Premicellar Accelerated Decarboxylation of 6-Nitrobenzisoxazole-3-carboxylate Ion and Its 5-Tetradecyloxy Derivative

Abstract

Didodecyldialkylammonium chloride and bromide (alkyl = Me, Et, n-Pr, n-Bu) accelerate the spontaneous decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion, 1,H, and its 5-tetradecyloxy derivative, 1,OTD. With most of these surfactants, first-order rate constants, kobs, go through maxima in very dilute surfactant and then decrease and go through minima as association colloids form. These phenomena are not explicable in terms of substrate-induced micellization. However, kobs increases in the N-alkyl sequence Me < Et < n-Pr < n-Bu, as is typical of decarboxylations in association colloids of single-chain surfactants. Reaction in premicelles is accelerated by an initial increase in 1,H. The factors that control relative rates of spontaneous reactions in premicelles and in the association colloids, in particular, depletion of water at the reaction center and association of substrate and quaternary ammonium centers, are discussed with respect to the roles of substrate and surfactant hydrophobicities.