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