ASSOCIATIVE AND DISSOCIATIVE PATHWAYS IN THE ALKALINE-HYDROLYSIS OF ARYL 2-HYDROXYCINNAMATES

Aryl 2-hydroxycinnamate esters hydrolyze in alkaline solutions (20% dioxane-water v/v) obeying the rate law k(obs) = k(2) + k(b)[OH-]/(1 + a(H)/K-a), where K-a is the ionization constant of the hydroxy group of the ester and kb is the second-order rate constant for the attack of hydroxide ion on the ionized ester. Kinetic data and activation parameters for the hydrolysis of the 2,4-dinitrophenyl ester show that the mechanism giving rise to the k(a) term cannot be a simple B(Ac)2 type process and suggest the occurrence of a E1cB mechanism involving an ''extended'' o-oxoketene intermediate. The Bronsted plot of the apparent second-order rate constants (k(a)K(a)/k(w)) versus the pK of the leaving group indicates that the reaction mechanism changes from E1cB to B(Ac)2 for esters with leaving groups having pK higher than about 6.