4516
J. Org. Chem. 1994,59, 4516-4528
Radical Bromination of Cyclohexene in CC4 by Bromine: Addition versus Substitution D. W. McMillen and John B. Grutzner* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-1393 Received July 27, 1993@
The radical reaction of bromine (10-2-10-5M) with cyclohexene in CC14 in the light has been investigated. The reactions have been found to be highly reversible and controlled by both thermodynamics and the availability of Brz and HBr as equilibrating agents. The selectivity of substitution over addition is controlled by [Brz]. Bromine addition to the double bond and allylic substitution occur a t comparable rates at room temperature. The limiting substitutiodaddition ratio was found to be 4.0 f 0.2for [Brz] less than M. While the p-bromocyclohexyl radical is generated very rapidly, its steady-state concentration is kept low by its rapid reversion to cyclohexene. Substitution via the allyl radical, while relatively slow, is irreversible and fast enough to maintain the concentration of bromine a t sufficiently low level to prevent significant addition. The equilibrium constant for the reaction 1 Br 2a is estimated to be 500 M-l, and the rate constant for substitution kz. as 2000 M-l s-l. The ratio of removal of hydrogen from cyclohexene, 3-bromocyclohexene, and 3,6-dibromocyclohexene by bromine radical was found to be 2.2:l.O:O.l. The ratio for addition of bromine to the double bond was 5.5:1.0:
