Hydrolysis of 2,4-dinitrophenyl phosphate in normal and reverse micelles

Abstract

The rate of spontaneous hydrolysis of the 2,4-dinitrophenyl phosphate dianion (DNPP^2–) is strongly increased by aqueous cationic micelles of cetyltrialkylammonium bromides (alkyl = Me, Pr, Bu) and tetradecylquinuclidinium bromide. First order rate constants reach limiting values in dilute surfactant where DNPP^2– is fully micellar bound. Rate enhancements increase with increasing bulk of the head group and are associated with decreases in activation enthalpies. Betaine sulfonate micelles (C₁₄H₂₉N⁺R₂[CH₂]₃SO₃^–, R = Me, Et, Pr, Bu) also speed hydrolysis, but first order rate constants continue to increase even up to 0.2 mol dm^–3 surfactant, because DNPP^2– is not strongly bound. Reverse micelles of cetyltrialkylammonium bromide (alkyl = Me, Bu) in dichloromethane give rapid hydrolysis, but rate constants decrease on addition of water approximately to the value observed with aqueous cationic micelles as ‘water pool’ reverse micelles of CTABr are formed. N,N-Dimethylhydroxylamine increases the rate of hydrolysis in water, but not in reverse micelles, and a (hypothetical) phosphoramidate is not trapped by F^–.