A Novel Dissociative Mechanism in Acyl Group Transfer from Aryl 4-Hydroxybenzoate in Aqueous Solvents.

Article abstract of DOI:10.1021/jo00204a011

The hydrolysis of ary 4-hydroxybenzoates esters exibits the kinetic rate law k_obsd=(k_a+k_b->/(1++>/K_a) where k_b is the second-order rate constant for hydroxide ion attack on the ionized ester and K_a is the ionization constant.The apparent second-order rate constant for the hydrolysis of the 2,4-dinitrophenyl ester (k_aK_a/K_w) is some 340-fold larger than that determined from the Hammett correlation for the alkaline hydrolysis of substituted 2,4-dinitrophenyl benzoates known to possess a B_Ac2 mechanism.A slightly positive entropy of activation for k_a and aniline trapping experiments for the 2,4-dinitrophenyl ester are consistent with a mechanism where a p-oxo ketene intermediate takes the reaction flux. Correlation of k_aK_a/K_w with the pK of the leaving phenol fits the equation k_aK_a/K_w-10^{(-1.33pK+9.57)}+10^{(-0.35pK+3.51)} which favors the B_Ac2 mechanism for poor leaving groups and the E1cB pathway for good leaving groups.There is no reason to postulate a borderline concerted displacement process in this case.