Amines and aromatic amines: preparation and properties
Compounds formed when hydrogen atoms of ammonia NH₃ are replaced by hydrocarbon radicals are amines. One H replaced — primary (CH₃NH₂ methylamine), two — secondary ((CH₃)₂NH dimethylamine), three — tertiary amine ((CH₃)₃N trimethylamine). If the radical is a benzene ring it is an aromatic amine: C₆H₅NH₂ aniline. Preparation: acting on a haloalkane with ammonia (C₂H₅Cl + NH₃ → C₂H₅NH₂ · HCl) and reducing nitro compounds (C₆H₅NO₂ + 3H₂ → C₆H₅NH₂ + 2H₂O). Because nitrogen has a lone electron pair, amines are bases: CH₃NH₂ + H₂O ⇌ CH₃NH₃⁺ + OH⁻ turns litmus blue and phenolphthalein crimson; with acids they form salts: CH₃NH₂ + HCl → CH₃NH₃Cl. Aliphatic amines are stronger bases than ammonia, whereas aniline is weaker because the nitrogen lone pair is drawn toward the benzene ring. Aniline gives a white precipitate with bromine water: C₆H₅NH₂ + 3Br₂ → C₆H₂Br₃NH₂ + 3HBr. Amines burn to CO₂, H₂O and N₂: 4CH₃NH₂ + 9O₂ → 4CO₂ + 10H₂O + 2N₂. Lower amines are gases or volatile liquids with a sharp fishy smell; aniline is poisonous, is absorbed through the skin and is used in the synthesis of dyes and drugs.
Draw on the board the series NH₃ → CH₃NH₂ → (CH₃)₂NH → (CH₃)₃N and write how many H remain on N in each. No experiments with amines or aniline — they are toxic and sharp-smelling.