Phenylamine and azo compounds
| English | Chinese | Pinyin |
|---|---|---|
| phenylamine | 苯胺 | běn àn |
| basicity | 碱性 | jiǎn xìng |
| diazonium salt | 重氮盐 | zhòng dàn yán |
| azo dye | 偶氮染料 | ǒu dàn rǎn liào |
| azo compound | 偶氮化合物 | ǒu dàn huà hé wù |
Colour from the ring
- Phenylamine 苯胺 has an –NH₂ joined to a benzene ring.
- This affects both its basicity 碱性 and its ring chemistry.
- Its diazonium salt 重氮盐 makes brightly coloured azo dyes 偶氮染料.
Phenylamine to azo dye route
Follow phenylamine from diazotisation to coloured azo compound.
Phenylamine can be used to make azo dyes via a diazonium salt.
Azo compounds are brightly coloured.
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.
Methyl orange, a bright orange azo dye; its strong colour comes from the -N=N- azo group, which is why azo compounds 偶氮化合物 are widely used as dyes
Phenylamine is made from benzene by:
Benzene → nitrobenzene (nitration) → phenylamine (reduction with tin and acid, then NaOH).
Match each term to its meaning.
Each item links the term to its correct meaning.
Basicity and azo dyes
- 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.
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.
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⁺.
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.
Making azo dyes
- Phenylamine forms a diazonium salt at low temperature, which couples to give azo dyes.
- Azo compounds are intensely coloured and widely used as dyes.
You've got it
- 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