Amines

Amines are organic compounds derived from ammonia by replacing one or more hydrogen atoms with alkyl or aryl groups. They are classified by the number of carbon-containing substituents on the nitrogen: primary amines have one substituent, secondary amines have two, and tertiary amines have three. When four substituents are present and the nitrogen bears no lone pair, the compound is a quaternary ammonium salt.

Most amines feature a trigonal pyramidal geometry at nitrogen due to the lone pair. They behave as bases, forming ammonium ions by protonation. Basicity generally increases with alkyl substitution because alkyl groups donate electron density, but aryl amines (such as aniline) are less basic because the lone pair can be delocalized into the aromatic ring. Amides, in contrast, have a carbonyl-adjacent nitrogen and are much less basic.

Physical properties vary with size and substitution. Primary and secondary amines often engage in hydrogen bonding and have higher boiling points than alkanes of comparable mass, whereas tertiary amines lack N–H bonds but can still accept protons. Many small amines are miscible with water; larger ones become less soluble as the hydrophobic carbon groups predominate.

Amine synthesis proceeds by several routes, including reduction of nitro compounds or nitriles, reductive amination of aldehydes or ketones, and alkylation of ammonia or existing amines. Several specialized methods exist, such as the Gabriel synthesis for primary amines. In reactions, amines act as nucleophiles or bases and form amides by acylation, imines by reaction with carbonyl compounds, or quaternary ammonium salts upon alkylation. Applications include intermediates in pharmaceuticals, polymers, dyes, and catalysts, making amines among the most versatile nitrogen-containing compounds.