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 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 ρ r σ r and polar ρ n σ n effects of the aromatic ring substituents on the reaction rate. Increases in the values of the Hammett reaction constants ρ r and ρ n are observed as the substituent constants σ r or σ 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.