Amines: Aliphatic and Aromatic, Basicity and Azo Dyes

This guide covers subtopics 34.1, Primary and secondary amines, and 34.2, Phenylamine and azo compounds, from Topic 34, Nitrogen compounds, of Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. This is A Level content, extending the AS-level halogenoalkane + ammonia route (19.1) and the basicity of ammonia (12.1).

Before studying this

This resource assumes the AS-level production of amines from halogenoalkanes and ammonia (19.1), from Nitrogen Compounds: Amines and Nitriles, and the basicity of ammonia (12.1), from Nitrogen and Sulfur, which applies the Brønsted–Lowry theory. Nitrile reduction and the basicity of amines are A Level content covered below. It also assumes benzene’s electrophilic substitution chemistry from Arenes and Halogenoarenes for phenylamine’s preparation below.

Syllabus coverage

CAMBRIDGE INTERNATIONAL AS & A LEVEL CHEMISTRY 9701 — A Level, Topic 34

34.1 Primary and secondary amines — recalling the production of primary and secondary amines: halogenoalkane + NH₃ in ethanol under pressure; halogenoalkane + a primary amine in ethanol, sealed tube/pressure; reduction of an amide with LiAlH₄; reduction of a nitrile with LiAlH₄ or H₂/Ni; describing the condensation reaction of ammonia or an amine with an acyl chloride to give an amide; describing and explaining the basicity of aqueous amine solutions.

34.2 Phenylamine and azo compounds — describing phenylamine’s preparation (nitration of benzene, then reduction with hot Sn/concentrated HCl, then NaOH(aq)); describing phenylamine’s reaction with Br₂(aq) and with HNO₂/NaNO₂ and dilute acid below 10 °C; describing and explaining the relative basicities of aqueous ammonia, ethylamine and phenylamine; recalling the coupling of benzenediazonium chloride with phenol in NaOH(aq) to form an azo compound, identifying the azo group, and that azo compounds are used as dyes formed via a similar route.

Producing primary and secondary amines

Building on the AS-level halogenoalkane + ammonia route, the syllabus lists four production methods at A Level:

Method	Reagents/conditions	Gives
Halogenoalkane + ammonia	excess NH₃, ethanol, heated under pressure	primary amine
Halogenoalkane + primary amine	ethanol, heated, sealed tube/pressure	secondary amine
Amide reduction	LiAlH₄	primary amine
Nitrile reduction	LiAlH₄, or H₂/Ni	primary amine

The halogenoalkane routes can, without care, keep reacting (a primary amine product is itself nucleophilic and can attack a further molecule of halogenoalkane) — using a large excess of ammonia favours the primary amine by making a second substitution on the same nitrogen statistically unlikely.

Amides from acyl chlorides. This reaction appears here, within the amines subtopic, because it is the standard route from an amine to an amide: ammonia or an amine reacts with an acyl chloride at room temperature in a condensation reaction (releasing HCl, or — with excess ammonia/amine present — the HCl is neutralised by a second equivalent to give the ammonium/aminium salt instead) to give an amide:

CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl

Amides and Amino Acids covers this reaction and amide chemistry fully.

Basicity of amines

Amines are basic because the nitrogen’s lone pair can accept a proton (the same Brønsted-Lowry idea met for ammonia at AS Level). How strongly basic an amine is depends on how available that lone pair is.

Alkyl groups increase basicity relative to ammonia: an alkyl group is electron-donating by induction, pushing electron density onto the nitrogen and making its lone pair more available to bond to an incoming H⁺ — so ethylamine is a stronger base than ammonia.

Phenylamine’s preparation

Phenylamine is produced from benzene in two steps:

Step 1 — nitration of benzene (concentrated HNO₃/H₂SO₄, 25–60 °C) gives nitrobenzene, as already met in Arenes and Halogenoarenes.

Step 2 — reduction of nitrobenzene with hot tin and concentrated HCl, followed by NaOH(aq) to free the amine from its protonated (ammonium salt) form, gives phenylamine:

C₆H₅NO₂ –[Sn, conc. HCl, heat]→ [C₆H₅NH₃]⁺ –[NaOH(aq)]→ C₆H₅NH₂

Reactions of phenylamine

With bromine water. Phenylamine reacts with Br₂(aq) at room temperature, substituting readily at the ring (an activated ring, for the same lone-pair-delocalisation reason as phenol) without needing a catalyst, giving an immediate white precipitate of 2,4,6-tribromophenylamine (2,4,6-tribromoaniline) — even more readily than phenol itself, since the –NH₂ group activates the ring more strongly than –OH.

