AN OPEN TRANSITION-STATE IN CARBONYL ACYL GROUP TRANSFER IN AQUEOUS-SOLUTION

The second-order rate constants have been measured for the reaction of substituted phenolate ions with 2,4-dinitrophenyl acetate, 2,4-dinitrophenyl 4-methoxy-2,6-dimethylbenzoate and acetic anhydride in aqueous solution at 25-degrees-C. The data are over a wide range of phenolate ion basicity and obey good Bronsted equations which have beta-nuc values of, respectively, 0.57 +/- 0.03, 0.15 +/- 0.07 and 0.59 +/- 0.05. The principal conclusion of this work is that the identity reaction of 2,4-dinitrophenolate ion with 2,4-dinitrophenyl 4-methoxy-2,6-dimethylbenzoate has an open transition state, namely one with very weak bonds to entering and departing ligands. The transition state possesses a Kreevoy tightness parameter (tau) of 0.18. The open transition state arises from the stabilising effect of the acyl group substituents on the benzoylium ion and their destabilising effect on the putative tetrahedral intermediate as well as the weak basicities of the nucleophile and nucleofuge. This is the first example of an open transition state in an acyl group transfer which does not require the assistance of a negatively charged internal nucleophile. The data for 2,4-dinitrophenyl acetate may be employed to calculate an identity rate constant (k(ii)) for the reaction of 2,4-dinitrophenolate ion with the ester. This data may be fitted to a theoretical Lewis-Kreevoy plot (log k(ii) vs. pK(i)) possessing both positive and negative values of beta-ii (slope of the line). Microscopic medium effects place a limit to the accuracy of predictions of rate constants, including k(ii), from linear free energy relationships.