| Abstract
| - The main question addressed in this paper is whether the nucleophilic substitution of the p-nitrophenoxygroup in (CO)5CrC(OC6H4-4-NO2)Ph (1-NO2) by a series of substituted phenoxide ions is concertedor stepwise. Rate constants, kArO, for these substitution reactions were determined in 50% MeCN−50%water (v/v) at 25 °C. A Brønsted plot of log kArO versus pis consistent with a stepwise mechanism.This contrasts with reactions of aryl oxide ions with p-nitrophenyl acetate and with similar acyl transferswhich are concerted. The reason for the contrast is that the tetrahedral intermediates formed in the reactionsof 1-NO2 are much more stable than those in acyl transfers and the intrinsic barriers to their decompositionare higher than for the ester reactions. The points on the Brønsted plots for which p≥ pdefine a straight line with βnuc = −0.39, suggesting that bond formation has made very little progress atthe transition state and that partial desolvation of the nucleophile is part of the activation process. Thehydrolysis of 1-NO2 and of the unsubstituted analogue (1-H) has also been studied over a wide pHrange, providing rate constants for nucleophilic attack by hydroxide ion (kOH), by water (kH2O), and bygeneral base-catalyzed reaction with water (kB). Furthermore, kH2O values were obtained for the hydrolysisof (CO)5CrC(OC6H4X)Ph (1-X) as a byproduct of the reactions of 1-NO2 with aryl oxide ions. Structure−reactivity relationships for these reactions are discussed in terms of inductive, π-donor, and steric effects.
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