Theoretical study of the gas phase decomposition of glycolic, lactic, and 2-hydroxyisobutyric acids

The reaction mechanism associated with the decomposition of three alpha-hydroxycarboxylic acids (glycolic, lactic, and 2-hydroxyisobutyric) in the gas phase to form carbon monoxide, water, and the corresponding carbonyl compounds has been theoretically characterized by using ab initio analytical gradients at the MP2 level of theory with the 6-31G** and 6-31++G** basis sets. A detailed characterization of the potential energy surface points out the existence of three competitive reaction pathways for the decomposition process. The first pathway describes a two-step mechanism, with water elimination and formation of an alpha-lactone intermediate, achieved by the nucleophilic attack of the carbonylic oxygen atom of the carboxyl group (mechanism A). The second pathway is also a two-step mechanism, but in this case the formation of the alpha-lactone is obtained by means of the nucleophilic attack of the hydroxylic oxygen atom of the carboxyl group (mechanism B). These two pathways share a common second step in which the alpha-lactone decomposes. The third pathway is a one-step process in which the decomposition of the corresponding alpha-hydroxy acid takes place in a concerted fashion (mechanism C). The geometrical parameters of the stationary points appearing along the three pathways and the components of the transition vectors associated-to the transition structures calculated using the 6-31G** basis set are similar to those calculated with the larger 6-31G++G** basis set. The decomposition is favorable along pathway A, and the first step can be considered as the rate-limiting step for the global process. The rate constant values for this step increase in the order of glycolic, lactic, and 2-hydroxyisobutyric acids due to the stabilization of the incipient carbocationic center on C-3 With the substitution of hydrogen atoms by methyl groups. The apparent first-order rate Constants calculated by transition state theory agree well with the experiments reported by Chuchani and co-workers.