Micellar Enhancements of Rates of SN2 Reactions of Halide Ions. The Effect of Headgroup Size

Article abstract of DOI:10.1021/j100341a061

Observed first-order rate constants for reaction of methyl naphthalene-2-sulfonate with Cl^- or Br^- in micelles of cetyltrialkylammonium halide (C16H33NR3X, R = Me,Et,n-Pr,n-Bu, X=Cl,Br) increase monotonically with increasing surfactant and halide ion concentrations and tend to limiting values.The variation of rate constants with concentrations of surfactant and halide ions can be fitted to an equation that accounts for the distribution of both reactants between water and micelles.Fractional micellar ionization, α, increases, i.e., the affinity of the micelles for the halide ion decreases, with increasing bulk of the N-alkyl groups, but the second-order rate constants at the micellar surface and the overall rate constants increase.The rate effects of the micelles of the bromide ion surfactants can be understood in terms of NMR evidence on the micellar binding of the reactants and their location at the micellar surface.The reactivity increase with increasing headgroup bulk is related to the disruption of the hydration of Br^- and to the electrostatic interaction of the naphthalene ?-system with cationic nitrogen.