Contrasting kinetic behaviour of allyl and crotyl alcohols towardsN-bromosuccinimide in aqueous methanol

The reactions of N-bromosuccinimide (NBS) with allyl and crotyl alcohols in aqueous methanol are first order in NBS. Allyl alcohol displays a zero-order dependence whereas crotyl alcohol exhibits a Michaelis–Menten-type dependence on substrate concentration. The NBS–allyl alcohol reaction is inhibited by H+ whereas the NBS–crotyl alcohol reaction is independent of [H+]. However, both are fast in the absence of acid. The rates are unaffected by the addition of neutral salt and succinimide. The reaction of NBS with crotyl alcohol exhibits a positive dielectric effect whereas allyl alcohol displays no such effect. Inhibition by HgII is observed in the crotyl alcohol reaction but not with allyl alcohol. 3-Bromo-2-methoxypropan-1-ol is the product of the allyl alcohol reaction but a mixture of 2-bromo-3-methoxybutan-1-ol and 2-bromobutane-1,3-diol in the crotyl alcohol reaction. In the allyl alcohol reaction the formation of solvated bromonium ion and of HOBr is suggested as rate-limiting steps. In the crotyl alcohol reaction, rate determining breakdown of an NBS–crotyl alcohol complex, formed in a fast pre-equilibrium, is proposed.