Acid and Base Reactions with Amines

Why are amines basic?

The nitrogen atom has a lone pair of electrons. A Brønsted–Lowry base is a proton acceptor, and the lone pair can form a dative covalent bond with an H⁺ ion. For methylamine: CH₃NH₂ + H⁺ → CH₃NH₃⁺. Amines are weak bases because they only partially accept protons from water, as in ammonia.

Write an equation to show the basicity of an amine in water.

CH₃NH₂ + H₂O ⇌ CH₃NH₃⁺ + OH⁻. The amine accepts a proton from water. This produces hydroxide ions, which is why aqueous solutions of amines are alkaline.

Why is methylamine a stronger base than ammonia, but phenylamine a weaker base than ammonia?

Base strength depends on how available the nitrogen lone pair is for accepting a proton.

• Methylamine: the alkyl group is electron-donating (a positive inductive effect). It pushes electron density onto the nitrogen, which makes the lone pair more available, so methylamine is a stronger base than ammonia.

• Phenylamine: the nitrogen lone pair is partially delocalised into the π system of the benzene ring. This delocalisation draws electron density away from the nitrogen, so the lone pair is less available to accept a proton(polarise). Phenylamine is therefore a weaker base than ammonia.

What happens when an amine reacts with a dilute acid?

The amine acts as a base, accepting a proton to form a soluble ammonium salt. The general equation is RNH₂ + HCl → RNH₃⁺Cl⁻.

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

• C₆H₅NH₂ + HCl → C₆H₅NH₃⁺Cl⁻ (phenylammonium chloride)

• 2CH₃NH₂ + H₂SO₄ → (CH₃NH₃⁺)₂SO₄²⁻

Secondary amines give dialkylammonium salts in the same way, e.g. diethylamine and HCl give diethylammonium chloride.

Why is this reaction useful in practice?

Amines are often only slightly soluble in water, but their ammonium salts are ionic and dissolve readily. Adding dilute acid can therefore dissolve an amine, and adding alkali reverses the reaction by regenerating the free amine. This is often used to separate an amine from a mixture.