aminenitrogen

Amine nitrogen refers to the nitrogen atom characteristic of organic amines. In these compounds, nitrogen is bonded to one or more carbon substituents and, depending on the degree of substitution, hydrogen atoms can also be present. The three broad classes are primary amines (R-NH2, one carbon substituent and two hydrogens), secondary amines (R2NH, two carbon substituents and one hydrogen), and tertiary amines (R3N, three carbon substituents and no hydrogens). Quaternary ammonium species (R4N+) arise when nitrogen bears four carbon substituents and carries a positive charge; these are typically salts rather than neutral amines.

The amine nitrogen is usually sp3 hybridized and adopts a pyramidal geometry due to the lone pair on nitrogen. The lone pair makes amines basic and nucleophilic: they readily accept protons to form ammonium ions (RNH3+ or R2NH2+) and participate in many substitution and condensation reactions. Aliphatic amines generally have conjugate acid pKa values around 9–11, reflecting their basicity; aromatic amines are less basic due to resonance with the ring.

Reactions involving amine nitrogen include alkylation, acylation (to form amides, when the nitrogen attacks carbonyl compounds), and reductive amination of carbonyl compounds to give primary, secondary, or tertiary amines. In biology, amino groups are central to amino acids and many neurotransmitters; at physiological pH they commonly exist as protonated ammonium forms and function in hydrogen bonding and metal coordination.

Spectroscopically, primary and secondary amines show N–H stretches in infrared spectra, whereas tertiary amines do not. Nuclear magnetic resonance studies, including 15N NMR, provide detailed information on the nitrogen environment in complex molecules. Amine nitrogen thus underpins a wide range of chemical reactivity and biological function.