Spontaneous decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion in CH2Cl2 is speeded by cationic surfactants (C16H33NR3Br, R = Me, Et, n-Pr, n-Bu; C16H33NMe3X, X = Br, ClO4, C12H25OSO3; CTABr, CTAClO4, CTADS, cetylquinuclidinium bromide, CQBr, and p-octyloxybenzyltrimethylammonium bromide, pOOTABr) and by Bu4NBr. First order rate constants reach limiting values (kmax) which increase with increasing bulk of the head group, and decrease on addition of water. Values of kmax in CH2Cl2 with and without a given amount of added water correlate with conductivities for most of the surfactants. It appears that reaction is fastest in small ionic clusters, and growth of these clusters and especially formation of “water-pool” reverse micelles slows reaction. Variations of NMR 1H chemical shifts on addition of H2O to CD2Cl2 show that CTABr readily generates “water-pool” micelles, but they do not form as the bulk of the head group is increased. There is little effect upon the conformations of the hexadecyl residues, based on 13C chemical shifts, which were separated for most of the methylene groups.
Surfactant Effects upon Decarboxylation of 6-Nitrobenzisoxazole-3-Carboxylate Ion in Dichloromethane. The Role of Head Group Size
GERMANI, Raimondo;SAVELLI, Gianfranco;
1991
Abstract
Spontaneous decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion in CH2Cl2 is speeded by cationic surfactants (C16H33NR3Br, R = Me, Et, n-Pr, n-Bu; C16H33NMe3X, X = Br, ClO4, C12H25OSO3; CTABr, CTAClO4, CTADS, cetylquinuclidinium bromide, CQBr, and p-octyloxybenzyltrimethylammonium bromide, pOOTABr) and by Bu4NBr. First order rate constants reach limiting values (kmax) which increase with increasing bulk of the head group, and decrease on addition of water. Values of kmax in CH2Cl2 with and without a given amount of added water correlate with conductivities for most of the surfactants. It appears that reaction is fastest in small ionic clusters, and growth of these clusters and especially formation of “water-pool” reverse micelles slows reaction. Variations of NMR 1H chemical shifts on addition of H2O to CD2Cl2 show that CTABr readily generates “water-pool” micelles, but they do not form as the bulk of the head group is increased. There is little effect upon the conformations of the hexadecyl residues, based on 13C chemical shifts, which were separated for most of the methylene groups.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
