Dialkylaminopyridine catalysed esterolysis of p-nitrophenyl alkanoates in different cationic microemulsions
The reactions of p-nitrophenyl alkanoate esters with dialkylaminopyridine (DAAP) and its related mono- and di-anionic water-soluble derivatives have been studied separately in three different microemulsion (ME) media. These were (a) oil-in-water ME (O/W), (b) water-in-oil ME (W/O) and (c) a bicontinuous ME, where oil and water are in nearly comparable amounts. All the ME systems were stabilized by cationic surfactant, cetyltrimethylammonium bromide (CTABr) and butanol as a cosurfactant. The second-order rate constants (k2) in the microemulsion media were also determined over a phase volume (φ) of approximately 0.13–0.46. In order to explain the contribution of effective concentration of the nucleophiles in the aqueous pseudophase, corrected rate constants k2φ=k2(1 –φ) were obtained. The rate constants of the corresponding hydrolytic reactions were also examined in CTABr micelles. While the DAAP catalysts were partitioned between the micellar and aqueous pseudophases in ME, the hydrophobic substrates were found to be mainly confined to oil-rich phases. Present results indicate that the main effect of ME media on the hydrolysis reaction is due to both electrostatic reasons and substrate partitioning.