Nucleophilic Attack of Salicylhydroxamate Ion at C═O and P═O Centers in Cationic Micellar Media

Abstract

The reaction between the salicylhydroxamate anion (SHA(-)) and p-nitrophenyl benzoate (PNPB), tris(3-nitrophenyl)phosphate (TRIS), and bis(2,4-dinitrophenyl)phosphate (BDNPP) have been examined kinetically. The α-nucleophile, SHA(-), incorporated into cetyltrimethylammonium bromide (CTAB) micelles accelerates dephosphorylation of tris(3-nitrophenyl)phosphate (TRIS) over the pH range 6.7-11.4. With a 1.0 mM of SHA in CTAB, the nucleophilicity of SHA followed the order of reactivity, PNPB (C=O, carboxylate ester) > TRIS (P=O, triester) > BDNPP (P=O, diester), and monoanionic SHA(-) and dianionic SA(2-) are the reactive species. The critical micelle concentration, cmc, of cetyltrimethylammonium bromide (CTAB) decreases and the fractional ionization constant, α, increases with increasing the concentration of SHA(-). Addition of 1 and 10 mM SHA under the reaction conditions (pH 9.2, borate buffer) led to saturation of the micellar surface and provided qualitative information for the micellar incorporation of hydroxamate ion. Plots of the pseudo-first-order rate constant, k(obs), log k(obs), fraction of hydroxamic acid ionized, α(SHA(-)) and α(SA(2-)), vs pH showed bifunctional nucleophilicity of hydroxamic acid under micellar condition. Plotting k(obs) vs [SHA](T) gave a straight line with intercept k(0). This indicates that hydroxamate ions are very strong nucleophiles for nucleophilic attack at the C and P center. The pseudo-first-order rate constant-surfactant profiles show micelle-assisted bimolecular reactions involving interfacial ion exchange between bulk aqueous media and micellar pseudophase.