KINETICS AND EQUILIBRIA OF REACTIONS BETWEEN ACETIC-ANHYDRIDE AND SUBSTITUTED PHENOLATE IONS IN AQUEOUS AND CHLOROBENZENE SOLUTIONS

Potassium acetate, solubilised in chlorobenzene-by 18-crown-6, displac es the phenolate ion from substituted phenyl acetates by a second-orde r (k(-2)(Cl)) process. Potassium phenolate ions, under similar conditi ons, react with acetic anhydride via a second-order (k(2)(Cl)) to yiel d the phenyl acetate. The concentration of the crown does not affect t he reactivity unless it is not sufficient to solubilise the reactants. The rate constants correlate with the ionisation of the substituted p henols 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 phe nolate 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 substitut ed phenolate ions with acetic an hydride in water and they obey the Br onsted 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 equ ilibrium constant in chlorobenzene (beta = 2.6) compared with that for aqueous solution (beta = 1.7) indicates a greater development of effe ctive charge consistent with the weaker solvating power of chlorobenze ne. The reaction of substituted phenoxide ion with acetic anhydride ha s 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 thoug h the reactions in chlorobenzene are faster than in water.