Primary and secondary amines

Amines are classified by the number of carbon groups on the nitrogen: primary (RNH₂), secondary (R₂NH) and tertiary (R₃N). (Note this differs from alcohols, which are classified by the carbon bearing the –OH.)

Two preparation routes for a primary amine:

• Halogenoalkane + excess ammonia (in ethanol, heated under pressure): nucleophilic substitution gives the amine. Because the amine is itself a nucleophile, excess NH₃ is used to limit further substitution to 2°/3° amines.
• Reduction of a nitrile (LiAlH₄, or H₂ with a Ni catalyst): $\text{CH}_3\text{CN} + 4[\text{H}] \rightarrow \text{CH}_3\text{CH}_2\text{NH}_2$ This gives a clean primary amine, but it adds a carbon (the nitrile was made with KCN).

Amines are bases because the nitrogen lone pair accepts a proton: $\text{C}_2\text{H}_5\text{NH}_2 + \text{HCl} \rightarrow \text{C}_2\text{H}_5\text{NH}_3^+\text{Cl}^-$

Base strength depends on how available the lone pair is:

• Ethylamine > ammonia: the alkyl group is electron-donating (+I), pushing electron density onto N and making the lone pair more available.
• Phenylamine < ammonia: the N lone pair is delocalised into the benzene ring, so it is less available.

Hence the order ethylamine > ammonia > phenylamine.

The same nitrogen lone pair that makes amines basic also makes them nucleophiles. They attack electron-deficient carbon centres. The most important example is with an acyl chloride, which forms an N-substituted amide: $\text{CH}_3\text{NH}_2 + \text{CH}_3\text{COCl} \rightarrow \text{CH}_3\text{CONHCH}_3 + \text{HCl}$

Amines also react with halogenoalkanes by nucleophilic substitution (which is why the NH₃ preparation gives a mixture — the product amine reacts further). This nucleophilic behaviour underlies amide and peptide synthesis.