Basic hydrolysis of substituted nitrophenyl acetates in beta-cyclodextrin/surfactant mixed systems. Evidence of free cyclodextrin in equilibrium withmicellized surfactant

The basic hydrolysis of o-, m-, and p-nitrophenyl acetates (NPA) in mixed s ystems consisting of beta-cyclodextrin (beta-CD) and a surfactant--sodium d odecyl sulfate (SDS), tetradecyltrimethylammonium hydroxide (TTAOH), or tet radecyltrimethylammonium bromide (TTABr)--has been studied. beta-CD was fou nd to catalyze the basic hydrolysis of NPA via the interaction of its hydro xyl group, in deprotonated form, with the carbonyl group in the complexed s ubstrate. In the presence of a surfactant, the rate of the reaction is init ially decreased. This inhibitory effect is the result of the surfactant mon omers being complexed by beta-CD and the NPA being in turn displaced from t he aqueous medium, which cancels the catalytic effect of the cyclodextrin. Once micellization starts, the system behaves Like a typical micellar syste m in the hydrolysis of a hydrophobic substrate. Plots of the ratios of pseu do-first-order rate constants in micelle-CD mixed systems and in micelles-k appa(obs)(SDS+CD)/k(obs)(SDS) and k(obs)(TTAOH+CD)/k(obs)(TTAOH) versus sur factant concentration revealed the ratio to be greater than unity even at h igh surfactant concentrations, which is consistent with a CD-catalyzed path way. This result is in turn consistent with the presence of a substantial c oncentration of free CD in equilibrium with the micellar systems. The const ancy of k(obs)(SDS+CD)/k(obs)(SDS) and k(obs)(TTAOH+CD)/k(obs)(TTAOH) at su rfactant concentrations above the critical micelle concentration suggests t he absence of interactions between CD and the micellar system once the latt er has been established.