34.2 Phenylamine and azo compounds

Benzene is first nitrated to nitrobenzene. Hot tin and concentrated hydrochloric acid reduce the nitro group to an ammonium salt; aqueous sodium hydroxide then liberates phenylamine.

Treat it as a three-stage route: electrophilic substitution, reduction in acid, then basification. Separating the stages prevents the acid–base work-up from being missed.

C₆H₆ → C₆H₅NO₂ → C₆H₅NH₃⁺Cl⁻ → C₆H₅NH₂. NaOH converts the ammonium salt to the free amine.

The reduction product in the acidic mixture is not immediately free phenylamine; it is protonated until alkali is added.

The –NH₂ group activates the aromatic ring: phenylamine reacts with bromine water at room temperature to give rapid substitution. With nitrous acid made from NaNO₂ and dilute acid below 10 °C, it forms a diazonium salt; warming with water gives phenol.

Keep the conditions separate: bromination is an aromatic substitution, while diazotisation requires cold acid and the next hydrolysis step requires warming.

Phenylamine + Br₂(aq) gives 2,4,6-tribromophenylamine; the diazonium salt route then releases N₂ as phenol forms.

Do not warm during diazotisation or use bromine conditions to infer the diazonium route.

In water the usual basicity order is ethylamine > ammonia > phenylamine. An ethyl group donates electron density towards nitrogen, while in phenylamine the lone pair is delocalised into the benzene ring.

A more available lone pair accepts H⁺ more readily. Solvation and the aqueous environment matter, so use the syllabus order rather than a gas-phase shortcut.

Ethylamine produces a higher equilibrium concentration of OH⁻ than ammonia at comparable conditions; phenylamine’s ring delocalisation makes proton acceptance less favourable.

Phenylamine is still basic. Delocalisation reduces its basicity; it does not remove the nitrogen lone pair.

Azo compounds contain an –N=N– group connecting carbon-containing groups, commonly two aromatic rings. Their extended conjugation allows strong absorption in the visible region and often produces intense colours.

In syllabus chemistry, azo compounds are linked to diazonium salts and coupling reactions; the N=N unit is the identifying structural feature.

An azo dye can be represented as Ar–N=N–Ar′, where substituents on either ring tune colour and solubility.

The azo link is not an amide or an amine: it has two nitrogens joined by a nitrogen–nitrogen double bond.