Amines
Amines are derivatives of ammonia in which one or more hydrogen atoms are replaced by alkyl or aryl groups. The nitrogen is sp3 hybridised with a lone pair, so amines are pyramidal (C-N-C angle about 108 degrees in trimethylamine).
Classification and nomenclature
- Primary (1): RNH2 or ArNH2; Secondary (2): R-NHR'; Tertiary (3): R3N.
- Simple amines have identical groups; mixed amines have different groups.
- IUPAC: primary amines = alkanamines (CH3NH2 = methanamine; C6H5NH2 = benzenamine/aniline). Use the locant N for substituents on nitrogen (CH3NHCH2CH3 = N-methylethanamine).
Preparation
- Reduction of nitro compounds (H2/Ni,Pd,Pt or Fe/HCl) gives amines.
- Ammonolysis of alkyl halides (gives a mixture of 1, 2, 3 amines and quaternary salt; reactivity RI > RBr > RCl).
- Reduction of nitriles (LiAlH4 or H2/catalyst) - ascent of series, one carbon more.
- Reduction of amides with LiAlH4.
- Gabriel phthalimide synthesis - primary amines only (not aromatic).
- Hoffmann bromamide degradation (amide + Br2 + NaOH) - amine has one carbon less.
Physical properties
Lower amines are fishy-smelling gases; solubility falls with molar mass. Boiling point order of isomers: primary > secondary > tertiary (H-bonding).Basic character
- Aliphatic amines are stronger bases than ammonia (+I effect); aromatic amines are weaker (lone pair in conjugation with ring).
- Gas phase: 3 > 2 > 1 > NH3. Aqueous order is irregular due to solvation and steric effects: (C2H5)2NH > (C2H5)3N > C2H5NH2 > NH3 and (CH3)2NH > CH3NH2 > (CH3)3N > NH3.
- In substituted anilines, electron releasing groups (-OCH3, -CH3) raise basicity; electron withdrawing groups (-NO2, -COOH, -X) lower it.