MICELLAR HEADGROUP SIZE AND ANION NUCLEOPHILICITY IN SN2 REACTIONS

Rates of reaction of OH- with methyl naphthalene-2-sulfonate (MeONs) in aqueous cationic surfactants (C16H33NR3OH, R = Me, Et, n-Pr, n-Bu, and cetylquinuclidinium hydroxide) increase with increasing surfactant and hydroxide ion concentrations. The variations of rate constants can be fitted to an equation that describes the distribution of both reactants between water and micelles in terms of Langmuir isotherms. The Langmuir coefficients for OH- decrease in the head-group sequence Me3N > quinuclidinium > Et3N > Pr3N > Bu3N. The second-order rate constants at the micellar surfaces are not very sensitive to the bulk of the head group, although they increase at high added OH-. Rate constants for attack of water on micellar-bound substrate increase with increasing bulk of the head group. © 1990 American Chemical Society.