What you'll learn

• How to make primary aliphatic amines from halogenoalkanes and nitriles.
• How aromatic amines are formed from nitrobenzenes.
• Why amines are basic, and why phenylamine is less basic than ammonia.
• How primary amines react to form amides, diazonium salts, and azo dyes.

1. What is an amine?

Amines contain a nitrogen atom with a lone pair of electrons. This lone pair controls much of their chemistry: it makes amines nucleophiles and bases.

Important wording:

• Aliphatic amine: the nitrogen is attached to an alkyl chain, for example CH₃CH₂NH₂.
• Aromatic amine: the nitrogen is attached directly to a benzene ring, for example phenylamine, C₆H₅NH₂.
• Phenylamine is also commonly called aniline.

2. Making primary aliphatic amines

From halogenoalkanes: substitution with ammonia

A halogenoalkane contains a polar carbon–halogen bond, such as C–Br. The carbon bonded to the halogen is electron-deficient, so ammonia can attack it as a nucleophile.

For bromoethane:

CH₃CH₂Br + 2NH₃(alc) → CH₃CH₂NH₂ + NH₄Br

Conditions:

• Excess ethanolic ammonia
• Heat in a sealed tube

The first product is an alkylammonium salt, which is then deprotonated by another ammonia molecule to form the primary amine.

The main aliphatic amine routes are summarised below.

From nitriles: reduction

A nitrile contains the functional group –C≡N. Nitriles can be reduced to primary amines:

RCN + 4[H] → RCH₂NH₂

Typical reagents:

• LiAlH₄ in dry ether
• Then dilute acid work-up

A useful synthetic point: if the nitrile was made from a halogenoalkane using CN⁻, the carbon chain becomes one carbon longer.

Halogenoalkane to nitrile:

CH₃CH₂Br + KCN → CH₃CH₂CN + KBr

Then reduction:

CH₃CH₂CN + 4[H] → CH₃CH₂CH₂NH₂

3. Making aromatic amines from nitrobenzenes

Aromatic amines are made by reducing nitroarenes. For example, nitrobenzene is reduced to phenylamine.

Overall equation:

C₆H₅NO₂ + 6[H] → C₆H₅NH₂ + 2H₂O

Typical reagents and conditions:

• Tin and concentrated hydrochloric acid
• Heat under reflux
• Then add NaOH(aq)

Reflux means heating a reaction mixture with a condenser, so volatile substances condense and return to the flask rather than escaping.

In acid, phenylamine is initially converted into the phenylammonium salt, C₆H₅NH₃⁺Cl⁻. Adding NaOH(aq) releases the free amine, C₆H₅NH₂.

4. Basicity of amines

A Brønsted–Lowry base is a proton acceptor. Amines are bases because the lone pair on nitrogen can accept H⁺.

In water:

RNH₂ + H₂O ⇌ RNH₃⁺ + OH⁻

With hydrochloric acid:

RNH₂ + HCl → RNH₃⁺Cl⁻

This forms an ammonium salt.

Comparing ammonia, aliphatic amines and phenylamine

Aliphatic amines are usually stronger bases than ammonia. Alkyl groups have a positive inductive effect: they push electron density towards nitrogen, making the lone pair more available.

Phenylamine is weaker than ammonia. The nitrogen lone pair is partly delocalised into the benzene ring, so it is less available to accept H⁺.

Typical basicity order:

ethylamine > ammonia > phenylamine

5. Ethanoylation using ethanoyl chloride

Ethanoylation is the introduction of the ethanoyl group, CH₃CO–, into a molecule. When a primary amine reacts with ethanoyl chloride, an amide is formed.

General equation:

RNH₂ + CH₃COCl → CH₃CONHR + HCl

Example with methylamine:

CH₃NH₂ + CH₃COCl → CH₃CONHCH₃ + HCl

The product is N-methylethanamide.

This reaction is a nucleophilic addition–elimination reaction. The amine attacks the carbonyl carbon, chloride leaves, and a proton is lost to give the amide.

6. Reaction with cold nitric(III) acid

Nitric(III) acid, HNO₂, is also called nitrous acid. It is unstable, so it is generated in the reaction mixture using sodium nitrite and hydrochloric acid:

NaNO₂ + HCl → HNO₂ + NaCl

The reaction with primary amines depends strongly on whether the amine is aliphatic or aromatic.

Primary aliphatic amines

Primary aliphatic amines react with cold HNO₂ to form an alcohol, nitrogen gas and water:

RNH₂ + HNO₂ → ROH + N₂ + H₂O

Example:

CH₃CH₂NH₂ + HNO₂ → CH₃CH₂OH + N₂ + H₂O

You would observe effervescence because nitrogen gas is produced.

Primary aromatic amines

Primary aromatic amines form diazonium salts at 0–10 °C.

For phenylamine:

C₆H₅NH₂ + HNO₂ + HCl → C₆H₅N₂⁺Cl⁻ + 2H₂O

The product is benzenediazonium chloride.

7. Coupling reactions and azo dyes

Benzenediazonium salts react with activated aromatic compounds such as phenols and aromatic amines. This is called a coupling reaction.

With phenol in alkaline solution:

C₆H₅N₂⁺Cl⁻ + C₆H₅OH → C₆H₅N=NC₆H₄OH + HCl

With phenylamine:

C₆H₅N₂⁺Cl⁻ + C₆H₅NH₂ → C₆H₅N=NC₆H₄NH₂ + HCl

The product usually forms at the para position if it is available. If the para position is blocked, an ortho product may form.

For phenols, alkaline conditions form the phenoxide ion, which is more strongly activating. For aromatic amines, conditions must not be too acidic, because protonating the –NH₂ group would deactivate the ring.

The coupling mechanism is an electrophilic substitution reaction: the diazonium ion behaves as the electrophile, and the activated aromatic ring attacks it.

Practical outline for making an azo dye

A typical preparation uses cold solutions:

1. Dissolve phenylamine in dilute HCl and cool in an ice bath.
2. Add cold NaNO₂(aq) to form the diazonium salt at 0–10 °C.
3. Add this slowly to cold alkaline phenol solution, with stirring.
4. A brightly coloured azo dye forms, often as a precipitate.

8. Chromophores and the origin of colour

Azo dyes are coloured because they contain an extended delocalised π-electron system. Electrons can absorb visible light and move to a higher energy level.

The energy of absorbed light is linked to wavelength by:

$$E=\frac{hc}{\lambda }$$

So a smaller energy gap means absorption at a longer wavelength.

The colour you see is the colour of light not absorbed. If a dye absorbs blue light strongly, it may appear orange or yellow, which is the complementary colour.