Theoretical study of decomposition mechanism of Pyruvic Acid

Abstract

In this work, the decomposition reaction of pyruvic acid has been studied, some mechanisms are based on the proposal that Gabriel da Silva presents in its theoretical study of this decomposition process. The study has been made from two viewpoints. The first one is an energetic and geometric analysis of the stationary points involved in the different pathways. The second one consists in a topological study of some mechanisms. The mechanisms proposed by Gabriel da Silva have been reproduced and similar geometric and energetic results have been found. The mechanism-A is the most favorable kinetically because the reactants have to pass through less energetic energy barriers to reach the products, in this case acetic acid and carbon monoxide. On the other hand, the mechanism-D is the most favorable thermodynamically because obtaining acetaldehyde has the most negative reaction free energy (-12.48 Kcal/mol). However, mechanism D has a high energy barrier (73.2 Kcal/mol). Furthermore, two alternatives to mechanism A (mechanisms A’ and A’’) have been characterized. All three mechanisms give acetic acid and carbon monoxide as products. These new alternatives have been calculated at the same theoretical level as the previous proposed mechanism and the energetic and geometric study has been extended with a topological analysis. Gibbs free energies of the mechanism A are similar to the mechanism A’, there is only a small difference of 2.21 kcal/mol in the first transition state, nevertheless, the energies of the other stationary points are the same in both mechanisms. These are stepwise mechanisms where the first transition state is associated to a hydrogen transfer between the alcohol and carbonyl groups and to a C-O bond formation that closes the epoxide ring. The difference between both mechanisms is the oxygen that forms the epoxide group, so both intermediates are enantiomers. In the second TS, the C-C and the other C-O bond of the epoxide ring are broken giving acetic acid and carbon monoxide. The geometric analysis reveals than in the first TS the hydrogen transfer in much more advanced than the C-O bond formation. In the same way, in the second TS the C-O bond distance is longer than the C-C one. 44 The description that emerges from the topological