The oxidation of cyclohexanol to adipic acid by nitric acid takes place by two routes. One of these, in which 2-nitro-2-nitrosocyclohexanone (5) and the nitrolic acid (6) are intermediates, is not influenced by the presence of vanadium(V). The other, involving cyclohexane-1,2-dione(8) and its tautomer, only gives a significant amount of adipic acid if the metal ion is present. The dione and ‘predione’(9), a dimeric species which can be isolated from the system, appear to be in equilibrium with the monohydrate (14). It is this last compound which is selectively oxidised by vanadium(V). The mono-enol (15) is formed by slow tautomerisation of the dione and is selectively oxidised in the presence of vanadium(V) to glutaric acid. The mono-oxime (10) appears to be the key intermediate linking the two routes.
