Phenylamine and azo compounds

· Phenylamine = aromatic primary amine, C₆H₅NH₂.
· Also called aniline or aminobenzene.
· Key reactions required: preparation from benzene, bromination, diazotisation, formation of phenol, azo coupling, and relative basicity.
· Exam focus: know reagents, conditions, observations, products, and why phenylamine is less basic than ethylamine/ammonia.

Preparation of phenylamine

· Step 1: benzene → nitrobenzene by nitration.
· Reagents/conditions: concentrated HNO₃ + concentrated H₂SO₄, 25–60°C.
· Equation: C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O.
· Step 2: nitrobenzene → phenylammonium ion by reduction.
· Reagents/conditions: hot Sn + concentrated HCl, heat under reflux.
· Acidic conditions form C₆H₅NH₃⁺, not free phenylamine.
· Step 3: add NaOH(aq) to deprotonate the phenylammonium ion.
· Equation: C₆H₅NH₃⁺ + OH⁻ → C₆H₅NH₂ + H₂O.
· Overall route: benzene → nitrobenzene → phenylamine.

This diagram shows the two-stage conversion of nitrobenzene to phenylamine. The reduction first forms the phenylammonium ion, which is then deprotonated by NaOH(aq) to give phenylamine. Source

Reaction of phenylamine with bromine water

· Reagent/condition: Br₂(aq) at room temperature.
· Observation: orange/brown bromine water decolourises and a white precipitate forms.
· Product: 2,4,6-tribromophenylamine.
· Phenylamine reacts more readily than benzene because –NH₂ activates the benzene ring.
· The nitrogen lone pair is partly donated into the ring, increasing electron density.
· The –NH₂ group directs substitution to the 2-, 4- and 6-positions.
· This is an electrophilic substitution reaction.

This diagram shows phenylamine undergoing bromination at the 2-, 4- and 6-positions. It explains why bromine water is decolourised and why a white precipitate forms. Source

Diazotisation of phenylamine

· Diazotisation = conversion of phenylamine into a diazonium salt.
· Reagents: HNO₂ or NaNO₂ + dilute acid.
· Conditions: keep below 10°C using ice.
· Product: benzenediazonium salt, commonly benzenediazonium chloride, C₆H₅N₂⁺Cl⁻.
· NaNO₂ + dilute HCl generates HNO₂ in situ.
· Essential condition: temperature below 10°C because diazonium salts are unstable and decompose on warming.
· General equation: C₆H₅NH₂ + HNO₂ + HCl → C₆H₅N₂⁺Cl⁻ + 2H₂O.
· The diazonium ion contains the –N₂⁺ group attached to the benzene ring.

This diagram shows the formation of a diazonium salt from phenylamine. The key exam condition is that the mixture must be kept cold, usually below 10°C. Source

Warming diazonium salts to form phenol

· If the diazonium salt is warmed with H₂O, it forms phenol.
· Conditions: warm with water after diazotisation.
· Product: phenol, C₆H₅OH.
· Nitrogen gas is also produced, so this reaction is driven by formation of stable N₂(g).
· Equation: C₆H₅N₂⁺Cl⁻ + H₂O → C₆H₅OH + N₂ + HCl.
· This links topic 34.2 to phenol preparation in topic 32.2.

Relative basicity: ethylamine, ammonia, phenylamine

· Basicity depends on how available the nitrogen lone pair is to accept H⁺.
· Ethylamine is the strongest base of the three.
· In ethylamine, the ethyl group donates electron density by the positive inductive effect, making the nitrogen lone pair more available.
· Ammonia has no alkyl group, so it is less basic than ethylamine.
· Phenylamine is the weakest base of the three.
· In phenylamine, the nitrogen lone pair is delocalised into the benzene ring.
· This makes the lone pair less available to accept H⁺.
· Basicity order: ethylamine > ammonia > phenylamine.
· Exam phrase: phenylamine is less basic because the nitrogen lone pair is delocalised into the benzene ring.

Azo coupling with phenol

· Azo compounds contain the azo group, –N=N–.
· Azo compounds are formed by coupling reactions of diazonium salts.
· Required reaction: benzenediazonium chloride + phenol in NaOH(aq).
· Phenol reacts with NaOH(aq) to form phenoxide ions, which are more reactive in electrophilic substitution.
· Conditions: alkaline solution, usually cold.
· Product: an azo compound, often an orange/yellow dye.
· The azo linkage joins two aromatic rings: Ar–N=N–Ar.
· The diazonium ion acts as the electrophile.
· Other azo dyes can be made using a similar route with different phenols or aromatic amines.

This diagram shows azo coupling between a diazonium salt and phenoxide ions. The product contains the characteristic –N=N– azo group, which is responsible for many dye structures. Source

This diagram shows how an aromatic compound couples with a diazonium compound to form an azo linkage. It is useful for identifying the –N=N– group in azo dyes. Source

Common exam observations and products

· Phenylamine + Br₂(aq): bromine water decolourises; white precipitate of 2,4,6-tribromophenylamine.
· Phenylamine + NaNO₂/dilute acid below 10°C: forms benzenediazonium chloride.
· Diazonium salt + warm water: forms phenol and N₂ gas.
· Diazonium salt + phenol/NaOH(aq): forms an azo dye, often yellow-orange.
· Azo group to identify: –N=N–.

High-value equations to memorise

· Nitration: C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O.
· Deprotonation: C₆H₅NH₃⁺ + OH⁻ → C₆H₅NH₂ + H₂O.
· Diazotisation: C₆H₅NH₂ + HNO₂ + HCl → C₆H₅N₂⁺Cl⁻ + 2H₂O.
· Hydrolysis of diazonium salt: C₆H₅N₂⁺Cl⁻ + H₂O → C₆H₅OH + N₂ + HCl.
· Azo compound functional group: Ar–N=N–Ar.