Base properties of aminesAQA A-Level Chemistry: Revision notes

A base is a proton acceptor. Amines are bases because the nitrogen atom has a lone pair of electrons that forms a dative covalent bond to H⁺.

In water: CH₃NH₂ + H₂O ⇌ CH₃NH₃⁺ + OH⁻

The equilibrium lies well to the left, so only a small proportion of molecules are protonated. Amines are therefore weak bases. They also react with acids to form salts:

CH₃CH₂NH₂ + HCl → CH₃CH₂NH₃⁺Cl⁻ (ethylammonium chloride)

For solutions of the same concentration, the stronger the base the higher the pH:

• primary aliphatic amine (e.g. methylamine, pH about 11.8 at 0.100 mol dm⁻³)
• ammonia (pH about 11.1)
• primary aromatic amine (phenylamine, pH about 8.8)

The order of base strength is primary aliphatic amine > ammonia > phenylamine.

Alkyl groups release electron density towards the nitrogen atom (a positive inductive effect).

This increases the electron density on nitrogen, so its lone pair is more available to accept a proton. Aliphatic amines are therefore stronger bases than ammonia, which has only hydrogen atoms attached.

In phenylamine the nitrogen's lone pair overlaps with the delocalised π system of the benzene ring and becomes partly delocalised into the ring.

The lone pair is therefore less available to accept a proton, which makes phenylamine a weaker base than ammonia.

Because amines are bases, they react with acids to form ionic salts, which are usually soluble in water even when the amine itself is only slightly soluble. Adding a stronger base such as NaOH then removes H⁺ from the ammonium ion and releases the free amine:

C₆H₅NH₃⁺ + OH⁻ → C₆H₅NH₂ + H₂O

Titrations of amines with acid are 1 : 1 for a primary amine. For example, 25.0 cm³ of 0.100 mol dm⁻³ ethylamine needs 20.0 cm³ of 0.125 mol dm⁻³ HCl.