Reagent Distribution and Micellar Catalysis of Carbocation Reactions1

Reactions of Malachite Green, MG+, and the tri-p-anisylmethyl cation, R+, with 1-benzyldihydronicotinamide (BDHNA) are catalyzed by anionic micelles of sodium lauryl sulfate (NaLS), and for MG+ weakly by cationic micelles of cetyltrimethylammonium bromide (CTABr). Added sodium bromide and tosylate speed reaction in CTABr, probably by increasing incorporation of MG+ into the cationic micelle, but they show the typical negative salt effects on the CTABr-cata-lyzed reaction of MG+ with hydroxide ion. Tosylate, but not bromide, ion also inhibits the CTABr-catalyzed reaction of MG+ with borohydride ion, suggesting that anion binding to micelles of CTABr follows the sequence OTos- > BH4- > Br- > OH-. For surfactant concentrations well above the critical micelle concentration, cmc, the rate constant-surfactant profile for reaction of MG+ with BDHNA can be interpreted in terms of the distribution of both reactants between water and the micelles, using the binding constants of 8000 and 10 for MG+ to NaLS and CTABr, and of 285 and 400 for BDHNA. Although the catalysis of this reaction is much greater for NaLS than CTABr (by factors of 15-fold as compared with twofold), this is the result of greater incorporation of MG+, not of a greater rate constant in the micelle. The binding constants of MG+ to CTABr are consistent with the rate-surfactant profiles of the reactions with hydroxide and borohydride ion. Second-order rate constants in the micellar pseudophase are compared with those in water.