Tertiary amines are formed by replacing another hydrogen atom in a secondary amine with an alkyl/aryl group. Amines are ‘simple’ when all the alkyl or aryl groups are the same and ‘mixed’ when they are different.

A secondary amine, such as R-NHR’, is formed when two hydrogen atoms of ammonia or one hydrogen atom of R-NH₂ are replaced by another alkyl/aryl group (R’).

Amines are classified as primary (1⁰), secondary (2⁰), and tertiary (3⁰) depending on the number of hydrogen atoms replaced by alkyl or aryl groups in the ammonia molecule.

The presence of an unshared pair of electrons on nitrogen causes the angle C–N–E (where E is C or H) to be less than 109.5°; for example, it is 108° in trimethylamine.