MICELLAR EFFECTS UPON RATES OF SN2 REACTIONS OF CHLORIDE-ION .2. EFFECTS OF CATIONIC HEADGROUPS

Observed first-order rate constants for reaction of methyl naphthalene-2-sulfonate with Cl in micelles of cetyltrialkylammonium chlorides (C16H33NR3Cl, R = Me, Et, n-Pr, n-Bu, n-Pc) increase monotonically with increasing concentrations of surfactant and Cl- and tend to limiting values. These variations can be fitted to a model that describes micelle-ion interactions in terms of Langmuir isotherms. The binding parameters, K′Cl, decrease with increasing headgroup bulk, but second-order rate constants at the surface increase. The value of K′Cl is higher with a quinuclidinium as compared with a N+R3 headgroup, but the second-order rate constant is similar to that for R = Et. Values of K′Cl are high with hydroxyethyl head groups N+R3 = N+Me2CH2CH2OH or N+(CH2CH2OH)3, but these groups decrease second-order rate constants. Values of K′Cl estimated from 35Cl NMR line widths agree with the kinetic values except with the N+(CH2CH2OH)3 headgroup for which the kinetic values are much higher. The effect of headgroup bulk upon the spontaneous reaction with water was examined by using sulfate as the surfactant counterion. Trends in reactivity are very similar for reactions of H2O and Cl-. © 1990 American Chemical Society.