NBS allylic bromination of cyclohexene; 3-Bromocyclohexene

Andrew McCarroll ([email protected]),
A contribution from the Walton group Group, St Andrews

Chemicals Used
Cyclohexene, N-Bromosuccinimide, Benzoyl peroxide, Carbon tetrachloride

Procedure
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%).

Author's Comments
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.

Data
¹H nmr (CDCl₃) 1.60-2.35 (6H, m), 4.85 (1H, m), 5.75-6.00 (2H, m)

Lead Reference
A. J. McCarroll, PhD Thesis, University of St Andrews, 2000.

Other References
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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