Effects of association colloids on elimination from 1,2-dihalo-1,2-diphenylethanes. The role of surfactant structure

Rates of E2 elimination of D,L-1,2-dichloro-1,2-diphenylethane (1) and the analogous dibromide (2) were measured in aqueous cetyltrimethylammonium chloride and hydroxide (CTACl and CTAOH), cetyltripropylammonium hydroxide (CTPAOH), didodecyldimethylammonium chloride and hydroxide (DDDACl and DDDAOH), and N,N-dimethyl-N-(2-hydroxyethyl)-n-hexadecylammonium hydroxide (DOH,OH). Micelles or other association colloids of these surfactants increase rates in solutions that contain OH-, and DOH,OH is much more effective than the nonfunctional surfactants. Variations of first-order rate constants, k(obs), with [surfactant] or [OH-] are fitted by a pseudophase model with water and micelles as distinct reaction media, to give second-order rate constants in the latter. These rate constants are in the sequence DOH,OH > CTPAOH approximate to DDDACl > DDDAOH approximate to CTAOH approximate to CTACl. Micelles favor reactions of 2 over 1, relative to reactivities in water, due to differences in amphiphilic interactions with the leaving halide ions.