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]dimethylammonium bromide (DHDBr) with p -nitrophenyl diphenyl phosphate in comicelles with inert surfactants, at pH such that the oximate zwitterion (DHD) is formed quantitatively, depend on the mole fraction of DHD to total surfactant. Second-order rate constants of reaction in the micellar pseudophase decrease in the following sequence of inert surfactant headgroups: phosphine oxide ≈ sulfoxide ≈ pyrrolidinone > Me 3 N + > sulfobetaine > Bu 3 N + > polyoxyethylene > OSO 3 - . 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 micellar charge per se. The simple relation between rate constants and nucleophile concentration given by the mole fraction of DHDBr applies reasonably well to mixtures of C 16 H 33 NMe 3 Br and the other inert surfactants. Reaction is relatively slow in comicelles of DHD and C 12 H 25 OSO 3 Na, probably due to different locations of reactants in the interfacial region. Samples of DHDBr prepared and purified by the literature method are contaminated by pyridine hydrochloride, which is removed by washing with aqueous NaBr.