Computational study of the thermal decomposition of 2-methylbutyraldehyde and 2-pentanone through retro-ene reactions

In this work, a theoretical study at the MP2/6-31G(d) level of the thermal decomposition retro-ene reaction of 2-methylbutyraldehyde was carried out at a pressure of 1.5 atm. and temperatures ranging from 1110 to 1190 K. The progress of the reaction has been followed by means of the Wiberg bond indices which in turn allowed the calculation of the reaction synchronicity. Transition state theory was used to calculate the theoretical rate constant at 1150 K which was compared with the previously reported experimental value at the same conditions. We found that both values show a close agreement. The obtained computational evidence allowed us to support a reaction mechanism which proceeds in two steps: the first one with the formation of ethylene and 1-propenol via a six-membered cyclic transition state and the second one involving keto-enol equilibrium of 1-propenol to propionaldehyde via a four-membered cyclic transition state. It was found that the reaction is a highly synchronous and concerted process. The results obtained for the thermolysis of 2-methylbutyraldehyde were compared with those obtained for the thermolysis of 2-pentanone. A comparison of our results with those reported for their corresponding β-hydroxy counterparts, 3-hydroxy-2-methylpropionaldehyde and 4-hydroxy-2-butanone has also been made. A study of the thermochemistry of the compounds involved in the reactions studied has been carried out at the G3 level.