Amines are classified as primary (1°), secondary (2°), or tertiary (3°) according to the number of alkyl or aryl groups attached to nitrogen. A quaternary ammonium salt features a nitrogen atom bonded to four substituents with a positive charge. The nitrogen atom in amines is sp³-hybridized with a lone pair, rendering amines both basic and nucleophilic. Nomenclature follows the suffix -amine for simple compounds, with the parent alkyl group named first (methylamine, ethylamine), while more complex amines are named as amino-substituted alkanes.
Basicity of Amines
The basicity of amines is measured by pKb or the conjugate acid pKa. Alkyl amines in water have conjugate acid pKa values around 10–11, making them stronger bases than ammonia (pKa ~ 9.25). Alkyl groups donate electron density through inductive effects, stabilizing the protonated form. Aromatic amines such as aniline (pKa ~ 4.6) are much weaker bases because the nitrogen lone pair is delocalized into the aromatic ring. Pyridine (a heteroaromatic amine) has a pKa of ~5.2, while pyrrole (pKa ~ 0.4) is essentially nonbasic — the lone pair is part of the aromatic sextet.
Synthesis of Amines
Reduction of nitro compounds (Sn/HCl, Fe/HCl, or H₂/Pd-C) provides a reliable route to primary aromatic amines from the corresponding nitroarenes. Reductive amination involves condensation of an aldehyde or ketone with ammonia or an amine to form an imine, which is reduced in situ by NaBH₃CN or NaBH(OAc)₃ to the corresponding amine. The Gabriel synthesis uses phthalimide as a protected ammonia equivalent: alkylation with an alkyl halide followed by hydrazinolysis releases the primary amine. The Hofmann rearrangement converts primary amides to primary amines with loss of one carbon.
Reactions of Amines
Amines undergo alkylation with alkyl halides, though over-alkylation to secondary, tertiary, and quaternary products is a practical challenge. Acylation with acyl chlorides or anhydrides produces amides, which are less basic and serve as protecting groups. Diazotization of primary aromatic amines with NaNO₂/HCl at 0°C generates aryl diazonium salts, which are versatile intermediates in the Sandmeyer reaction (replacement of N₂⁺ by Cl, Br, CN, OH) and in azo coupling to form colored azo dyes.
Hofmann Elimination and Phase-Transfer Catalysis
Hofmann elimination (exhaustive methylation followed by elimination with Ag₂O/heat) converts a primary amine to an alkene. This E2 elimination follows the Hofmann rule, giving the less substituted alkene as the major product because the large trimethylammonium leaving group sterically hinders access to the more substituted β-hydrogens. Quaternary ammonium salts (e.g., tetrabutylammonium bromide, TBAB) are widely used as phase-transfer catalysts, shuttling inorganic anions (CN⁻, OH⁻, MnO₄⁻) into organic solvents to facilitate reactions between immiscible phases.
Applications
Amines are ubiquitous in pharmaceuticals — approximately 80% of marketed drugs contain an amine functional group. Alkaloids (nicotine, morphine, quinine) are naturally occurring amines with potent biological activity. Aromatic amines are intermediates in dye manufacturing (aniline dyes), and polyfunctional amines such as ethylenediamine and hexamethylenediamine are monomers for polyamide (nylon) production. In biochemistry, amines appear in amino acids, neurotransmitters (dopamine, serotonin), and nucleic acid bases.
