Preparation of Amines

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Preparation of Amines

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1. Reduction of Nitro Compounds

  • By passing H2 gas in presence of finely divided Ni, Pd, or Pt

 

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  • By reduction with metals in acidic medium

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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.

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  • Primary amine obtained behaves as a nucleophile and further reacts as

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  • 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

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  • Catalytic hydrogenation of nitriles

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4. Reduction of Amides

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5. Gabriel Pthalimide Synthesis

  • Used for the preparation of primary amine
  • Reactions involved in the preparation of primary amine from phthalimide are as follows:

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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

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  • 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