3.11.2 Amines: Base Properties

3.11.2 Amines: Base Properties: Explain why amines act as Brønsted–Lowry bases, compare the base strength of ammonia, alkylamines and phenylamine, and write equations for their reactions with acids.

Lone pair

The nitrogen atom of an amine has a lone pair of electrons that can accept a proton by forming a dative covalent bond.

Brønsted–Lowry base

A base is a proton acceptor, so amines react with acids and also take protons from water to a small extent.

Salt formation

C₂H₅NH₂ + HCl → C₂H₅NH₃⁺Cl⁻ forms the salt ethylammonium chloride, which is a solid and is soluble in water.

Reaction with water

CH₃NH₂ + H₂O ⇌ CH₃NH₃⁺ + OH⁻ gives an alkaline solution, so the amine turns red litmus blue.

Release of amine

Adding NaOH to the salt displaces the amine: C₂H₅NH₃⁺Cl⁻ + NaOH → C₂H₅NH₂ + NaCl + H₂O.

Comparing Base Strength

Alkylamines

Alkyl groups push electron density towards nitrogen,
- so the lone pair is more available to accept a proton and the base is stronger than ammonia.

Ammonia

With only hydrogen atoms attached, ammonia has no electron-donating groups and sits in the middle of the order.

Phenylamine

The nitrogen lone pair overlaps with the π system of the ring and is partly delocalised, so it is less available and the base is weaker.

Order

Basic strength decreases in the order ethylamine > ammonia > phenylamine.

Compound	Effect on nitrogen lone pair	Relative basicity
Ethylamine	density increased by the alkyl group	strongest
Ammonia	no donating or withdrawing group	medium
Phenylamine	lone pair delocalised into the ring	weakest

Small amines are soluble in water because they can form hydrogen bonds with water molecules using the lone pair on nitrogen.

Solubility falls as the carbon chain gets longer, since the non-polar part of the molecule cannot hydrogen bond to water.

Smaller amines are gases or volatile liquids with a fishy smell, which is produced when proteins in fish decompose.

Washing hands with lemon juice or dilute acid neutralises the amine and removes the smell by forming a non-volatile salt.

3.11.2 – Key Points for the Exam

Brønsted–Lowry base: A species that accepts a proton.
Dative covalent bond: A bond in which both shared electrons come from one atom.
Ammonium salt: An ionic compound containing a positively charged nitrogen, such as C₂H₅NH₃⁺Cl⁻.
Delocalisation: Spreading of electrons over several atoms, which reduces availability.

Common Mistakes

Saying phenylamine is the strongest base; its lone pair is delocalised into the ring.
Writing the salt as C₂H₅NH₂Cl instead of C₂H₅NH₃⁺Cl⁻.
Forgetting that the lone pair, not the N–H bond, accepts the proton.
Explaining basicity by size of molecule instead of electron density on nitrogen.