Nucleophilic displacement at the benzoyl centre: a study of the change in geometry at the carbonyl carbon atom

The second-order rate constants for the reaction between hydroxide ion and phenoxide ion with 4-nitrophenyl esters of substituted benzoic acids in 10% acetonitrile–water (v/v) solution obey Hammett σ correlations. The values of the Hammett ρ of 1.67 (kArO) and 2.14 (kOH) are consistent with a large change in hybridization at the central carbon by comparison with the ρ value for a standard reaction where a full sp2 to sp3 change occurs. The transition state for the concerted reaction thus has a substantially tetrahedral geometry. The observation of the anti-Hammond effect whereby the ρ value for the hydroxide ion exceeds that of the less reactive phenoxide ion is consistent with a concerted, ANDN, mechanism for these reactions. A stepwise mechanism, AN + DN, is unlikely to yield a measurable break in the Hammett correlation for a change in the benzoyl substituent if the partitioning of the putative tetrahedral intermediate involves forward and reverse reactions with Hammett correlations possessing similar ρ values.