34.2 Phenylamine and azo compounds

Learning objectives

34.2.1Preparation of phenylamine via the nitration• Describe the preparation of phenylamine via the nitration of benzene to form nitrobenzene followed by reduction with hot Sn/conc. HCl followed by NaOH(aq)34.2.2Describe• Describe:: (a) the reaction of phenylamine with Br₂(aq) at room temp.; (b) the reaction of phenylamine with HNO₂ or NaNO₂ and dil. acid below 10 °C to produce the diazonium salt; further warming of the diazonium salt with H₂O to give phenol 34.2.3Basicities of aq. ammonia, ethylamine• Describe/explain: the relative basicities of aq. ammonia, ethylamine and phenylamine 34.2.4About azo compounds• Recall the following about azo compounds:- (a) describe the coupling of benzenediazonium chloride with phenol in NaOH(aq) to form an azo compound- (b) identify the azo group- (c) state that azo compounds are often used as dyes- (d) that other azo dyes can be formed via a similar route

Phenylamine is prepared from benzene through nitrobenzene. The order matters: install -NO2 by nitration, reduce it in acid, then add aqueous alkali to release the free amine from its phenylammonium salt.

Stage	Conversion	Reagents and conditions	Purpose
1 nitration	benzene -> nitrobenzene	concentrated HNO3 and concentrated H2SO4, 25-60 °C	electrophilic substitution installs -NO2
2 reduction	nitrobenzene -> phenylammonium chloride in the acidic mixture	hot Sn and concentrated HCl, heat/reflux	reduce -NO2 to the amine oxidation level; acid protonates the amine
3 alkaline work-up	phenylammonium chloride -> phenylamine	NaOH(aq)	remove H+ and liberate C6H5NH2

$\ce{C6H5NO2 + 6[H] -> C6H5NH2 + 2H2O}$

$\ce{C6H5NH3+Cl- + NaOH -> C6H5NH2 + NaCl + H2O}$

Do not stop at the acidic reduction mixture: it contains protonated phenylamine. NaOH(aq) is an essential final step, not the reducing agent.

Phenylamine has two distinct reaction patterns here. Its electron-rich ring undergoes rapid 2,4,6-substitution with bromine water, while its -NH2 group can be converted into a diazonium salt only under cold conditions.

Reaction	Reagents and conditions	Organic product	Key consequence
bromination	Br2(aq), room temperature	2,4,6-tribromophenylamine	bromine water is decolourised and a white precipitate forms
diazotisation	HNO2, or NaNO2 plus dilute acid, below 10 °C	benzenediazonium salt	the unstable diazonium ion is preserved by keeping it cold
hydrolysis of diazonium salt	H2O, then warm	phenol	N2 is released as the diazonium group is replaced by -OH

The nitrogen lone pair donates electron density into the benzene ring. This activates the ring and directs electrophilic substitution to the 2, 4 and 6 positions, so no halogen carrier is needed for bromine water at room temperature.

$\ce{C6H5N2+ + H2O ->[warm] C6H5OH + N2 + H+}$

Keep the temperature stages separate: below 10 °C forms and preserves the diazonium salt; warming it with water deliberately decomposes it to phenol. Do not warm during diazotisation.

$\text{aqueous basicity: ethylamine} > \text{ammonia} > \text{phenylamine}$

All three accept H+ through the nitrogen lone pair. Their relative basicity depends on how available that lone pair is for forming a dative covalent bond to a proton.

Base	Effect on the nitrogen lone pair	Relative result
ethylamine	the ethyl group donates electron density by the positive inductive effect	lone pair is more available than in NH3; strongest of the three
ammonia	no ethyl group donates electron density and no benzene ring delocalises the lone pair	intermediate
phenylamine	the lone pair overlaps with and is delocalised into the benzene pi system	less available to accept H+; weakest of the three

Phenylamine is still a base; delocalisation makes it less basic than ammonia rather than preventing protonation altogether. Compare lone-pair availability, not the number of hydrogen atoms on nitrogen.

An azo compound contains the azo group -N=N-. In the products here, this link joins two aromatic carbon groups, giving the general pattern Ar-N=N-Ar'. Azo compounds are often used as dyes.

Step	Material and condition	Chemical role or outcome
1	benzenediazonium chloride, kept below 10 °C	supplies the diazonium electrophile without allowing it to hydrolyse
2	phenol dissolved in NaOH(aq)	forms an alkaline, electron-rich phenoxide coupling component
3	add the cold diazonium solution to the alkaline phenol	electrophilic substitution couples the two aromatic rings through -N=N- and forms an azo compound

To identify the azo group in any structure, locate N=N with a carbon-containing group bonded on each side. Do not confuse the neutral -N=N- link in an azo product with the charged -N2+ group in a diazonium ion.

The same route can make other azo dyes: change the substituted aromatic diazonium salt or the activated aromatic coupling component while retaining diazotisation followed by coupling. Different substituents can change the dye produced.

Coupling is not the warm-water reaction from the previous objective. The diazonium salt is kept cold and reacted with phenol in NaOH(aq); warming it with water instead gives phenol and N2.

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—34.3 AmidesRead

34.2 Phenylamine and azo compounds

Learning objectives

Prepare phenylamine by nitration, reduction and alkaline work-up

Separate phenylamine bromination from cold diazotisation and warm hydrolysis

Ethylamine is more basic than ammonia, while phenylamine is weaker

Couple benzenediazonium chloride with alkaline phenol to make an azo dye

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