QUANTUM CHEMICAL STUDY OF OXIDATION REACTIONS IN UNSATURATED HYDROCARBONS

In this article quantum-chemical calculations using DFT B3LYP/cc-pvdz method was used for the investigation of changes in thermodynamic functions of reactions of oxidation of unsaturated hydrocarbons such as heptane and heptadiene as low-molecular models of hydrocarbon residues of lipids. The effect of the position of the reaction center relative to the double bonds and conformations of double bonds on reactivity of the compounds in reactions of hydrogen abstraction by the hydroxyl radical, dioxygen accession and chain propagation were analyzed. By comparison of changes in thermodynamic functions of reactions it was shown that hydrocarbons with cis- conformations of double bonds are characterized with higher reactivity in reactions of hydrogen abstraction but peroxi-radicals of these conformers are more stable. The changes in thermodynamic functions of reaction of hydrogen abstraction for diene according to the calculation are smaller comparing with olefins. This is due to the difference in the stability of the radicals formed. The stability of hydrocarbon radicals of dienes in comparison with olefins is explained by their planar structure with electron density of unpaired electron delocalized between five carbon atoms. The emergence of such pentadienil-type radicals is the cause of a higher oxidation of dienes compared with olefins. The analysis of molecular structures of peroxi-radicals of dienes shows that after accepting dioxygen by hydrocarbon radical the isomerization takes place. According calculations it is preferable for the dioxygen molecule not to join with the central carbon atom from which the hydrogen atom has been abstracted but to attack the double bond joining with C2 carbon atom. During the isomerization the double bond moves to the center of the molecule forming thus the conjugated pair with the other double bond. Comparison of thermodynamic functions of reaction for cis- and trans- isomers shows that cis-trans isomerization is possible during the dioxygen accession to the hydrocarbon radical. These results are in good agreement with the experimental data published earlier.Forcitation:Novikova A.A., Soloviev M.E. Quantum chemical study of oxidation reactions in unsaturated hydrocarbons. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 2017. V. 60. N 7. P. 14-20.