A Quantitative Analysis of the Effects of Head Group Bulk on SN2 and E2 Reactions in Cationic Micelles

Micelles of cetyltrialkylammonium hydroxide (alkyl = Me (CTAOH), n -Pr (CTPAOH)) increase rates of S N 2 substitution of methyl benzene- and 4-nitrobenzenesulfonate ( 1a,b ) and E2 elimination from phenethyl and 4-nitrophenethyl bromide ( 2a,b ). For the S N 2 reactions in 0.1 M surfactant, rate increases over reactions in aqueous 0.1 M OH - are by factors of 6−15, but for the E2 reactions, increases are by factors of 17−25 ( 2a ) and 100−240 ( 2b ). Second-order rate constants in the micellar pseudophase decrease slightly in going from CTAOH to CTPAOH for reaction of 1a and increase for reaction of 1b, 2a, and 2b . Second-order rate constants in the micellar pseudophase increase with head group bulk for reactions in which charge is dispersed in the transition state, and charge dispersion in the transition state formation increases second-order rate constants in micelles relative to those in water.