Reduce nitrobenzene to phenylamine, watch bromine water give an instant white precipitate, then turn phenylamine into a diazonium salt in an ice bath and couple it with phenol to make a bright orange azo dye.
Phenylamine, C6H5NH2, is made by reducing nitrobenzene with tin and concentrated HCl, heated under reflux, then adding NaOH to release the free amine from its salt. Its N lone pair is partly delocalised into the ring, which makes it a weak base but activates the ring: with aqueous bromine at room temperature it gives a white precipitate of 2,4,6-tribromophenylamine. With nitrous acid (NaNO2 + HCl) below 10 °C it forms benzenediazonium chloride, which couples with phenol in alkaline solution to give an azo dye containing –N=N–. The azo link joins two benzene rings into one extended delocalised system, which absorbs visible light, so azo compounds are used as dyes and indicators.
Fig. 1: from nitrobenzene to an azo dye, with the bromine test on phenylamine
Section 01
Making and testing phenylamine.
Phenylamine cannot be made from a halogenoarene and ammonia, because the C–Cl bond in chlorobenzene is too strong. It is made by reduction instead.
Step
Reagents and conditions
Equation or result
reduction
Sn + conc. HCl, heat under reflux
C6H5NO2 + 6[H] → C6H5NH2 + 2H2O
free the amine
add excess NaOH(aq)
C6H5NH3+ + OH− → C6H5NH2 + H2O
bromination
Br2(aq), room temperature, no catalyst
C6H5NH2 + 3Br2 → C6H2Br3NH2 + 3HBr
Why NaOH? In the acid the product is present as the salt C6H5NH3+Cl−; alkali removes the H+.
Why bromine reacts so easily: the delocalised N lone pair raises the electron density of the ring, especially at the 2, 4 and 6 positions, so bromine water is decolourised and a white precipitate forms at once, just as with phenol. Benzene needs a halogen carrier.
Yield: 12.3 g of nitrobenzene (Mr 123.0, 0.100 mol) can give at most 9.30 g of phenylamine (Mr 93.0). Obtaining 6.51 g is a 70.0% yield.
Precipitate mass: 0.930 g of phenylamine (0.0100 mol) reacts with 0.0300 mol Br2 to give 3.30 g of 2,4,6-tribromophenylamine (Mr 329.7).
Stage 1 · Learn
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Which reagents reduce nitrobenzene to phenylamine?
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What is seen when bromine water is added to phenylamine?
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Why must diazotisation be carried out below 10 °C?
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Which group is present in every azo dye?
Section 02
Diazotisation and coupling.
Two steps make an azo dye. Temperature control in step 1 and alkaline conditions in step 2 are the details examiners ask for.
Step
Reagents and conditions
Equation
make nitrous acid
NaNO2(aq) + HCl(aq), in situ
NaNO2 + HCl → HNO2 + NaCl
diazotisation
HNO2 + HCl, below 10 °C (ice bath)
C6H5NH2 + HNO2 + HCl → C6H5N2+Cl− + 2H2O
coupling
phenol in NaOH(aq), cold
C6H5N2+ + C6H5OH → C6H5N=NC6H4OH + H+
Below 10 °C: above this the diazonium ion decomposes, C6H5N2+ + H2O → C6H5OH + N2 + H+, and fizzing N2 is seen.
Why alkaline? NaOH turns phenol into the phenoxide ion, whose ring is even more electron-rich, so the weakly electrophilic diazonium ion attacks it, usually at the 4-position.
Colour: the –N=N– group links two rings into one extended delocalised system. The energy gap between orbitals is small enough for visible light to be absorbed, so the product (4-hydroxyazobenzene) is orange-yellow. Azo compounds are used as dyes for fabrics and food, and as indicators such as methyl orange.
Yield: 0.930 g of phenylamine (0.0100 mol) can give at most 1.98 g of dye (C12H10N2O, Mr 198.0); 1.58 g is a 79.8% yield.
Examiner trap
The reducing agent for nitrobenzene is tin and concentrated HCl, followed by NaOH; do not write NaBH4 or LiAlH4. Diazotisation must be kept below 10 °C (but not frozen), otherwise phenol and N2 form. Coupling with phenol needs alkaline conditions. Phenylamine with bromine water needs no catalyst and gives three Br atoms on the ring.
Stage 2 · Exam
Exam-style questions
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Why is benzenediazonium chloride coupled with phenol in ALKALINE solution?
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12.3 g of nitrobenzene (Mr 123.0) is reduced and 6.51 g of phenylamine (Mr 93.0) is obtained. What is the percentage yield?
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What mass of 2,4,6-tribromophenylamine (Mr 329.7) forms from 0.930 g of phenylamine (Mr 93.0) with excess bromine water?
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Why are azo compounds coloured?
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