Question

Carboxylic acid esters can undergo acid catalyzed hydrolysis, neutral hydrolysis and base catalyzed hydrolysis. Below is a table with three structurally related carboxylic acid esters and the second order reaction rate constants for the acid-catalyzed mechanism (k_A) and the rate constant for the base-catalyzed mechanism (k_B). The difference between the three compounds is mainly the substitution of hydrogen atoms on R_1 with the more electronegative chlorine atoms. a) Explain why there is only a small change in k_A for the three compounds. Why has the introduction of CI only a minor effect on k_A? b) Explain why there is a large change in k_B for the three compounds. Why does the introduction of CI increase k_B significantly? (For the discussion of this question it is reasonable to neglect the difference in R_2: ethyl versus methyl)

Answer 1

The substitution of Cl results in the inductive effect on the carbonyl carbon. The nucleophilic attack takes place in this carbon and which determines the kinetics of the reaction. In acid catalyzed reaction nucleophile is water. water is a weak nucleophile since it is neutral. A small increase in the partial charge on the carbonyl carbon will not affect the attack of neutral water to it. Whereas in base-catalyzed reaction, hydroxyl ion attacks the carbonyl carbon. Hydroxyl ion is a charged (strong) nucleophile and any charge difference in the carbonyl carbon will affect the binding kinetics of this nucleophile. Thus in presence of chloride ion, the carbonyl carbon is becoming more and more charged due to inductive effect and which results in the increase in kinetics in base catalyzed reactions.

Mechanism of these reactions are given in the attachment.