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 -C 14 H 29 O, 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 k obs 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 k obs = 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.