Para toluidine is treated with \(\mathrm{HNO}_{2}\) at ice cold conditions and then boiled with water. The final product obtained is (a) anthranilic acid (b) \(\mathrm{p}\) -cresol (c) toluic acid (d) phenol

Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. They are derivatives of ammonia, where one or more hydrogen atoms are replaced by alkyl or aryl groups. Amines react with acids to form salts and undergo alkylation and acylation to form amides. A unique property of aromatic amines, like para toluidine, is their ability to take part in electrophilic substitution reactions.

When an aromatic amine is treated with a nitrosating agent such as nitrous acid (HNO_{2}), usually in acidic conditions, it undergoes diazotization. This reaction is an integral part of synthetic chemistry and is widely used in the preparation of various aromatic compounds, including dyes and pharmaceuticals. The process transforms the amine group into a diazonium group, which can further react in several ways, depending on the reaction conditions, to form different products.

Diazonium salt formation is a key reaction in organic chemistry involving the conversion of primary aromatic amines into diazonium salts. This transformation is achieved by treating the amine with nitrous acid at low temperatures, generally 0 to 5 °C, which prevents the decomposition of the diazonium compound formed. The diazonium group (N_{2}^{+}) is a good leaving group, which makes it a versatile intermediate for various organic reactions. The diazonium salt is relatively stable at these low temperatures, but as the temperature is increased, it becomes more reactive and can participate in multiple displacement reactions or can decompose to produce phenols or other aromatic compounds.

The aromatic hydrolysis reaction is a process where the diazonium group of an aryl diazonium salt reacts with water. This typically occurs under increased temperature conditions, such as boiling water. The reaction involves the replacement of the diazonium group by a hydroxyl group (OH), leading to the formation of a phenol. It is essential to note that the diazonium salt must be heated gently during the reaction to avoid any explosive decomposition. The hydrolysis of diazonium salts provides a straightforward path to synthesizing phenolic compounds directly from aniline derivatives. This is an important step in many industrial applications, including the preparation of antiseptics, polymers, and various other chemical compounds.

The synthesis of p-cresol illustrates the fundamental concepts of diazonium salt chemistry. Para toluidine is an aromatic amine where the amino group is on the para position relative to the methyl group on the benzene ring. When para toluidine reacts with nitrous acid under cold conditions, it forms a diazonium salt at the para position. Subsequent heating of this diazonium salt in water leads to hydrolysis and replaces the diazonium group with a hydroxyl group. The product of the reaction is p-cresol, a compound with a hydroxyl group para to the methyl group on the benzene ring.

This particular synthesis is of great interest because p-cresol is used in the manufacture of antioxidants, pharmaceuticals, and perfumes, highlighting the vast importance of diazonium chemistry in industrial applications.