Hydroxylation of alkanes by hydrogen peroxide in superacid: A superelectrophilic active intermediate and an ionization–hydration process

Abstract

The density functional theory (DFT) at the B3LYP/6-311G(d,p) level was used to study the mechanism of hydroxylation of alkanes by hydrogen peroxide in superacid. Calculation showed that the hydroxylation of alkanes is an ionization–hydration process. The protonation of HOOH in superacid is the key to oxidation because it produces a superelectrophilic active intermediate (HOOH 2 +). The LUMO of HOOH 2 + has much lower energy (−9.18 eV) than that (0.21 eV) of neutral HOOH. The energy (−9.18 eV) is approximately the same as or even at a lower level than the HOMO energy (−10.8 to −8.0 eV) of alkanes, which facilities nucleophilic transfer of σ electrons of alkanes toward the σ * orbital of peroxo bonds. The electrophilic oxygen atom in HO–OH 2 + is the oxygen in the –OH group. The attack target of the oxygen atom is the hydrogen atom of C–H. An IRC calculation revealed that the hydroxylation process is as follows: alkane is first ionized into a carbocation through a hydride transfer toward HOOH 2 + and two H 2O molecules are formed. Then hydration of the carbocation with one H 2O molecule produces a protonated alcohol. The IRC also revealed that the microscopic nature of the hydroxylation is nucleophilic transfer of σ-electrons of a –C–H bond of alkanes toward the σ * orbital of the peroxo bond of HOOH 2 +. For the hydroxylation of methane and ethane, the activation barriers are 2.58 and 1.40 kcal/mol, respectively. However, for hydroxylation of the secondary carbon of propane and the tertiary carbon of isobutane, the hydroxylation is a spontaneous process because the activity barriers have not appeared. For the hydroxylations of the primary carbons of propane and isobutene, although their activation barriers exist, they are extremely low (in the range of 0.1–1.4 kcal/mol). These results can explain why at very low temperature, even at −78 °C, alkanes could be oxidized by hydrogen peroxide in superacid and could produce various oxides.