Oxidation by Loss of Electrons

One of the best examples of this type of process is seen in the copper(u)-catalysed oxidation of ascorbic acid (vitamin C). Co-ordination of ascorbic acid (in the enediolate form) to copper(n) is a very favourable process and results in the formation of complex 9.2 containing a five-membered chelate ring (Fig. 9-2). 

The radical form 9.4 has an unpaired electron and may undergo fast reactions with redox partners that also undergo one-electron processes. Such a redox partner is the triplet radical, dioxygen. The copper complex of ascorbic acid undergoes rapid aerial oxidation to give the dione, dehydroascorbic acid, which may be viewed as being derived by electron loss from the radical (Fig. 9-4). 

The oxidation of benzene- 1,2-diols to benzo-l,2-quinones is also a process of considerable biological importance, and solutions of copper compounds in organic solvents frequently act as catalysts for the aerial oxidation of such compounds (Fig. 9-5). These reactions almost certainly involve sequential one-electron processes, as indicated in Fig. 9-6. In some cases, the semiquinone forms may be isolated. 

Similar processes are observed with transition metal dithiolene complexes, in which a range of formal oxidation states of metal ions may be accommodated. In reality, this is 

Another common type of reaction involving sulfur compounds is the oxidation of thiols or thiolates to disulfides. This process is found to be very sensitive to the presence of metal ions. The metal can act as the primary oxidant, or dioxygen may be involved in a reaction with a co-ordinated thiol (Fig. 9-10). Very often, oxidation reactions involving metal ions and thiols are catalytic in the metal. 

---