Spontaneous hydrolyses in sulfobetaine micelles. Dependence of micellar charge effects upon mechanism

Rate constants of spontaneous hydrolyses in zwitterionic micelles of N-hexa decyl-N,N-dimethyl-3-ammonio-propanesulfonate (SB3-16) are compared with th ose in cationic (n-C16H33NMe3X, X = Cl, Br, OMes; CTACl, CTABr, CTAOMes) an d anionic (n-C12H25OSO3Na, SDS) micelles. Substrates are methyl benzenesulf onate, 2-adamantyl and pinacolyl 4-nitrobenzenesulfonate, 4-bromo- and 4-ni trobenzoyl chloride, phenyl and 4-nitrophenyl chloroformate and bis(4-nitro phenyl) carbonate. Hydrolyses are micellar inhibited, except for the nitro substituted acid chlorides. Reactions with extensive bond-breaking in the t ransition state (S(N)1 hydrolyses) are faster in SDS than in cationic and s ulfobetaine micelles, but the other hydrolyses, which involve significant b ond-making, are slower in SDS. Rate constants are similar in cationic and s ulfobetaine micelles. These micellar charge effects are ascribed to interac tions of the polar transition states with the asymetrically charged interfa cial region which complement effects of the lower polarities of micelles re lative to water.