(600bv) New Pathways for Formation of Acids and Carbonyl Compounds in Low Temperature Oxidation

Low temperature oxidation of hydrocarbons is important in understanding kinetic phenomena related to fuel ignition, liquid phase autoxidation, lipid peroxidation and atmospheric chemistry. The kinetics of these systems is dominated by peroxy radicals and typically involves the following sequence of reactions ROO.↔.QOOH↔.OOQOOH↔HOOQ=O + .OH↔.OQ=O + 2.OH¹. Ketohydroperoxides (HOOQ=O) have long been postulated as important intermediates in low temperature oxidation and recent gas phase experiments² have confirmed their existence. In addition, these experiments have also reported the formation of oxygenated products like carboxylic acids, carbonyl compounds and diketones which cannot be explained by existing pathways. Over thirty years ago, a similar problem was reported in autoxidation experiments on hexadecane performed by Korcek and co-workers.³Through a complex analytical scheme they established that commonly assumed pathways were insufficient to explain the large yields of methyl ketones and acids which were major cleavage products. Their thermal decomposition experiments on HOOQ=O species led directly to acids/methyl ketones suggesting that these were precursors to their formation.

In this work, we discuss experimental and theoretical evidence for pathways leading from HOOQ=O species to acids and carbonyl compounds using 2-formyl ethylhydroperoxide (HOOCH2CH2CHO) as a model compound. The first step in the reaction sequence involves the isomerization of the ketohydroperoxide to the more stable 1,3-hydroxy dioxolane (5-membered cyclic peroxide) which can occur via unimolecular intramolecular H-migration or through double hydrogen shift reactions catalyzed by carboxylic acids. The 1,3-hydroxy dioxolane then undergoes concerted dissociation/rearrangement reactions to yield primarily formic acid/acetaldehyde or acetic acid/formaldehyde products in H-bonded complexes. Thermochemical and kinetic parameters for this reaction sequence have been obtained using high-level calculations for electronic energies and partition functions. Kinetic modeling of the Korcek experiment with the new reactions for HOOQ=O thermal decomposition provides a qualitative explanation for their observations establishing a new pathway for acid/methyl ketone formation in liquid phase autoxidation. A more general theoretical investigation of the newly discovered reaction classes is in progress and may help resolve issues in low temperature gas phase oxidation relevant to combustion and atmospheric chemists.

References

(1)        Zádor, J.; Taatjes, C. A.; Fernandes, R. X. Progress in Energy and Combustion Science 2011, 37, 371.

(2)        Herbinet, O.; Battin-Leclerc, F.; Bax, S.; Gall, H. L.; Glaude, P.-A.; Fournet, R.; Zhou, Z.; Deng, L.; Guo, H.; Xie, M.; Qi, F. Physical Chemistry Chemical Physics 2011, 13.

(3)        Jensen, R. K.; Korcek, S.; Mahoney, L. R.; Zinbo, M. Journal of the American Chemical Society 1981, 103, 1742.