Solvent effects on the solvolysis of 1-(4-methoxyphenyl)-1-phenyl-2,2,2-trifluoroethyl chloride. Influence of an electron-withdrawing alpha-substituent on carbonium ion center

Solvolysis rates of 1-(4-methoxyphenyl)-1-phenyl-2,2,2-trifluoroethyl chloride (1) and 1-(4-methoxyphenyl)-1-phenylethyl chloride (2) were measured in a variety of aqueous binary solvents, and the solvent effect was treated with the Grunwald-Winstein equation. The solvent effect on the solvolysis of 1 failed to give a single linear correlations using the ordinary Y or Y-Cl, but exhibited the wide split pattern which could not be related to the solvent nucleophilicity. The improved correlations with Y-BnCl and extended dual-parameter treatment, log (k/k(o)) = mY(Cl) + hI (m(Delta)Y(Delta)), were observed for the solvolysis of 1. These results suggest that the incipient cationic charge in the solvolysis of 1 is delocalized strongly into the aryl-rings in the transition state. While the solvent effect on the solvolysis of 2 is better correlated with Y or Y-Cl than Y-BnCl but the linearity is not satisfactory. The correlation is comparably improved by the use of the extended Grunwald-Winstein equation, log (k/k(o)) = 0.81Y(Cl) + 0.26N(OTs) (R=0.994, SD=+/-0.12), indicating the cationic charge of reaction center of 2 was localized mostly in the transition state.