Oxidation of cyclohexane and cyclopentane

Some reference to the oxidation of cycloalkanes is merited in view of their significant presence in conventional fuels, particularly as relatively little attention has been paid to the chemistry involved. Recently Walker and co-workers [93, 94], by separate addition to slowly reacting mixtures of H2 + O2, have made a quantitative analysis of the oxidation products of cyclohexane (753 K) and of cyclopentane (673-783 K). They also determined rate constant ratios /cei/ 62 and /cec/ is for OH and H attack 

Temperature coefficients for H and OH attack on cyclo-pentane and cyclo-hexane based on fc = A T e [97,98] 

Initial yield of conjugate alkene for alkyl radicals at 753 K and 70 Torr O2 

Relative initial yields of products from c-hexane at 753 K [97] 

It must be emphasized that the ratio is independent of the value of the equilibrium constant for C5H11 + 02 C5Hn02. The relative amount of 1,2- to 1,4 isomer is, however, much larger than expected (Table 1.17) for alkyl radicals, k(l,4s)/k(l,6s) 0.07, compared with the value of [1,2 cyclohexane oxide]/[l,4 cyclohexane oxide] = 0.60 0.10. Walker and Gu-lati point out that although about 99.5% of the cyclohexane molecules adopt the chair form at 753 K, the O—O group at either an axial or an equatorial position in the chair form is not favourably structured for a low strain energy in the transition state, as shown by 

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Several tris(pyrazol-l-yl)metliane complexes of V(in, IV, or V), Fe(II), Cu(II), and Re(III or VII) (Fig. 22.2) have been found to be catalyst precursors for the peroxidative oxidation of cyclohexane (and cyclopentane in the cases tested) to give, in a single pot, the corresponding alcohols and ketones (Scheme 22.1). The reactions are usually carried out in acetonitrile, with aqueous H2O2 as the oxidizing agent, in acidic medium, at room temperature. They proceed via radical mechanisms with possible involvement of both C-centered and O-centered radicals as indicated by radical trap experiments. 

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