Unsaturated and benzylic alcohols

These are generally oxidised with the same kinetics as saturated alcohols but more rapidly. In addition to the example of allyl alcohol - discussed previously (p. 377), several reports exist of oxidations of benzylic alcohols. The importance of a 1 1 complex K = 0.8 0.2 at 25 °C, [HCIO4] = 0.525 M) in the Ce(IV) oxidation is clear from agreement of spectroscopic and kinetic data . For a 

The oxidations of these by Ce(IV), Mn(III) and V(V) have been studied extensively . Kinetic data are summarised in Table 13. The main tasks are those of (/ ) discriminating between C-C cleavage and oxidation of CHOH groups to C-0 and (//) assessing the role of cyclic complexes. 

The acidity dependences of V(V) oxidations are significant. That of pinacol , which undergoes 100% C-C cleavage, is a+bh ). The first (acid-independent) term is rare in V(V) oxidations and implies that V02 is the active oxidant the second term implies, on the basis of the Zucker-Hammett hypothesis, that the transition state has the structure (J5), the mechanism being 

This contrasts with the V(V) oxidation of cyclohexanol which exhibits an [H30 ] dependence and involves an extra water molecule in the transition 

Evidence concerning the relative extents of C-C and C-H fission is less well defined for Ce(IV) and Mn(III) as compared with V(V). Pinacol is cleaved to acetone in all cases, but while Mn(IlI) pyrophosphate [like V(V)] oxidises pinacol much faster than butane-2 3-diol, the rate ratio with Ce(IV) is only approximately 3 and the production of acetaldehyde from butane-2 3-diol by Ce(IV) oxidation demonstrates C-C cleavage . It is probable, therefore, that Mn(III) oxidises the disecondary glycol by C-H fission. 

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