Temperature effects upon aqueous micellar-assisted decarboxylation of 6-nitrobenzisoxazole-3-carboxylate and its 5-methyl derivative.
An investigation of temperature effects upon first-order rate constants in the micellar pseudophase for decarboxylation of 6-nitrobenzisoxazole-3-carboxylate (6-NBIC) and its 5-methyl derivative (6-NBIC-5-Me) was carried out. Surfactants used were cationic cetyltrialkylammonium bromide with alkyl = methyl (CTABr), ethyl (CTEABr), n-propyl (CTPABr), and n-butyl (CTBABr). The investigation shows that micelles speed up reactions by decreasing enthalpies of activation. Increase in head group bulk further speeds reactions still by a small decrease in the enthalpies, for both substrates. Values of DeltaH# and DeltaS# for 6-NBIC in the various surfactants give linear isokinetic plot, with CTBABr as outlier.
Decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion in surfactant self assemblies
Decarboxylation of the 6-nitrobenzisoxazole-3-carboxylate ion is speeded more by small assemblies of didodecyl(dimethyl)ammonium chloride (DDDACl) than by fully formed assemblies or by normal cationic micelles. The first-order rate constants, k′M, of reaction of fully micelle-bound substrate increase with decreasing surface charge density of normal cationic micelles with a change from hydrophilic to less hydrophilic counter-ions, e.g., in going from CTAOH to CTAOTos (CTA = C16H33NMe3 and Tos = toluene-p-suIphonate), or from cationic to zwitterionic micelles. These changes are ascribed to changes in transfer free energies of the initial-state carboxylate ion and the charge-delocalized transition state so that small assemblies of cationic amphiphiles, e.g., of DDDA or (C8H17)3N+R, are better catalysts than cationic micelles because of less initial-state stabilization. A similar explanation can be applied to catalysis of decarboxylation by synthetic cationic vesicles.
Dephosphorylation in zwitterionic micelles of amine oxide or betaine sulfonate surfactants
Etude de la dephosphorylation du phosphate de diphenyle et de p-nitrophenyle car il se fixe fortement aux micelles