A diamine is an amine with exactly two amino groups. Diamines are used as monomers to prepare polyamides, polyimides, and polyureas. The term diamine refers mostly to primary diamines, as those are the most reactive. In terms of quantities produced, 1,6-diaminohexane (a precursor to Nylon 6-6) is most important, followed by ethylenediamine. Vicinal diamines (1,2-diamines) are a structural motif in many biological compounds and are used as ligands in coordination chemistry. Geminal diamines (1,1-diamines) are usually reactive intermediates in transimination reactions and the reduction of amidines, in aqueous conditions they preferentially eliminate the less basic amine to leave an iminium ion. Some stable geminal diamines have been isolated.

Aliphatic diamines

Linear

Branched

Derivatives of ethylenediamine are prominent:

Cyclic

Xylylenediamines

Xylylenediamines are classified as alkylamines since the amine is not directly attached to an aromatic ring.

Aromatic diamines

Three phenylenediamines are known: Various N-methylated derivatives of the phenylenediamines are known: Examples with two aromatic rings include derivatives of biphenyl and naphthalene: