Effects of headgroup structure on dephosphorylation of p-nitrophenyl diphenyl phosphate by functional oximate comicelles

Rate constants of reaction of n-dodecyl [2-(hydroximino)-2-phenylethyl]dime thylammonium bromide (DHDBr) with p-nitrophenyl diphenyl phosphate in comic elles with inert surfactants, at pH such that the oximate zwitterion (DHD) is formed quantitatively, depend on the mole fraction of DHD to total surfa ctant. Second-order rate constants of reaction in the micellar pseudophase decrease in the following sequence of inert surfactant headgroups: phosphin e oxide approximate to sulfoxide approximate to pyrrolidinone > Me3N+ > sul fobetaine > Bu3N+ > polyoxyethylene > OSO3-. Except for reaction in anionic comicelles these second-order rate constants vary within a factor of 5 and appear to depend on the local structure of the interface rather than on mi cellar charge per se. The simple relation between rate constants and nucleo phile concentration given by the mole fraction of DHDBr applies reasonably well to mixtures of C16H33NMe3Br and the other inert surfactants. Reaction is relatively slow in comicelles of DHD and C12H25OSO3Na, probably due to d ifferent locations of reactants in the interfacial region. Samples of DHDBr prepared and purified by the literature method are contaminated by pyridin e hydrochloride, which is removed by washing with aqueous NaBr.