Compounds in which an amino group, -\mathrm{NH_2}, is bonded to a hydrocarbon group are called amines. Aniline, \mathrm{C_6H_5NH_2}, in which the amino group is bonded to a benzene ring, is the simplest aromatic amine.
When nitrobenzene is heated with tin and concentrated hydrochloric acid, it is reduced, and anilinium chloride (aniline hydrochloride) forms.
2\mathrm{C_6H_5NO_2} + 3\mathrm{Sn} + 14\mathrm{HCl} \longrightarrow 2\mathrm{C_6H_5NH_3Cl} + 3\mathrm{SnCl_4} + 4\mathrm{H_2O}
When sodium hydroxide solution is added to the solution of anilinium chloride, aniline is set free as an oily liquid. In industry, aniline is made by reducing nitrobenzene with hydrogen, using a catalyst.
Aniline is a colourless, oily liquid that boils at 184 °C and dissolves only a little in water. It is easily oxidized, so it slowly turns brown in air.
Aniline is a weak base. It reacts with hydrochloric acid to form a salt, anilinium chloride, and dissolves.
\mathrm{C_6H_5NH_2} + \mathrm{HCl} \longrightarrow \mathrm{C_6H_5NH_3Cl}
When a strong base such as sodium hydroxide is added to this solution, aniline is set free again, and the solution turns cloudy.
\mathrm{C_6H_5NH_3Cl} + \mathrm{NaOH} \longrightarrow \mathrm{C_6H_5NH_2} + \mathrm{NaCl} + \mathrm{H_2O}
When bleaching powder solution is added to aniline, a purple colour appears. This reaction is used to detect aniline.
When potassium dichromate solution acidified with sulfuric acid is added to aniline, aniline is oxidized, and a black substance called aniline black forms. Aniline black is used as a black dye and pigment.
When sodium nitrite solution is added to a solution of aniline in hydrochloric acid while it is cooled in ice, benzenediazonium chloride forms. This reaction is called diazotization.
\mathrm{C_6H_5NH_2} + 2\mathrm{HCl} + \mathrm{NaNO_2} \longrightarrow \mathrm{C_6H_5N_2Cl} + \mathrm{NaCl} + 2\mathrm{H_2O}
Benzenediazonium chloride is unstable. Above about 5 °C, it reacts with water, giving off nitrogen and forming phenol, so the reaction is carried out at 5 °C or below.
\mathrm{C_6H_5N_2Cl} + \mathrm{H_2O} \longrightarrow \mathrm{C_6H_5OH} + \mathrm{N_2} + \mathrm{HCl}
When a solution of benzenediazonium chloride is added to a solution of sodium phenoxide, an orange-red compound, 4-(phenyldiazenyl)phenol (4-hydroxyazobenzene), forms. This reaction is called a coupling reaction.
\mathrm{C_6H_5N_2Cl} + \mathrm{C_6H_5ONa} \longrightarrow \mathrm{C_6H_5{-}N{=}N{-}C_6H_4OH} + \mathrm{NaCl}
The group -\mathrm{N{=}N}- is called an azo group, and compounds that contain it are called azo compounds. Many azo compounds are brightly coloured and are used as dyes, called azo dyes. Methyl orange, which is used as an indicator, is also an azo compound.