Amines as bases and nucleophiles. Preparation of aliphatic amines by reaction of halogenoalkanes with ammonia and by reduction of nitriles. Preparation of aromatic amines by reduction of nitro compounds. The relative base strength of ammonia, primary aliphatic and aromatic amines. Amines as nucleophiles in further substitution.

What this dot point is asking

AQA wants you to prepare aliphatic amines from halogenoalkanes and nitriles and aromatic amines from nitro compounds, explain why amines are bases, rank the base strengths of ammonia and primary aliphatic and aromatic amines, and explain amines acting as nucleophiles.

Preparation of amines

From halogenoalkanes (excess ammonia): nucleophilic substitution gives a primary amine, then further substitution gives secondary, tertiary amines and quaternary ammonium salts. Excess ammonia favours the primary amine.

By reduction of nitriles: a nitrile is reduced (using $LiAlH_4$, or $H_2$ with a nickel catalyst) to a primary amine. This usefully adds a carbon to the chain.

Aromatic amines: nitrobenzene is reduced with tin and concentrated hydrochloric acid, then made alkaline with $NaOH$, to give phenylamine.

The choice of preparation depends on the target. Using excess ammonia with a halogenoalkane is cheap but gives a mixture of primary, secondary and tertiary amines plus a quaternary salt, because each amine product is itself a nucleophile that can react further; excess ammonia simply makes the primary amine the largest fraction. The nitrile route is cleaner and is the one to choose when the carbon chain needs to be extended by one, since the cyanide adds a carbon and the subsequent reduction converts the nitrile group to a primary amine without further substitution.

Base strength

• Aliphatic amines are stronger bases than ammonia: the alkyl group is electron-releasing (positive inductive effect), increasing electron density on the nitrogen so the lone pair is more available.
• Aromatic amines (phenylamine) are weaker bases than ammonia: the nitrogen lone pair is partly delocalised into the benzene ring, so it is less available to accept a proton.

Amines as nucleophiles

The lone pair lets amines act as nucleophiles. They react with halogenoalkanes (further alkylation, adding alkyl groups one at a time up to a quaternary ammonium salt) and with acyl chlorides to form N-substituted amides, e.g.

The same basicity explains a simple reaction with acids: an amine reacts with hydrochloric acid to form a soluble salt, for example $\text{CH}_3\text{CH}_2\text{NH}_2 + \text{HCl} \rightarrow \text{CH}_3\text{CH}_2\text{NH}_3^+\text{Cl}^-$ (ethylammonium chloride). This is reversed by adding a stronger base such as sodium hydroxide, which deprotonates the salt and regenerates the free amine, and is the step used to liberate phenylamine after its preparation. Quaternary ammonium salts with a long hydrocarbon chain act as cationic surfactants, used in fabric softeners and hair conditioners, which is a common applied context in AQA questions.

Try this

Q1. Why is a primary aliphatic amine a stronger base than ammonia? [2 marks]

• Cue. The alkyl group pushes electron density onto N, making the lone pair more available to accept $H^+$.

Q2. Name the reagents used to reduce nitrobenzene to phenylamine. [1 mark]

• Cue. Tin and concentrated hydrochloric acid (then $NaOH$).