Mechanistic and kinetic study on the ozonolysis of 2,4-hexadienedial

Abstract

The formation and unimolecular reactions of primary ozonides and carbonyl oxides arising from the O₃-initiated reactions of 2,4-hexadienedial (HDE) have been investigated using the density functional theory and ab initio method. The activation energies of O₃ cycloaddition to the >C=C< and >C=O bonds of HDE for the formation primary ozonides (POZ1 and POZ2) are 4.79 and 21.37 kcal mol⁻¹, respectively, implying that the initial O₃ to the >C=C< bond is favorable pathway. Cleavage of POZ1 to form carbonyl oxides occurs with a barrier of 12.19–21.35 kcal mol⁻¹, and the decomposition energies range from −1.09 to −15.75 kcal mol⁻¹. The CHOCHOO radical, the hydroxyl radical (OH) formation via H-migration is more favorable than the dioxirane formation via rearrangement. However, the CHOCH=CHCHOO radical, the dioxirane formation via rearrangement is more favorable than OH formation. Using the transition state theory, the rate constants of formation of POZ1 and POZ2 are 1.49 × 10⁻¹⁹ and 6.03 × 10⁻²⁵ cm³ molecule⁻¹ s⁻¹ at 300 K, respectively. This study shows that the hyperconjugative effect makes O₃ addition to >C=C< and >C=O bonds of HDE more difficult than to >C=C< bond of ethylene and isoprene and to >C=O bond of formaldehyde. The largest rate constants of OH formation and dioxirane formation in the unimolecular reactions of carbonyl oxides are 6.13 × 10⁻⁴ and 7.93 × 10⁻¹ s⁻¹ at 300 K, respectively. The dioxirane is main product in the unimolecular reaction of the carbonyl oxides arising from the O₃-initiated reaction of HDE.