Structure, classification and nomenclature

The essentials

In amines the nitrogen atom is sp³ hybridised. Three of its hybrid orbitals form bonds with hydrogen or carbon, and the fourth contains a lone pair of electrons. The molecule is therefore pyramidal, like ammonia, and the C–N–C or C–N–H angle is a little less than 109.5° (about 108° in trimethylamine). The lone pair is responsible for the basic and nucleophilic behaviour of amines.

Key points

• Primary (1°): one H of NH₃ replaced, R–NH₂. Secondary (2°): two replaced, R₂NH. Tertiary (3°): three replaced, R₃N.
• Simple amines have both alkyl groups the same; mixed amines have different groups.
• Common names: alkyl group + "amine", e.g. methylamine (CH₃NH₂), dimethylamine, triethylamine.
• IUPAC names: replace the "-e" of the alkane by "-amine", e.g. ethanamine (C₂H₅NH₂), propan-2-amine. For 2° and 3° amines the largest group is the parent and others are shown with the prefix N-, e.g. N-methylethanamine (CH₃NHC₂H₅), N,N-dimethylmethanamine ((CH₃)₃N).
• Diamines: ethane-1,2-diamine (H₂N–CH₂–CH₂–NH₂), hexane-1,6-diamine.
• Arylamines: C₆H₅NH₂ is aniline, also named benzenamine. In aniline the lone pair is partly delocalised into the ring.
• The classification depends on the number of carbon groups on nitrogen, not on the carbon to which nitrogen is attached (unlike alcohols).

Exam tips

• When naming, choose the longest carbon chain attached to N as the parent; other groups on N get the locant "N".

Common mistakes

• Calling (CH₃)₃C–NH₂ a tertiary amine; it is a primary amine because N carries only one carbon group.