MICELLAR EFFECTS UPON RATES OF SN2 REACTIONS OF CHLORIDE-ION .1. EFFECTS OF VARIATIONS IN THE HYDROPHOBIC TAILS

Interactions of 35Cl- with trimethylammonium surfactants increase with increasing length of the hydrophobic alkyl group on the basis of NMR line widths. The dependence of line width upon [surfactant] and [NaCl] can be fitted by a Langmuir isotherm, and for alkyl = C8H17-C18H37 the binding parameters, K′Cl, agree reasonably well with those estimated kinetically from rate constants of the reaction of Cl- with methyl naphthalene-2-sulfonate. The second-order rate constants for reaction in the micellar pseudophase decrease modestly with decreasing length of the alkyl group and are similar to those in water. For C8H17NMe3Cl there appear to be interactions of Cl- with both micelles and monomeric surfactant, or small clusters of it. This nonmicellar interaction is also kinetically significant with C6H13NMe3Cl and Me4NCl, but interactions with substrate and Cl- are weak. © 1990 American Chemical Society.