The nitrogen lone pair
Amines are weak bases. The nitrogen atom carries a lone pair of electrons that can form a dative covalent bond with a proton. An amine can therefore act as a Brønsted–Lowry proton acceptor.
Methylamine establishes the following equilibrium in water:
Only a low concentration of hydroxide ions is produced, so methylamine behaves as a weak base.
Reaction with acids
Amines accept protons from acids and form ammonium salts. For example, methylamine reacts with hydrochloric acid to produce methylammonium chloride:
For a primary amine, the proton-transfer step can be represented generally as:
Before protonation
The amine contains an available lone pair on the nitrogen atom.
Methylamine is .
After protonation
The nitrogen has accepted and the resulting ammonium ion carries a positive charge.
Amine salts are ionic compounds. Those containing small alkyl groups are water-soluble, with solubility decreasing as the hydrocarbon chain becomes longer.
Phenylamine is only slightly soluble in water, whereas phenylammonium chloride is soluble. Treatment of an amine salt with sodium hydroxide converts it back into the amine.