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

Abstract

Nucleophilic second-order rate constants ( k n ) for the reactions of piperidine (Pip) with phthalimide (PTH) in aqueous reaction mixture containing 2% (v/v) MeCN remain unchanged with change in total concentration of cetyltrimethylammonium bromide ([CTABr] T) from 0 to 1×10 −4 at 0 and 0.005 M NaOH. The presence of 0.01 M CTABr apparently ceased the piperidinolysis reaction at 0.005 M NaOH and the rate of cleavage of PTH is controlled by hydrolysis reaction only. Pseudo-first-order rate constants ( k obs) obtained for the CTABr micellar-mediated cleavage of PTH at different [Pip] in the presence of a constant [NaBr] and at different [NaBr] in the presence of a constant [Pip] are qualitatively explained in terms of pseudophase and pseudophase ion-exchange (PIE) models of micelle. The lack of apparent pipperidinolysis of PTH in the presence of 0.01 M CTABr and 0.005 M NaOH is attributed to the existence of fast equilibrium between PTH and N-piperidinylphthalamide (1) in the micellar pseudophase due to presence of large effective concentration of OH − in the Stern layer ([OH M −]). The large effective [OH M −] is produced through ion-exchange between OH − (in aqueous pseudophase) and counterions (Br −) of CTABr micelles.