Functional micellar catalysis. Part 2. Ester hydrolysis promoted by micelles containing the imidazole ring and the hydroxy-group

Abstract

The catalytic effects on the hydrolysis of p-nitrophenyl esters due to micelles of quaternary ammonium salts containing the imidazole ring (Ia), the hydroxy-group (IIa), and both functions (IIIa) have been investigated in the pH range 7–9. Both functions have comparable pK values when at the head of cationic micelles and their basic forms are the effective nucleophilic sites. Bifunctional micelles made of (IIIa) or of mixed (Ia) and (IIa) are better catalysts than the monofunctional ones, with (Ia) being much more effective than (IIa). Co-operative interaction between the two functional groups is inter- rather than intra-molecular in character. A mechanism is suggested for bifunctional micelles involving acylation of the imidazole ring followed by a relatively rapid acyl transfer to the hydroxy-function in the micellar phase.