Kinetic study on the reaction of N-bromosuccinimide with β,γ-unsaturated alcohols in aqueous acetic acid

133-137The reaction of N-bromosuccinimide (NBS) with allyl, crotyl and cinnamyl alcohols in 50% v/v aq. acetic acid is acetoxybromination, and exhibits first order dependence on [NBS], Michaelis-Menten type of behaviour with [substrate], and displays H+ inhibition. Added succinimide and Hg (II) have no significant effect on the reaction rate. There is no primary salt effect, and the dielectric effect is positive. The reaction is shown to proceed via two paths: (i) formation of NBS-substrate complex in a pre-equilibrium followed by its rate-limiting breakdown to give a bromonium or carbonium ion intermediate, which in turn reacts with acetate ion in a fast step to yield the product, and (ii) fast interaction between the substrate and HOBr formed by the rate-determining hydrolysis of NBS. Linearity of Exner's plots suggests a common mechanism. There is no correlation between log k and Taft's polar substituent constants, σ, σ, σ. A bridged bromonium ion for aliphatic alcohols, and for the aromatic alcohol a resonance stabilised bromo carbocation are suggested as intermediates