Oxygen atoms, organic radical ions

Following stages of the transformation are dissociation of the radical-ions and recombination of reaction product (anion from radical-anion, or proton from radical-cation) with surface sites. These reactions may be first steps in catalytic transformations of organic compounds with participation of the oxygen atom of oxides lattice. 

The reduction of nitrobenzene to aniline is a typical oxidation-reduction reaction in which tin metal, Sn°, is oxidized to stannic ion, Sn, in the form of stannic chloride, SnCl4 hydrochloric acid serves as the source of protons. A plausible mechanism of this reaction is outlined in Scheme 21.3. Generally, the reduction occurs by a sequence of steps in which an electron is first transferred from a tin atom to the organic substrate to give an intermediate radical ion that is then protonated. The oxygen atoms on the nitro group are eventually removed as water molecules. It is left as an exercise to write a balanced equation for the overall reaction and to provide a mechanism for the reduction of N-phenylhydroxylamine (11) into aniline (see Exercises 13 and 14 at the end of this section). 

As with electrolytic reductions, the information that has accumulated in the past is now being supplemented by more searching studies of mechanism. The primary step in an oxidation may be the direct transfer of an electron, to yield a carbonium ion or other free radical which then reacts with other species in the adsorption layer or it may be the direct oxidation of a carrier , such as the manganese(II) ion or some other substance that readily enters into a reversible oxidation-reduction process. Also, the solvent may be involved, and in aqueous media the OH ion may lose an electron to become an OH radical, which may then give rise to hydrogen peroxide or to an adsorbed oxygen atom, or may react directly with the organic substrate. Finally, at a lead dioxide electrode, which is often used as anode, the electrode surface itself may be involved in the oxidation. 

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