Charmian J. O'Connor

Catalysis by reversed micelles in non-polar solvents: aquation and electron-transfer reactions of chromium(III) and cobalt(III) complexes in benzene

Rate constants for aquation of the ion [Cr(C2O4)3]3– by octylammonium tetradecanoate-solubilized water in benzene and for aquations of [Cr(C2O4)3]3–, [CO(C2O4)3]3–, and cis-[Co(en)2(N3)2]+(en = ethylenediamine) by dodecylammonium propionate-solubilized water in benzene are factors of up to 5 × 106, 1 × 106, ca. 1 500, and 11, respectively, greater than those for the same reactions in water. At constant surfactant concentration there is a linear dependence of the aquation rate on the concentration of solubilized water, while at constant water concentration increasing surfactant concentration results in an exponential rate decrease. These results are discussed in terms of favourable substrate orientation in polar cavities of alkylammonium carboxylate aggregates, formed from these surfactants in benzene, where hydrogen bonding facilitates proton transfer and enhanced water activity accelerates synchronous M–O bond-breaking and nucleophilic attack by water. An implication of these results is that, in the hydrophilic environment of the reversed micelle, aquations of CoIII and CrIII complexes follow rate-determining bimolecular mechanisms subsequent to formation of ‘one-ended dissociated’ species. No micellar effects have been observed for the electron-transfer process in the aquation of the ion [Co(C2O4)2(H2O)2]–. Rate enhancement by surfactant-solubilized water in non-aqueous solutions and the utility of these systems in elucidating the mechanistic roles of water are discussed.

The acid catalysed hydrolysis of N-t-butylbenzaldoxime and 2-t-butyl-3-phenyloxaziridine

The rates of hydrolysis of N-t-butylbenzaldoxime and 2-t-butyl-3-phenyloxaziridine have been measured over a wide range of acidities in HCl, H2SO4, and HClO4. Interpretation of the rate data by use of the Bunnett criteria of mechanism, deuterium isotope effects, and Arrhenius parameters are consistent with a bimolecular specific acid catalysed mechanism for hydrolysis of both substrates reacting through the protonated nitrone as a common intermediate.

Reactivity of co-ordinated amino-acids. Part 1. Oxygen-18 exchange studies on the trans(fac)-bis(N-methyliminodiacetato)chromate(III) anion and N-methyliminodiacetic acid

The rate of oxygen exchange of the trans(fac)-bis(N-methyliminodiacetato)chromate(III) anion and of N-methyliminodiacetic acid with solvent water varies with hydrogen-ion concentration. At 100 °C the latter reaction is 39 times faster than the former. A mechanism for 18O exchange of the co-ordinated ligand through acid-independent and acid-dependent paths is postulated. Both paths appear to involve one-ended dissociated species.

Micellar effects on the hydrolysis of benzylidene-t-butylamine n-oxide and 3-phenyl-2-t-butyloxaziridine

Micellar sodium dodecyl sulphate (NaLS) enhances the rate constants for the hydrolysis of benzylidene-t-butyl-amine N-oxide and -phenyl-2-t-butyloxaziridine by factors of 40 and 68, respectively. Hexadecyltrimethyl-ammonium bromide and 3-(dodecylmethylammonio)propane-1-sulphonate have no effects on the rates while polyoxyethylene(15)nonylphenol (lgepal CO-730) and 3-(dodecyldimethylammonio)propane-1-sulphonate enhance them slightly. Catalysis by NaLS of these two compounds is quite similar and originates from binding to the negatively charged micelle where the common intermediate, the positively charged protonated nitrone, and the transition state for the rate-limiting steps are stabilized.