: 200
Amines are classified by the number of groups on nitrogen: primary (RNH2), secondary (R2NH), tertiary (R3N), and quaternary ammonium salts (R4N+X-). This classification refers to nitrogen substitution, not carbon type — a common point of confusion.
Nitrogen in amines is sp3 hybridised with pyramidal geometry. The lone pair on nitrogen is the source of both basicity (proton acceptance) and nucleophilicity (attack on electrophilic carbon). In aniline, this lone pair is partially delocalised into the benzene ring (+M effect), giving N partial sp2 character and reducing both basicity and nucleophilicity compared to aliphatic amines.
Amines are further classified as aliphatic (alkyl groups on N) or aromatic (aryl group directly on N). Key examples: methylamine (1° aliphatic), aniline (1° aromatic), N-methylaniline (2° aromatic), N,N-dimethylaniline (3° aromatic). Benzylamine (C6H5CH2NH2) is classified as 1° aliphatic despite having a phenyl group — the -CH2- insulates the amino group from the ring.