D. 1. Tuleen and B. A. Hess. JI. Vanderbilt University Nbshville, Tennessee
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Free-Radical Bromination of p-Toluic Acid A n experiment in organic chemistry
Free-radical halogenation is described in some detail in most textbooks of elementary organic chemistry (1). Many of these accounts include discussions of the mechanistic aspects of these typical chain reactions. Few experiments which illustrate these principles af free-radical substitution appear in standard laboratory texts. One notable exception (3) (in which'benzyl bromide is prepared) requires the use of a hood. This paper describes the synthesis of abromo-p-toluic acid (11) and the subsequent displac'& ment of bromide ion by three nucleophiles. Bromination of substituted toluenes by N-bromosuecinimide (NBS) is believed to proceed by a freeradical chain mechanism in which bromine atom functions as the chain carrier (3, 4). Bromine is continuously regenerated by the rapid, ionic reaction of NBS with the HBr which is produced by hydrogen abstra~tionby the bromine atom. .
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The bromination of p-toluic acid, I, by NBS proceeds to completion within an hour in refluxing carbon tetrachloride (using benzoyl peroxide as an initiator). Some of the students achieved inferior results due to insufficient swirling of the heterogeneous reaction mixture. A11 of the operations were performed at the bench. Benzylic halide I1 is non-lachrymatory; students were, however, advised to exercise caution, as it may be a skin irritant. Purification of I1 was not effected (5); nuoleophilic displacement was performed on crude materid. Students selected one of the three nucleopbiles listed and performed the displacement reactions. The use of 476
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p-tert-butylbenzenethio! allows the preparation of a sulfide using a readily available, nonmalodorous thiol. Yields reported in the Experimental Section are based on p-toluic acid and are somewhat greater than those typical of student preparations.
Journal of Chemical Education
Experimental Section a-Bromo-p-toluic Acid(I1). To a mixture of p-toluic acid (2.72 g, 0.02 mole, Aldrich) and q-bromosuccinimide (3.6 g, 0.0202 mole, Aldrich) was added bensoyl peroxide (0.2 g ) in such s manner that none of it remained on the ground joint. Carbon tetrachloride (25 ml) was added and the mixture was refluxed for 1 hr with acc+ond swirling. The cantents of the flask were chilled in an ice bath, then filtered with suction. The resultant solid was washed with three 1 0 d portions of pentsne, pressed dry, snd transferred to s beaker. Weter (50 ml) was added and the resultant slurry was stirred briefly to dissolve succinimide. The crude product was separated by suction filtration and dowed to dry. a-Methoxy-p-toluic Acid(II1). A solution of the crude abramo-p-toluic acid in methyl dcohol (50 ml) containing potassium hydroxide (2.24 g, 0.04 mole) was refluxed for 45 min. Methyl alcohol was removed by distillation and the residue was dissolved in 50 ml of water and cooled. Acidification with 1 N HCI followed by suction filtrstion afforded the crude acid (2.2 g, 0.0133' mole, 66%) mp 106-111% Recrystallization from water gave 1.6 g, mp 111-113'C (lit (6) mp 108°C). Nmr (CDCl,): 7.3-8.3 (m, 4H, Ar-H), 11.1 (s, COIH), 4.57 (s, CHd, 3.44 (8, CH1) ppm. u-Phenaxy-p-toluio Acid(1V). A solution of the crude obromo-v-toluic acid in aaueous (50 ml) ~otassiumhvdroxide
the precipitation of a solid, which was dissoived by the addition of water (200 ml). Acidifiorttian with 1 N HCI and filtratiaii gave the crude produce, mp 191-205'C. Recrystallieation from ethanol-water afforded 2.2 g (48%) of the desired material, mp 206-212°C. One further recrystallization from ethanol-water afforded material of mp 211-214T (lit. (8)mp 216°C). Nmr (DMSO): 6.9-8.1, (m, 9H, h-H), 5.26 (s, 2H, CHz)ppm. cr(p-Ter~butylphenylthid)~Iuie Acid(V). A solut~onof the crude.a-bromo-p-toluic acid in aqueous (50 ml) potassium hy; droxide (2.8 g, 0.05 mole) containing p-twt-butylbeneenethiol (3.3 g, 0.02 mole) was heated with steam for 45 min. Aeidification using 1 N IECl and suction filtration afforded the crude product; recrystallization from ethanol-water gave 3.8 g (63%) of V, m.p. 177-180°C. Further recrystallization afforded m* terial ofmp 185-1860~. iqmr(CDCL): 7.2-8.1 ( m , 8 H , ~ r - ~ ) , 4.13.(s, 2H, CHA 1.32 (S, 9H, (CHdaC), 10.3 (broad, lH, COOH).
Anal. Calc'd for QsHsoOS: C, 72.00; H, 6.66. 71.86; H, 6.69.
Fotmd: C,
Literature Cited
MonRiaon, R. T . , , * N ~ B O ~ R., N., "organic cdemiatry"(ind Ed.]. Allyn and Bacon,,Iho., Boston, 1966, pp. 1 1 6 1 3 0 and pp. 385-390. (b) ROBERTB, J. D., AND CASBRIO, M. C., "Modern Organio Chemistry," W. A. Benjamin. Ino., New York, 1967, PO. 6 6 7 7 and p. 629. H ~ ~ a r n ~G a .r K., ~ . aso JOHNBO*, H. w.. JR.. "selkcted ~kperiments in Organic Chdiativ" (2nd ~ d . 1 .W. H. Freeman and c o . . sari ~ranoiaao,1968, p. 172. P ~ R S O N . R. E.. A " M A ~ ~ J. N c.. . J . d m . c&_ sac.. 85, 354 (19631. RuasmL. G . A,. DEBOER, C., ano ~kslaolto,K. M., J . A m r . c h m . Soc.. 85,365 (1963). KING. F. E.. *Nn G r m = m M. F..J . Chern. ~ O E . .354j (1950); D ~ B , G . H.. A N D C * s = ~ e R. , N., J. Ow. Chcm., 19,1571 (1954). EXNEB,0..ANDJONIB. J., COU. C ~ C Ec ~h .m . commm.. 2?,2296 (19621.
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(2) (8) (4) (5) (61
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Volume 48, Number 7, July 1971
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