A QUANTITATIVE-ANALYSIS OF THE EFFECTS OF HEAD GROUP BULK ON S(N)2 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 (la,b) and E2 elimination from phenethyl and 4- nitrophenethyl bromide (2a,b). For the S(N)2 reactions in 0.1 M surfac tant, rate increases over reactions in aqueous 0.1 M OH- are by factor s 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 ps eudophase decrease slightly in going from CTAOH to CTPAOH for reaction of la and increase for reaction of Ib, 2a, and 2b. Second-order rate constants in the micellar pseudophase increase with head group bulk fo r reactions in which charge is dispersed in the transition state, and charge dispersion in the transition state formation increases second-o rder rate constants in micelles relative to those in water.