Theoretical study of a reaction path via a hydrogen-bonded intermediate for the alkaline hydrolysis of esters in the gas phase

Abstract

Two steps are considered for the alkaline hydrolysis of methyl acetate in the gas phase. The first step is the formation of a tetrahedral intermediate and the second is its decomposition. In the present paper, the mechanism for the second step is discussed by using ab initio molecular orbital calculations. The path releasing acetic acid and methoxide anion is an energetically unfavourable path, as easily expected by their pK_a values. The final products, acetate ion and methanol, are more stable than the tetrahedral intermediate. We found a hydrogen-bonded acetate–methanol molecule as the intermediate that was more stable than both the tetrahedral intermediates and the final products. We searched and found the transition state which connected the tetrahedral intermediate with the hydrogen-bonded intermediate. The activation energy of the path through the transition state was calculated to be 8.7 kcal mol^–1(MP2/6–31 + G**//RHF/6–31G: 5.8). This mechanism explains well the alkaline hydrolysis of esters in the gas phase.