Changes in mechanism and transition state structure for solvolysis reactions of ring substituted benzyl chlorides in aqueous solution

Abstract

Rate and product data are reported for the solvolysis reactions of 27 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 × 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 ( ${\rho }_r{\sigma }_r$) and polar ( ${\rho }_n{\sigma }_n$) effects of the aromatic ring substituents on the reaction rate. Increases in the values of the Hammett reaction constants ${\rho }_r$ 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.