What are tarry oxidation products?

When nitric acid oxidizes aniline, it can produce unwanted tarry oxidation products if the aniline is not properly protected during nitration. In its unprotonated form, aniline acts as a nucleophile, attacking protonated nitric acid and initiating this undesirable oxidation pathway. A nucleophile is a species that donates a pair of electrons to form a new covalent bond.

During direct nitration, in addition to the desired nitro derivatives, a significant amount of tarry by-products may form. In a strongly acidic medium, aniline becomes protonated to the anilinium ion, which is meta-directing. As a result, nitration produces not only ortho and para derivatives but also a considerable amount of the meta derivative.

If unprotected aniline is subjected to nitration in the presence of concentrated sulfuric acid, both nitric acid and aniline can be protonated, leading to further formation of tarry oxidation products. In such a reaction mixture, species like aniline, anilinium ions, sulfuric acid, nitric acid, and protonated nitric acid coexist, increasing the chance of unwanted reactions.

Note

Oxidation processes are also applied in environmental chemistry. They reduce the biochemical oxygen demand (BOD) in wastewater by converting harmful impurities into carbon dioxide, water, and bio-solids. Chemical oxidation is frequently used for disinfection as well.