Deprotonation of the alpha-(N,N-dimethylcarbamoyl)-alpha-methyl-4methoxybenzyl carbocation by alkanecarboxylate and halide ions

The product rate constant ratios k(elim)/k(az) [N-3(-)] for partitioning of the alpha-(N,N-dimethylcarbamoyl)-alpha -methyl-4-methoxybenzyl carbocatio n (4(+)) in aqueous methanol between deprotonation of the alpha -methyl car bon to form an alkene and nucleophilic addition of azide ion increase for r eactions in the presence of carboxylate and halide ions. An analysis of the se data gave second-order rate constants k(B) for deprotonation of 4(+) by carboxylate ions which are correlated by a Bronsted coefficient of beta = 0 .17. The second-order rate constant for deprotonation of 4(+) by solvent wa ter shows a 40-fold negative deviation from this correlation. Values of k(e lim)/k(az) [N-3(-)] in 20% methanol in water increase by less than or equal to 30% as the halide ion concentration is increased from 0.0-0.80 mol dm(- 3) (I = 0.80, NaClO4) due either to direct deprotonation of 4(+) by the hal ide ion or to a specific salt effect on the partitioning of 4(+). The value for k(B) calculated with the assumption that this increase is due to depro tonation of 4(+) by chloride ion shows a small positive deviation from the Bronsted correlation of rate data for the reaction of carboxylate ions. How ever, all of these data show a satisfactory fit to a single Bronsted correl ation with slope beta = 0.12. These results are consistent with the conclus ion that chloride ion shows a normal reactivity toward deprotonation of thi s alpha -methyl carbocation for a base of pK(a) approximate to -8 compared to substituted carboxylate anions, and an enhanced reactivity compared to s olvent water.