Surfactant effects on decarboxylation of alkoxynitrobenzisoxazole-3-carboxylate ions. 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 k(obs) for reaction of 1,OMe increase monotonically with [surfactant] and become constant when the substrate is fully bound and k(obs) = k'(M). The values of hobs 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.