Secondary Valence Force Catalysis. VII. Catalysis of Hydrolysis of p-Nitrophenyl Hexanoate by Micelle-Forming Cationic Detergents

The hydroxide ion dependent hydrolysis of p-nitrophenyl hexanoate and p-nitrophenyl laurate is subject to marked catalysis by alkyltrimethylammonium detergent salts. In contrast, the rate of the corresponding reaction of p-nitrophenyl acetate is rather insensitive to the concentration of these species. Evaluation of the pertinent equilibrium constants for association of the acetate and hexanoate with micelles formed from tetradecyltrimethylammonium chloride through molecular sieve chromatography on Sephadex G-10 reveals that the distinct behavior exhibited by these esters is principally a reflection of the greater tendency of the latter to associate with the catalyst in solution. Catalysis of the basic hydrolysis of p-nitrophenyl hexanoate by alkyltrimethylammonium salts is accentuated as the length of the alkyl chain is increased from 8 to 18. Monovalent anions are potent inhibitors of the tetradecyltrimethylammonium chloride catalysis for the basic hydrolysis of p-nitrophenyl hexanoate; an increase in bromide ion concentration from zero to 0.20 M inhibits the detergent-dependent reaction by more than an order of magnitude. The increasing order of effectiveness of anions as inhibitors of this reaction is : F^- < Cl^- < Br^- < NO₃^-. These facts suggest that the inhibition is largely a consequence of displacement of hydroxide from the surface of the micelle as a consequence of binding of other anions.