Derivatives of Benzene
Senior 6 · Chemistry
Derivatives of Benzene
Lesson 5 of 5
25 min
In 1856 an eighteen-year-old student in London, William Perkin, was trying to make the malaria drug quinine from phenylamine (aniline). Instead he obtained a purple solid that dyed silk beautifully: mauveine, the first synthetic dye. Within decades, dyes made from phenylamine had replaced most plant dyes. Many of today's dyes, food colours and indicators, including the methyl orange you use in titrations, are azo dyes made by the route in Figure 5.1.
Figure 5.1: From benzene to dyes. Each step uses a reaction met in this unit.
Nitrobenzene, made by nitrating benzene (Unit 3), is a pale yellow, oily liquid with a smell of almonds. It is denser than water (1.20 g cm⁻³) and boils at 211 °C. It is toxic: it passes through the skin and stops haemoglobin from carrying oxygen. Almost all of it is reduced to phenylamine.
Reduction to phenylamine. In the laboratory, nitrobenzene is heated under reflux with tin and concentrated hydrochloric acid. The amine forms as its salt, phenylammonium chloride, so sodium hydroxide solution is then added to release the free amine, which is separated by steam distillation:
Industry uses hydrogen with a nickel catalyst: .
Definition
Aromatic amine: a compound with an group joined directly to a benzene ring. Example: phenylamine (aniline), .
Diazonium ion: an ion containing the group, written for the benzenediazonium ion.
Azo compound: a compound in which two carbon groups (usually aromatic rings) are joined by an azo group, –N=N–. Example: 4-hydroxyazobenzene.
Phenylamine is a colourless oily liquid (boiling point 184 °C) that turns brown on standing, as air oxidises it. Like ammonia, it has a lone pair on nitrogen and accepts a proton, so it dissolves in hydrochloric acid:
But it is a much weaker base than ammonia or ethylamine.
Table 5.1: Base strength ( of the conjugate acid: the higher the value, the stronger the base)
| Base | of conjugate acid | Reason |
|---|---|---|
| ethylamine, | 10.7 | the ethyl group pushes electrons onto N: lone pair more available |
| ammonia, | 9.25 | reference |
| phenylamine, | 4.6 | the lone pair on N is partly delocalised into the ring: less available to accept |
The same delocalised lone pair strongly activates the ring. Like phenol, phenylamine decolourises bromine water at once, giving a white precipitate of 2,4,6-tribromophenylamine: .
Step 1: diazotisation. Sodium nitrite and hydrochloric acid make nitrous acid in the solution (), which reacts with phenylamine below 10 °C (in practice at 0–5 °C, in ice):
The benzenediazonium ion is unstable. If the solution is warmed above 10 °C, it decomposes, giving off nitrogen and forming phenol, a useful laboratory preparation:
Step 2: coupling. The cold diazonium solution is added to phenol dissolved in sodium hydroxide. The diazonium ion is a weak electrophile, but the phenoxide ion is very strongly activated, so electrophilic substitution happens at carbon 4, opposite the O⁻:
A yellow-orange precipitate of 4-hydroxyazobenzene forms at once. Coupling with naphthalen-2-ol instead gives an intensely orange-red dye.
Figure 5.2: An azo dye. The coupling happens at carbon 4 of phenol, opposite the OH group.
Why azo compounds are coloured. The –N=N– group joins the π systems of the two rings into one long delocalised system. The larger a delocalised system, the smaller the energy gap its electrons must jump, until the gap matches visible light. Azo compounds absorb blue-violet light and look yellow, orange or red; different groups on the rings shift the colour.
Table 5.2: Uses of phenylamine and its products
| Product | Use |
|---|---|
| azo dyes | colouring cotton, wool and leather; some food colours; indicators such as methyl orange |
| diisocyanates made from phenylamine | polyurethane foams for mattresses and insulation |
| 4-aminophenol, then paracetamol | medicine for pain and fever |
| sulfonamides | antibacterial drugs |
| rubber chemicals | speeding up vulcanisation and protecting tyres from ageing |
Worked example
Problem. 24.6 g of nitrobenzene is reduced to phenylamine with a 65.0% yield. What mass of phenylamine is obtained? (: nitrobenzene 123.11, phenylamine 93.13)
Solution. One mole of nitrobenzene gives one mole of phenylamine.
Answer: 12.1 g of phenylamine.
Worked example
Problem. Outline the synthesis of 4-hydroxyazobenzene from benzene, giving the reagents and conditions for each step. Explain why two of the steps must be kept cold.
Solution. (1) Nitrate benzene: concentrated and , at or below 55 °C → nitrobenzene. (2) Reduce: tin and concentrated HCl under reflux, then NaOH → phenylamine. (3) Diazotise: and HCl at 0–5 °C → benzenediazonium chloride. (4) Couple: add to phenol in NaOH(aq), below 10 °C → 4-hydroxyazobenzene. Steps 3 and 4 are cold because the diazonium ion decomposes above about 10 °C, giving phenol and nitrogen instead of the dye.
Warning
Safety. Phenylamine and nitrobenzene are toxic by skin contact, by breathing the vapour and by swallowing; they reduce the blood's ability to carry oxygen, and phenylamine is a suspected carcinogen. Wear gloves and use a fume cupboard. Solid diazonium salts can explode, so they are never isolated: they are made and used at once, in cold solution.
Tip
Remember the dye route as "nitrate, reduce, diazotise, couple", and the temperature rule: diazonium below 10 °C. Warm it, and you make phenol instead.
Allery Chemistry: Making azo dyes (5 min). Watch for why the diazotisation is done in ice, and how coupling with a phenol produces the coloured product.
Practice
Further reading
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