Surfactant Effects on Decarboxylation of Alkoxynitrobenzisoxazole-3-Carboxylate Ion. Acceleration by Premicelles

Decarboxylation of 6-nitro-5-alkoxybenzisoxazole-3-carboxylate ion (1,OMe and 1,OTD, alkoxy ) MeO, n-C14H29O, respectively) is accelerated by cationic micelles of cetyltrialkylammonium bromide (CTABr, CTEABr, CTPABr, CTBABr, alkyl ) Me, Et, n-Pr, n-Bu respectively). The first-order rate constants kobs for reaction of 1,OMe increase monotonically with [surfactant] and become constant when the substrate is fully bound and kobs ) k′M. The values of kobs for 1,OTD increase sharply with increasing [surfactant] and reach well-defined maxima at [surfactant] at or below the cmc, before decreasing to values corresponding to kobs ) k′M. The magnitude of the rate maxima and the increase in k′M are in the sequence CTABr < CTEABr < CTPABr < CTBABr. The rate maxima are due to formation of premicellar complexes of substrate with one or a few surfactant monomers, but they “dissolve” in micelles at higher [surfactant]. A rationalization is provided for acceleration by very dilute surfactants.