Changes in Mechanism and Transition State Structure for Solvolysis Reactions of Ring Substituted Benzyl Chlorides in Aqueous Solution

Rate and product data are reported for the solvolysis reactions of twenty-seven mono, di (3,4) and tri (3,4,5) ring-substituted benzyl chlorides. The first order rate constant for solvolysis in 20% acetonitrile in water decrease from k _solv = 2.2 s^-1 for 4-methoxybenzyl chloride to 1.1 x 10^-8 s^-1 for 3,4-dinitrobenzyl chloride. The product rate constant ratios k _MeOH/k _TFE for solvolysis in 70/27/3 (v/v/v) HOH/TFE/MeOH range from a minimum of k _MeOH/k _TFE = 8 to a maximum of 110. The rate data were fit to a four-parameter Hammett equation that separates the resonance $\left({\rho }_{\text{r}}{\sigma }_r\right)$ and polar $\left({\rho }_{\text{n}}{\sigma }_n\right)$ effects of the aromatic ring substituents on the reaction rate. Increases in the values of the Hammett reaction constants ${\rho }_{\text{r}}$ and ${\rho }_n$ are observed as the substituent constants ${\sigma }_r$ or ${\sigma }_n$ are increased. A sharp decrease in the product selectivity k _MeOH/k _TFE = 26 for stepwise solvolysis of 4-methoxybenzyl chloride is observed as electron-withdrawing meta-substituents are added to 4-methoxybenzyl ring due to a Hammond-effect on the position of the transition state for solvent addition to the substituted 4-methoxybenzyl carbocation reaction intermediates. Sharp increases in the selectivity k _MeOH/k _TFE are observed with decreasing reactivity of other 3,4,5-subsituted benzyl chlorides due to anti-Hammond shifts on a two-dimensional More-O'Ferrall reaction coordinate diagram in the position of the transition state for a concerted solvolysis reaction.