Effects of cetyltrimethylammonium bromide (CTABr) micelles on the rate of the cleavage of phthalimide in the presence of piperidine

Nucleophilic second-order rate constants (k(n)) for the reactions of piperi dine (Pip) with phthalimide (PTH) in aqueous reaction mixture containing 2% (v/v) MeCN remain unchanged with change in total concentration of cetyltri methylammonium bromide ([CTABr](T)) from 0 to 1 x 10(-4) at 0 and 0.005 M N aOH. The presence of 0.01 M CTABr apparently ceased the piperidinolysis rea ction at 0.005 M NaOH and the rate of cleavage of PTH is controlled by hydr olysis reaction only. Pseudo-first-order rate constants (k(obs)) obtained f or the CTABr micellar-mediated cleavage of PTH at different [Pip] in the pr esence of a constant [NaBr] and at different [NaBr] in the presence of a co nstant [Pip] are qualitatively explained in terms of pseudophase and pseudo phase ion-exchange (PIE) models of micelle. The lack of apparent pipperidin olysis of PTH in the presence of 0.01 M CTABr and 0.005 M NaOH is attribute d to the existence of fast equilibrium between PTH and N-piperidinylphthala mide (1) in the micellar pseudophase due to presence of large effective con centration of OH- in the Stern layer ([OHM-]). The large effective [OHM-] i s produced through ion-exchange between OH- (in aqueous pseudophase) and co unterions (Br-) of CTABr micelles. (C) 2001 Elsevier Science B.V. All right s reserved.