NBS allylic bromination of cyclohexene; 3-BromocyclohexeneSyntheticPage 144 (2001)
A contribution from the Walton group Group, St Andrews
Cyclohexene, N-Bromosuccinimide, Benzoyl peroxide, Carbon tetrachloride
Benzoyl peroxide (0.35 g) was added to a mixture of cyclohexene (35 g; 0.43 mol) and N-bromosuccinimide (24.9 g; 0.14 mol) in carbon tetrachloride (100 cm3). The mixture was stirred for 2 hours, then heated slowly to reflux, and maintained at reflux for 3.5 hours. The mixture was cooled, filtered and concentrated, then distilled at reduced pressure (b.p. 61 oC @ 12 mmHg) to give 3-bromocyclohexene as a colourless oil (15.71 g; 70%).
Unfortunately, carbon tetrachloride is the only suitable solvent. The slow start to the reaction is necessary, as it can go out of control if heated too rapidly. The method is a good one for symmetric molecules. Regioselectivity can be poor if the allylic positions are not equivalent. We only performed this reaction once.
1H nmr (CDCl3) 1.60-2.35 (6H, m), 4.85 (1H, m), 5.75-6.00 (2H, m)
A. J. McCarroll, PhD Thesis, University of St Andrews, 2000.
B. S. Furniss, A. J. Hannaford, P. W. G. Smith and A. R. Tatchell, "Vogel's Textbook of Organc Chemistry, 5th Edition," Longman, Harlow, 1989.
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