AQA A-Level Chemistry - Amines

Main Ideas

1. What Amines Are

Amines are organic compounds containing a nitrogen atom with a lone pair. A simple primary amine has an –NH₂ group attached to an alkyl group.

Amines are classified by the number of carbon-containing groups attached to the nitrogen:

• Primary: one group
• Secondary: two groups
• Tertiary: three groups
• Quaternary ammonium ion: four groups, with a positive charge and no lone pair on nitrogen

2. Naming Amines

Names are based on the groups attached to nitrogen, with “amine” at the end. Examples include:

• Methylamine: one methyl group
• Dimethylamine: two methyl groups
• Ethylmethylamine: one ethyl group and one methyl group; list the groups alphabetically
• Trimethylamine: three methyl groups

Structural formulas show how many hydrogen atoms remain on nitrogen. For example, a primary amine has –NH₂, while a secondary amine has –NH–.

3. Making Amines from Haloalkanes

A haloalkane can react with ammonia by nucleophilic substitution:

1. The lone pair on ammonia attacks the carbon bonded to the halogen.
2. The carbon–halogen bond breaks, and the halide leaves.
3. Loss of a hydrogen produces a primary amine.

The newly formed amine still has a lone pair, so it can react with more haloalkane. Further substitution can produce secondary and tertiary amines, and eventually a quaternary ammonium ion.

To favour the primary amine, use excess ammonia. This reduces the chance that the product amine will react further.

Ammonium salts are also formed during these reactions. The halide ion and the species accepting a proton contribute to the salt.

4. Quaternary Ammonium Ions and Conditioners

Quaternary ammonium compounds have four carbon-containing groups attached to nitrogen. Because nitrogen has no available lone pair, these compounds do not undergo further nucleophilic substitution in the same way as the earlier amines.

Some are used as cationic surfactants in hair and fabric conditioners. Their long hydrocarbon chains help them associate with hair or fabric, forming a layer that reduces static build-up.

5. Making Amines by Reducing Nitriles

Nitriles contain the –C≡N group and can be reduced to primary amines. The nitrile carbon becomes part of the amine’s carbon chain.

Two reduction methods are described:

• Lithium aluminium hydride (LiAlH₄) in ether
• Hydrogen gas with a nickel catalyst

Sodium borohydride is not strong enough for this reduction. A mechanism is not required for this topic; the reactants and conditions should be known.

6. Amines as Bases

Amines act as Brønsted–Lowry bases: they accept a proton using nitrogen’s lone pair.

The relative basicity presented is:

Primary amine > ammonia > aromatic amine

The key concept for explaining this comparison is the availability of the nitrogen lone pair: the more available it is, the more readily it can bond to a proton.

• In an alkyl amine, the alkyl group’s inductive effect pushes electron density towards nitrogen, making the lone pair more available than in ammonia.
• In an aromatic amine, such as phenylamine, the nitrogen lone pair is partly delocalised into the benzene ring. It is therefore less available to accept a proton, making the amine a weaker base.

When explaining the comparison, explicitly discuss the availability of the lone pair and, for aromatic amines, its delocalisation.

7. Making an Aromatic Amine

Phenylamine can be made by reducing nitrobenzene:

1. Reflux nitrobenzene with tin and concentrated hydrochloric acid.
2. This forms a phenylammonium salt as an intermediate.
3. Add sodium hydroxide to remove a proton and obtain phenylamine.

Speaker or Source Featured

• Eliot Rintoul — the presenter and sole speaker identifiable from the subtitles and video context.