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1 2008

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Quantum Chemical Study of Hydroxylation of Alkanes by Hypofluorous Acid

Oncak M., Srnec M., Zahradnik R.

Polish Journal of Chemistry

2008

Vol. 82, nr 4

649--659

The experimentally observed hydroxylation of alkanes by hypofluorous acid (one of Rozen's oxidation reactions) was investigated using the methods of quantum chemistry. It was shown that the high efficiency of the reaction may be explained by self-catalysis. The oxidizing HOF molecule transfers the oxygen atom to a substrate, which is accompanied by the HF formation, while the second hypofluorous acid molecule stabilizes the oxidizing HOF molecule by a hydrogen bond. The hydroxylation barriers were found to decrease with increased coordination of the oxidized carbon atom by methyl groups, in agreement with the experiment. In the gas phase, the calculated DFT/B3LYP reaction barriers amount to 22.5, 14.5, 9.0, and 6.4 kcal/mol for oxidation of methane, ethane, central carbon atoms of propane, and 2-methylpropane, respectively; for a terminal C-H propane bond, a barrier was enumerated to 13.9 kcal/mol. It was also found that the reaction can be catalyzed by the product molecule, hydrogen fluoride (as first suggested for ethylene in Sertchook R., Boese A.D., Martin J.M.L., J. Phys. Chem. A, 110, 8275 (2006)), and common features of the H-bond assisted catalysis were investigated. The analogous but very much less favorable hydroxylation by hypochlorous acid molecule was also briefly discussed.