Preparation of Amines
1. Reduction of Nitro Compounds
- By passing H2 gas in presence of finely divided Ni, Pd, or Pt
- By reduction with metals in acidic medium
2. Ammonolysis of Alkyl Halides
- The carbon−halogen (C−X) bond in alkyl or benzyl halides can be easily cleaved by a nucleophile.
- The process of cleavage of C−X bond by ammonia molecule is called ammonolysis.
- The reaction is carried out in a sealed tube at 373 K.
- An alkyl or benzyl halide reacts with an ethanolic solution of NH3 as follows.
- Primary amine obtained behaves as a nucleophile and further reacts as
- Order of reactivity of halides with amines − RI > RBr > RCl
- Disadvantage − Mixture of primary, secondary, tertiary amines and quaternary ammonium salts is produced.
3. Reduction of Nitrites
- Reduction of nitrites with LiAlH4
- Catalytic hydrogenation of nitriles
4. Reduction of Amides
5. Gabriel Pthalimide Synthesis
- Used for the preparation of primary amine
- Reactions involved in the preparation of primary amine from phthalimide are as follows:
Aromatic primary amines cannot be prepared by this process.
Reason − Aryl halides do not undergo nucleophilic substitution with the anion formed by phthalimide.
6. Hoffmann Bromamide Degradation Reaction
- Preparation of primary amines by treating an amide with Br2 in an aqueous or ethanolic solution of NaOH
- Amine formed contains one carbon less than that present in the amide.
- Lower aliphatic amines are gases with fishy odour.
- Primary amines with three or more carbon atoms are liquid.
- Higher ones are solid.
- Aniline and other arylamines are colourless, but get coloured on storage due to atmospheric oxidation.
- Lower aliphatic amines are soluble in water. Reason: Form H-bonds with water molecules
- Solubility decreases with increase in molar mass.
- Higher amines are insoluble in water.
- Amines are soluble in organic solvents like alcohol, ether and benzene.
- Order of boiling points of isomeric amines is: Primary > Secondary > Tertiary