Kinetics and Equilibria of Reactions Between Acetic-Anthydride and Sbstituted Phenolate Ions in Aqueous and Chlorobenzene Solutions

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 (k(2)(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 k(2)(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 Bronsted beta(eq)(Cl) of 2.6 measured against pK(a)(ArOH(aq)). The second-order rate constants (k(2)(aq)) have been measured for the reaction of substituted phenolate ions with acetic an hydride in water and they obey the Bronsted equation: log (k(2)(aq)) = 0.56 +/- 0.06 pK(a)(ArOH(aq)) - 2.52 +/- 0.51 Comparison of the value of the Bronsted exponent for the equilibrium constant in chlorobenzene (beta = 2.6) compared with that for aqueous solution (beta = 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 alpha 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.