cell is sketched in the figure. This cell is our final modification of the Lingane H-type cell (10)which has been used for several years in our laboratory for semimacro work. It can be made from standard Pyrex glassware by any skilled glassworker and is very versatile, hut has a rather large electrical resistance which in practice puts an upper limit of about 3 A on the current. The conical flasks are 500 ml in size. Electrical connection to the magnetically stirred mercury electrode is made through a short platinum wire sealed into the wall. The reference electrode which should he placed as near to the mercury surface ss possible is simply a piece of silver wire in saturated aqueous potassium chloride solution; it is shout 50 mV negative versus the SCE commonly used. The agar plug in the referenoe electrode (2% agar in aqueous half-saturated potassium chloride solution) may be omitted. An agar plug in the middle compartment of the cell is also unnecessary. The capillary of the dropping mercury electrode (DME) shown allows the concentration of electrosctive substances to he followed polarographically in the cell during electrolysis. The potentiostttt was a. 40 V/25 A instrument from Tage Juul Electronics, Copenhagen, but a 8maller apparatus would suffice (e.g., 40 V/3 A); even a. conventional rectifier giving a direct current with rt small ripple could be used. The amount of electricity is conveniently measured with a conventional dc-wattmeter which is provided with a suitable shunt for the proper current range and then calibrated by means of a constant current source. Reduction of trichloroacetic acid. The cathode and middle compartment of the cell were filled with s. solution of 30 g of ammonium chloride and 10 ml of 25% aqueous ammonia in 350 ml of water. The contents of the anode compartment could either be the same or an aqueous sodium chloride solution. Evolution of chlorine at the anode was avoided by adding sodium nitrite to the anolyte. Sixteen and four tenths grams (0.1 mole) of trichloroacetic acid were added to the catholyte. The cell was surrounded by water in a plastic box and the reduction done overnight a t a potential not below -1.15 V versus the Ag/AgCl reference electrode. Mild cooling was necessary to prevent the solution in the middle compartment from boiling. After a little less than the theoretical amount of 5.36 A-hr had been passed through the cell (the electromechanicd integrator may not be able to count both a t about 2 A in the beginning of the electrolysis and a t 20-30 mA towsrds the end of the reduction), the catholyte was trsnsferred to a continuous extraction apparatus. Twenty-five ml of concentrated hydrochlorio acid were added and the solution extracted with ether overnight (20-22 hr). A longer extraction time (2 da) only resulted in a slightly increased yield. The ethereal extract was dried over anhydrous sodium sulfate, filtered, the solvent stripped off, and the residue (11-12 g) fractionated in v ~ l through o s smell Vigreux column collecting 9.7-10.8 g (75-84%) of dichloroacetie acid with bp 8445°C (9 mm) and nosa 1.4620-1.4630. A reference sample (puriss.) obtained from Fluka (Buchs, Switzerland) hsd nozB1.4648. Alternatively, the product could he isolated as the ammonium salt (7).
Litemture Cited (1) Lnm, H., i n "Chemistry of the Carbon-Nitrogen Double Bond"
(Editor: PAT*&6.) Interacienoe (division of John Wilay &Sons. Ino.) London. 1970, P. 505. (2) E ~ ~ n s o l L., i , in "Chemistry of the Carhorylia Aoida and Estus" (Editor: PAT& S.) I n t e m i e n ~ e(division of John Wiley & Sona. Ino.). London. 1969, p. 55. (3) Umm, J. H.P., AnnudRcporta, 65B.231 (1969). 56,405.(1969). (4) BAIZER,M. M., Natu~wi~~cndchdten.
(6) W E I N B E RN. ~ , L., AND WEINBERO,H. R., Chem. Ran., 68,449 (1968). (7)Un*m, N.. Smr~v*, T..m n SAHAI,W., J. Eleelrochem. Sac. Jap. (overseasSu~n1.Ed.). 28. E 69 (19601. (8) I Y ~ B ~E., N&D . + L. ~ D H., , A& Chm. Scond.. 19,2303 (1965). (9) VOBEL.E.. "Practical Organic Chemiatru," (3rd Ed.). Lonpmana, London I O R Z, n . 411. ~~~, -. (10) LDND,H., "Studier over elektrodereaktioner i organisk polarografi og voltammetri." Asrhua. Denmark, 1961. p. 88.
Volume 48, Number 2, February 1971
/
137
