Carboxylic Acids from Halides and Carbon Dioxide A Facile, Fast, Inexpensive Electrochemical Synthesis J. Pwliquen Laboratoire d e Photochimie Solaire du C. N. R. S. 2. Rue Henri Dunant. 94320-Thiais. France M. Heintz, 0. Sock, M. Troupel Laboratoire d'Electrochimie, Catalyse et Synthese Organique. 2, Rue Henri Dunant. 94320-Thlais. Although electrochemistry has produced a great deal of detailed mechanistic results, it is not fully recognized as a useful synthetic tool, most often because it is believed to require sophisticated and expensive laboratory equipment and also because those now practicing chemistry have not been taught its technique. In this paper, we present an electrosynthesis that takes place in a one compartment cell and necessitates only a very simple constant current generator; the experiment can be run in every undergraduate organic laboratory. A new method bas been reported recently (I)for the electrocarboxylation of organic halides R X ( X = I, Br, C1) a t temperatures close to ambient and under atmnsoheric vressure-of carbon dioxide (Con). The concentrationbofthe ieact a n t is unusuallv. high for a n electrochemical synthesis, and the reaction can he achieved in a short time by the use of high current densities.' T h e results presented in ref 1show that electrocarboxylation can be suc~essfullyapplied tovarious classes of compounds, substituted or not, provided the substituents are not reductible, including those Grignard's derivatives which are difficult to prepare (e.g., vinylic halides) or unstable (e.g., benzylic or CO and COOR substituted halides). Valuable arene-2 propanoic acid derivatives are thus prepared as starting compounds for drug synthesis. As an example, we have chosen the carboxylation of benzvlic chloride (BC) since the startina material is readilv available and the product, phenyl acetic acid, can be eas& identified. T h e equation reaction can be written as PhCH,CI
+ Mg + CO,
the anodic process being Mg
-
-
PhCHnCOO- + Mg2++ Cl-
France
tion or reduction since the electrode potential can be tuned within a large range, high reducing or oxidizing power (and no dangerous chemicals to handle!), and easy work-up. On the other hand, the choice of the electrodes and the cell design can he critical since electrochemical reactions are surface processes that take place a t the eledrode-solution interface. I n addition to Laboratory-scale synthesis, as demonstrated here, organic electrochemistry is also carried nut on industrial scale (e.g., acrylonitrile dimerization, PbEta synthesis). It provides, too, a very convenient method for the preparation of reactive intermediates as anions, cations, or radicals. Exerlmental The electrochemical cell (Fig. 1)is made of a 60-mL beaker covered by a plastic cap with the four holes for the passage of the magnesium rod, the COzinlet and outlet tubes, and the wire to the stainless steel gauze. The magnesium rod is held at the center of the cell and is surrounded by the gauze. This geometrical configuration allows the best electricfield distribution hetween the two electrodes. The rod and the gauze are connected, respectively, to the positive and negative poles of the eonstant-current generator. Dimethyformamide (DMF) and BC are stirred overnight over copper sulfate and calcium chloride, respectively, and distilled. In a typical run, 2 gof BC are dissolved in 30 mLof DMF along with 0.55 g of the tetrahutyl ammonium iodide as the supportingeledrolytez. The magnesium bar is previously activated by heing dipped in 1M hydrochloride solution and carefully wiped. Additionally,0.5 mL of dodecane is added as a reference for GC analvsis. Con is bubbled through the solution. The rolurion is stirred maynctirally. and the cell is ~.oulcdin an ice bath The current is set at 0.1 A, the voltage t e q within the L 1 . i V range.
Mg2++ 2e-
and the cathodic one PhCH&l+ COI + 2e-
-
PhCH2COO-
+ Cl-
The reaction is formally very similar t o the carboxylation of Grignard's reagents,
but i t proceeds in a different way since the carboxylation stops as soon as the current is turned off. By-products such as bihenzyl or toluene are detected only in a very small ratio. In addition to this introduction to the technique itself, the students can he exposed to the available literature (2-7), emphasis being put on the advantages and disadvantages as compared to the classical techniques of organic chemistry. Among the advantages one finds high selectivity in oxida-
'The current density (A/dm2)is the current intensity that flows through an electrode surface of 1dm2. This small amount is sufficient to give rise to the necessary electric conductivity at the very beginning: after that the conductivity is mainly due to the in situ-generated inns.
Figure 1. Diagam ot the elecbochemic~lcell: 1, magnesium rod (Ism diameter):2, clampto hold (1):3, copper wireto stainless steel gauze: 4. COI inlet: 5, stainless steel gauze; 6. magnetic stirrer: 7 , plastic cap: 8. 60-mL beaker: 9. COz outlet. Volume 8 3 Number 11 November 1986
1013
The throreural reaction time iscalculatedasfollows.Since 1.58 X mol ot BC has to be reduced and since the reduction needs two electrons per molecule, the theoretical number Q t b of Coulomba ra 10.'
and the theoretical reaction time
The BC consumption is followed by GC (Fig. 2) (column length, 4 m; stationary phase, SE 30,15%; oven temperature, 160 "C); BC concentration is estimated by comparison with the dodecane reference. The plot of BC concentration versus time shows that the reduction is over when 2.1-2.2 mol of electron ner mole of BC have been delivered, Q... = 2.1-2.2 ~-~~~ Qth. For ihis purpose 0.2-mL aliquots are takenbut with a svrinee and added to solutions made UD of 1mL 2N HCI and are shaken and alZ"mcof ether. Solutions thus lowed to decant in the test tube, and sampling for GC is made from the organic layer. When the r e a c h comes to an end. 90 rnl. of 2N HCI are ndded and the acidic solution is carefully extracted three times with ether. The organic layer is decanted and treated with 30 mL of 1 N KOH; the alkaline solution is extracted with ether. After acidification of the aqueous solution, the phenyl acetic acid is extracted with ether and evaporated to dryness using suction. The yield is 80-9070.
Time Ihours)
~
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Journal of Chemical Education
Figure 2. Benzyl chloride concentration variatioo versus time.
Literature Clted (1) (2) (3) (41 (51 (6)
Soek, 0.: Troupel, M.: Perichon,J. Tetrahedron Left. 1986,26,1509. Baizer, M. M. "Organic Eleetroeherni%try";Dekkar: New York, 1973. Kyriamu, D."Basis ofElectraorganie Syntheses": Wiley: NouYork. Chausaard, J.L'Acfuolil€Chim. 1985.6.55. Sirnonet, J. L'Actuolil6 Chim. 1582.9.19. Periehon, J. L'Arluolith Chim. 1982.9.25. zr. 1983
