Phenylamine and azo compounds
Phenylamine and azo compounds
- Phenylamine has an –NH₂ joined to a benzene ring.
- This affects both its basicity and its ring chemistry.
- Its diazonium salt makes brightly coloured azo dyes.
Making and reacting
- Make it: nitrate benzene → nitrobenzene, then reduce (hot Sn / conc. HCl, then NaOH) → phenylamine.
- with bromine water (RT) → 2,4,6-tribromophenylamine (white precipitate) — the ring is activated.
- with HNO₂ (below 10 °C) → a diazonium salt; warming with water → phenol.
Practice
Phenylamine is made from benzene by:
Benzene → nitrobenzene (nitration) → phenylamine (reduction with tin and acid, then NaOH).
Basicity and azo dyes
$$\text{phenylamine} < \text{ammonia} < \text{ethylamine}$$
- ethylamine is the strongest (its alkyl group pushes electrons onto N); phenylamine is the weakest (its lone pair is delocalised into the ring).
- a diazonium salt couples with phenol (in NaOH) → an azo compound (the –N=N– group) — brightly coloured dyes.
Practice
The order of basicity is:
Ethylamine's alkyl group makes its lone pair most available; phenylamine's lone pair is delocalised into the ring, so it is weakest.
Practice
Phenylamine is the weakest base because its nitrogen lone pair is:
Delocalisation into the ring means the lone pair is less available to accept H⁺.
Practice
An azo compound is formed when a diazonium salt couples with phenol; it contains the:
The –N=N– azo group gives strong colours, which is why azo compounds are used as dyes.
You've got it
Key idea
- phenylamine = –NH₂ on a benzene ring; made by nitrating then reducing benzene
- basicity: phenylamine < ammonia < ethylamine (ring pulls the lone pair away; alkyl pushes it in)
- diazonium salt + phenol → azo compound (–N=N–), used as dyes