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

Micellar rate effects on hydrolyses of substituted benzoyl chlorides, 1, de pend on headgroup charge and electron donation or withdrawal by substituent s. Micellized sodium dodecyl sulfate, SDS, inhibits hydrolyses, and first-o rder 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 trimethylamm onium 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 lik e 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 pol arities than cationic micelles, but micellar and solvent effects do not cor respond 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-(NO 2)(2), and 4-NO2 and negative for the other substrates. The micellar effect s correspond to differing extents of bond-making and -breaking in the trans ition 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 si milar 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 react ion center for hydrolyses of the benzoyl chlorides.