Class 12 Chemistry Amines Preparation of Amines

Preparation of Amines

  • Amines are prepared by the following methods:
  • 1. Reduction of nitro compounds
  1. Ammonolysis of alkyl halides
  2. Reduction of nitriles
  3. Reduction of amides
  4. Gabriel phthalimide synthesis
  5. Hoffmann bromamide degradation reaction

 

  1. 1. Reduction of nitro compounds
  • Nitro compounds are reduced to amines by passing hydrogen gas in the presence of finely divided nickel, palladium or platinum and also by reduction with metals in acidic medium.

R- NO2 + 3H2 à R-NH2 + 2H2O   {In the presence of Ni, Pt or Pd}

Ar- NO + 3H2à Ar- NH2 + 2H2O

  • Nitro alkanes can also be similarly reduced to the corresponding alkanamines.
  • Reduction with iron scrap and hydrochloric acid is preferred because FeCl2 formed gets hydrolysed to release hydrochloric acid during the reaction.
  • Thus, only a small amount of hydrochloric acid is required to initiate the reaction.
  • Nitro compounds can also be reduced with active metals such as Fe, Sn, Zn etc. with conc. HCl

R- NO2 + 3H2 à R- NH2 + 2H2O   {In the presence of Sn/HCl}

Ar- NO2 + 3H2 à Ar- NH2 + 2H2O

 

  1. Ammonolysis of alkyl halides
  • The carbon - halogen bond in alkyl or benzyl halides can be easily cleaved by a nucleophile.

Class_12_Chemistry_Amines_Preparation_Of_Amines_Ammonylsis

  • Therefore, an alkyl or benzyl halide on reaction with an ethanolic solution of ammonia undergoes nucleophilic substitution reaction in which the halogen atom is replaced by an amino (–NH2) group.
  • This process of cleavage of the C–X bond by ammonia molecule is known as ammonolysis.

Class_12_Chemistry_Amines_Preparation_Of_Amines_Ammonylsis1

  • The reaction is carried out in a sealed tube at 373 K. The primary amine thus obtained behaves as a nucleophile and can further react with alkyl halide to form secondary and tertiary amines, and finally quaternary ammonium salt.
  • The free amine can be obtained from the ammonium salt by treatment with a strong base:

Class_12_Chemistry_Amines_Preparation_Of_Amines_Ammonylsis2

  • Disadvantages:-
  • When ammonia is taken in excess primary amine is formed as main Product
  • When alkyl halide is used in excess quaternary ammonium salt is formed as main product
  • Note: - This method is not suitable for preparation of aryl amines because aryl amines are relatively less reactive than alkyl halides towards nucleophilic substitution reactions.
  1. Reduction of nitriles
  • Nitriles on reduction with lithium aluminium hydride (LiAlH4) or catalytic hydrogenation produce primary amines.
  • This reaction is used for ascent of amine series, i.e., for preparation of amines containing one carbon atom more than the starting amine.

Class_12_Chemistry_Amines_Preparation_Of_Amines_Reaction1

  1. Reduction of amides
  • The amides on reduction with lithium aluminium hydride yield amines.

Class_12_Chemistry_Amines_Preparation_Of_Amines_Reaction2

  1. Gabriel phthalimide synthesis
  • Gabriel synthesis is used for the preparation of primary amines.
  • Phthalimide on treatment with ethanolic potassium hydroxide forms potassium salt of phthalimide which on heating with alkyl halide followed by alkaline hydrolysis produces the corresponding primary amine.
  • Aromatic primary amines cannot be prepared by this method because aryl halides do not undergo nucleophilic substitution with the anion formed by phthalimide.

 Class_12_Chemistry_Amines_Preparation_Of_Amines_GabrielReaction

  1. Hoffmann bromamide degradation reaction
  • Hoffmann developed a method for preparation of primary amines by treating an amide with bromine in an aqueous or ethanolic solution of sodium hydroxide.
  • In this degradation reaction, migration of an alkyl or aryl group takes place from carbonyl carbon of the amide to the nitrogen atom.
  • The amine so formed contains one carbon less than that present in the amide.

Class_12_Chemistry_Amines_Preparation_Of_Amines_Hoffmann_BromamideReaction

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