The effect of alpha-(N,N-dimethylcarbamoyl) and alpha-(N,N-dimethylthiocarbamoyl) substituents on the rate constants for partitioning of alpha-substituted 1-(4-methoxyphenyl)ethyl carbocations between nucleophilic addition of 50:50 (v:v) MeOH/H2O (k(s), s(-1)) and deprotonation by this solvent (k(e), s(-1)) have been investigated. The data show that these alpha-amide and alpha-thioamide substituents result in 80-fold and greater than or equal to 30 000-fold decreases, respectively, in k(s) for capture of the 4-methoxybenzyl carbocation by solvent, but that they lead to much smaller changes in k(e) for deprotonation of the corresponding alpha-substituted 1-(4-methoxyphenyl)ethyl carbocations by solvent. The large effect of the alpha-thioamide substituent on the partitioning of alpha-substituted 1-phenylethyl carbocations between formation of the products of solvolysis and elimination is therefore due primarily to the effect of this alpha-substituent on k(s) for capture of the carbocation by solvent, The results of experimental and computational studies are consistent with the conclusion that the relative magnitude of the rate constants k(s)and k(e) for partitioning of alpha-substituted 1-phenylethyl carbocations is strongly controlled by the relative thermodynamic stabilities of the neutral products of these reactions.
