Micellar charge effects upon hydrolyses of substituted benzoyl chlorides. Their relation to mechanism

Micellar rate effects on hydrolyses of substituted benzoyl chlorides, 1, depend on headgroup charge and electron donation or withdrawal by substituents. Micellized sodium dodecyl sulfate, SDS, inhibits hydrolyses, and first-order rate constants in the micellar pseudophase, k'(M), decrease, relative to those in water, k'(W), over a range of ca. 10, but in cetyl trimethylammonium chloride, CTACl, k'(M)/k'(W) > 1 for hydrolyses of 1,3,5-(NO2)(2) and 1, 4-NO2, and decreases steeply with electron-donating substituents in the following sequence: 1,4-Cl approximate to 4-Br > 4-H > 4-Me > 4-OMe, over a range of > 10(3). Cetyl trimethylammonium bromide and mesylate behave like CTACl. Fits to the Hammett equation give rho approximate to 1 in SDS and rho approximate to 4 in CTACl. Anionic micelles have higher interfacial polarities than cationic micelles, but micellar and solvent effects do not correspond because over a range of solvents, H2O to H2O-MeCN, 1:1 w/w plots of log k'(W) against sigma go through minima with positive rho for 1, 3,5-(NO2)(2), and 4-NO2 and negative for the other substrates. The micellar effects correspond to differing extents of bond-making and -breaking in the transition state. Values of k(+)/k(-) (rate constants in CTACl and SDS) decrease strongly with increasing electron donation by substituents. Micellar rate effects in hydrolyses of benzyl bromide and 4-methoxybenzyl chloride are similar to those with the benzoyl chlorides. Although data were analyzed by a pseudophase treatment, application of transition state theory and reported micellar surface potentials allows estimation of local charge at the reaction center for hydrolyses of the benzoyl chlorides.