With nitrous acid, below 10 °C. Phenylamine reacts with HNO₂ (from NaNO₂ and dilute acid) below 10 °C to form a diazonium salt, benzenediazonium chloride, [C₆H₅N₂]⁺Cl⁻ — the low temperature is essential, since the diazonium salt decomposes above about 10 °C. Warming this diazonium salt with water displaces the diazonium group to give phenol, as already met in Phenol: Reactions and Acidity.

Relative basicity: ammonia, ethylamine, phenylamine

Ranking from most to least basic:

Ethylamine (most basic) — the ethyl group’s electron-donating inductive effect increases electron density on nitrogen beyond ammonia’s, making the lone pair most available to accept H⁺.

Ammonia (intermediate) — no substituent effect either way; the reference point.

Phenylamine (least basic, and markedly so) — the nitrogen’s lone pair delocalises into the aromatic ring’s π system (similar in kind to a phenoxide ion’s stabilisation, but here reducing basicity rather than increasing acidity, since the delocalised lone pair is less available to accept a proton in the first place). This delocalisation pulls electron density away from nitrogen rather than adding to it, making phenylamine a distinctly weaker base than even plain ammonia.

Azo compounds

Benzenediazonium chloride couples with phenol in NaOH(aq) at low temperature to form an azo compound:

[C₆H₅N₂]⁺Cl⁻ + C₆H₅OH + NaOH → C₆H₅–N=N–C₆H₄–OH (4-hydroxyazobenzene, an azo dye) + NaCl + H₂O

(C₁₂H₁₂ClN₂NaO₂ on both sides.) Note that the second ring carries four hydrogens, not five, once it bears both the azo linkage and the hydroxyl group — a ring carbon is used up by each substituent. Sodium hydroxide is consumed as a reactant here (not merely a condition), deprotonating phenol to the more strongly activating phenoxide ion before coupling.

The characteristic azo group, –N=N–, links the two aromatic rings. This extended, conjugated system of alternating and delocalised bonding across both rings and the azo linkage absorbs strongly in the visible region, which is exactly why azo compounds are intensely coloured and widely used as dyes. Other azo dyes are made by the same diazonium-coupling route, varying the aromatic partner coupled to the diazonium salt to tune the colour and properties of the resulting dye.

Common mistakes

Predicting phenylamine is more basic than ammonia by analogy with ethylamine. The direction of the substituent effect flips between an alkyl group (electron-donating, increases basicity) and an aromatic ring (lone pair delocalises away, decreases basicity) — these are opposite effects, not variations on the same trend.

Forgetting the temperature control needed for diazonium salt formation. Below 10 °C is essential for both making the diazonium salt (from phenylamine) and, separately, for the diazonium coupling reaction with phenol — above this, the diazonium salt decomposes rather than reacting as intended.

Confusing diazonium salt formation with diazonium coupling. Forming the salt reacts phenylamine with HNO₂/NaNO₂ and dilute acid; coupling reacts the already-formed diazonium salt with a second aromatic compound (like phenol) in alkaline conditions to build the azo dye — two distinct steps, easily conflated.

Missing that phenylamine’s ring reactions (bromination) are activated, like phenol’s, for the same lone-pair-delocalisation reason — some students expect phenylamine to behave like an unsubstituted benzene ring because it’s an amine, not a phenol, but the same delocalisation logic applies to nitrogen’s lone pair just as it does to oxygen’s.

Quick revision checklist

• Primary amine: halogenoalkane + excess NH₃ (pressure), or nitrile/amide reduction (LiAlH₄, or H₂/Ni for nitriles)
• Secondary amine: halogenoalkane + primary amine (pressure)
• Amide from acyl chloride + ammonia/amine (condensation, releases HCl)
• Basicity: ethylamine > ammonia > phenylamine (alkyl donates; ring delocalises the lone pair away)
• Phenylamine: benzene → (HNO₃/H₂SO₄) → nitrobenzene → (Sn/HCl, then NaOH) → phenylamine
• Diazonium salt: phenylamine + HNO₂/NaNO₂ + dilute acid, below 10 °C
• Azo dye: diazonium salt + phenol (in NaOH) → azo compound, –N=N– linkage

Related resources

• Nitrogen Compounds: Amines and Nitriles — the AS-level amine production this topic extends
• Phenol: Reactions and Acidity — phenol’s production from phenylamine, and the coupling partner in azo dye formation
• Arenes and Halogenoarenes: Electrophilic Substitution and Reactivity — the nitration step in phenylamine’s preparation
• Amides and Amino Acids — the amide product of amines reacting with acyl chlorides
• Cambridge AS & A Level Chemistry hub

Written against Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. Always check the current syllabus for your examination year.