The hydrolysis of substituted benzyl nitrates in water. II. Effect of ortho-substitution

Abstract

The relative effects of methyl substitution on the rates of hydrolysis of benzyl nitrates in water showed but small differences from earlier reports involving the displacement of Cl− in mixed solvents. The values of the α-deuterium isotope effects for the hydrolysis of the 2,4- and 2,6-dimethyl homologs were similar to that for the p-methylbenzyl nitrate suggesting an approach to limiting hybridization in each case. In spite of this similarity, there was a large negative shift in the value of ΔCP≠ for the hydrolysis of the 2,6-dimethylbenzyl nitrate compared to the values for the 2-methyl and 4-methyl members of the series. The value of ΔCP≠ for the 3-methyl was about the same as for the unsubstituted member. The mechanistic implications of these results while in agreement with the Swain–Thornton reaction rule emphasize the importance of external as well as internal factors in the determination of charge development and separation at the transition state. Steric hindrance or charge dispersal in the cation, or both, in limiting the rate of product formation, delay the attainment of the transition state and hence favor more negative values of ΔCP≠.