The effect of 2-halogeno-substituents on the rates of bromination of olefins

Abstract

Second-order rate coefficients have been determined for the reaction of bromine with CH₂C(X)CH₂Cl for X = H, F, Cl, and Br, in both trifluoroacetic acid and 50% methanol–water. In both solvents there was a large decrease in the rate coefficient for X = F, Cl, and Br compared with that for X = H. The fluoro-olefin however reacted considerably faster than did the chloro- and bromo-olefin, resulting in the reactivity order H F > Cl ∼ Br. These substituent effects are compared with those obtained for the addition of trifluoroacetic acid to 2-halogenopropenes (the only other known case of the kinetic effect of vinyl halogen substituents in addition reactions) and more generally with those found in electrophilic aromatic substitution. The results are discussed in terms of the relative importance of bridged bromonium ion structures and classical carbonium ion structures in the transition states of electrophilic additions to olefins.