Kinetics and equilibria of reactions between acetic anhydride and substituted phenolate ions in aqueous and chlorobenzene solutions

Abstract

Potassium acetate, solubilised in chlorobenzene by 18-crown-6, displaces the phenolate ion from substituted phenyl acetates by a second-order (k_–2^Cl) process. Potassium phenolate ions, under similar conditions, react with acetic anhydride via a second-order (k₂^Cl)to yield the phenyl acetate. The concentration of the crown does not affect the reactivity unless it is not sufficient to solubilise the reactants.

The rate constants correlate with the ionisation of the substituted phenols in water: log k₂^Cl= 1.60 ± 0.23 pK_a^ArOH(aq)– 9.06 ± 1.4, log k_–2^Cl=–0.97 ± 0.12 pK_a^ArOH(aq)+ 4.78 ± 0.78 The equilibrium constant for transfer of the acetyl group between phenolate ions and acetic anhydride in chlorobenzene has a Brønsted β_eq^cl of 2.6 measured against pK_a^ArOH(aq).

The second-order rate constants (k₂^aq) have been measured for the reaction of substituted phenolate ions with acetic anhydride in water and they obey the Brønsted equation: log(k₂^aq)= 0.56 ± 0.06 pK_a^ArOH(aq)– 2.52 ± 0.51 Comparison of the value of the Brønsted exponent for the equilibrium constant in chlorobenzene (β= 2.6) compared with that for aqueous solution (β= 1.7) indicates a greater development of effective charge consistent with the weaker solvating power of chlorobenzene.

The reaction of substituted phenoxide ion with acetic, anhydride has a Leffler α value of 0.33 and 0.62 for aqueous and chlorobenzene solutions, respectively, indicating a more advanced bond formation in the transition state of the reaction in the latter solvent even though the reactions in chlorobenzene are faster than in water.