Comparisons and analyses of theoretical treatments of micellar effects upon ion-molecule reactions. Relevance to amide exchange

Reported second-order rate constants of H+ and OH- catalyzed amide exchanges in micellar solutions, relative to those in water, have been claimed to be inexplicable in terms of pseudophase treatments of micellar rate effects, and the Bronsted-Bjerrum treatment was invoked to fit the data (Perrin, C. L.; et al. J. Am. Chem. Sec. 1999, 121, 2448). Rate constants of amide exchange, based on pH measurements in surfactant solutions, are only approximate, but examination of the data and comparisons with evidence on other reactions show that they are qualitatively consistent with pseudophase treatments. Micellar effects upon rate constants of deacylations by OH- and acid hydrolyses of dioxolanes, analyzed with pseudophase treatments, were considered for the purpose of comparison. The pseudophase and Bronsted-Bjerrum formalisms are equivalent in rationalizing micellar rate data, although the former appears to be descriptively more useful as applied to reactions of apolar organic compounds. Interrelations between the two approaches are analyzed in terms of transfer free energies of reactants and transition states. Potentiometric titrations of HCl with NaOH were made in solutions of sodium dodecyl sulfate and cetyl trimethylammonium chloride in order to estimate the autoprotolysis constant of water and to establish the relation of pH to concentrations of H+ and OH- in unbuffered micellar solutions.