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 sigma correlations, The values of the Hammett rho of 1.67 (k(ArO)) and 2.14 (k(OH)) are consistent with a large change in hybridization at the central carbon by comparison with the rho value for a standard reaction where a full sp(2) to sp(3) change occurs, The transition state for the concerted reaction thus has a substantially tetrahedral geometry, The observation of the anti-Hammond effect whereby the rho value for the hydroxide ion exceeds that of the less reactive phenoxide ion is consistent with a concerted, A(N)D(N), mechanism for these reactions, A stepwise mechanism, A(N) + D-N, 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 rho values.