Theoretical studies of fundamental pathways for alkaline hydrolysis of carboxylic acid esters in gas phase

Fundamental reaction pathways for the alkaline hydrolysis of carboxylic aci d esters, RCOOR', were examined through a series of first-principle calcula tions. The reactions of six representative esters with hydroxide ion were s tudied in the gas phase. A total of three competing reaction pathways were found and theoretically confirmed for each of the esters examined: bimolecu lar base-catalyzed acyl-oxygen cleavage (B(AC)2), bimolecular base-catalyze d alkyl-oxygen cleavage (B(AL)2), and carbonyl oxygen exchange with hydroxi de. For the two-step B(AC)2 process, this is the first theoretical study to consider the individual sub-steps of the reaction process and to consider substituent effects. For the carbonyl oxygen exchange with hydroxide and fo r the one-step BAL2 process, we report here the first quantitative theoreti cal results for the reaction pathways and for the energy barriers. The ener gy barrier calculated for the second step of the B(AC)2 process, that is, t he decomposition of the tetrahedral intermediate, is larger in the gas phas e than that of the first step, that is, the formation of the tetrahedral in termediate, for all but one of the esters examined. The exception, CH3COOC( CH3)(3), does not have an alpha hydrogen in the leaving group. The highest energy barrier calculated for the B(AC)2 process is always lower than the b arriers for the oxygen exchange and for the B(AL)2 process. The difference between the barrier for the B(AL)2 process and the highest barrier for the B(AC)2 process is only similar to 1-3 kcal/mol for the methyl esters, but b ecomes much larger for the others. Substitution of an alpha hydrogen in R' with a methyl group considerably increases the energy barrier for the B(AL) 2 process, and significantly decreases the energy barrier for the second st ep of the B(AC)2 process. The calculated substituent shifts of the energy b arrier for the first step of the B(AC)2 process in gas phase are in good ag reement with the observed substituent shifts for the base-catalyzed hydroly sis of alkyl acetates in aqueous solution. All of the calculated results ar e consistent with the available experimental results and lead to a deeper u nderstanding of previously reported gas-phase experimental observations.