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Monday, 27 February 2017

Amines - I

Amines are regarded as derivatives of ammonia in which one, two or all three hydrogen atoms are replaced by alkyl or aryl group.

Classification of amines:

Preparation of amines:
(i) By reduction of nitro compounds: Nitro compounds can be catalytically reduced by passing hydrogen gas in presence of Raney Ni, finely divided Pt or Pd as catalyst at room temperature.

Nitro compounds can also be reduced with active metals such as Fe, Sn, Zn etc. with conc. HCl.

(ii) By Hoffmann’s method (Ammonolysis of alkyl halides): Reaction of alkyl halides with an ethanolic solution of ammonia in a sealed tube at 373 K forms a mixture of primary, secondary and tertiary amine and finally quarternary ammonium salt. Process of cleavage of C-X bond by ammonia is called ammonolysis.

The free amine can be obtained from the ammonium salt by treatment with a strong base:

Order of reactivity of halides is: RI> RBr> RCl

Larger the size of halogen atom easier is the cleavage of R-X bond

Limitations of Hoffmann’s method:
Method gives mixture of amines which are difficult to separate in a laboratory.

Methods to get only one product by Hoffmann’s method:
(i) When ammonia is taken in excess primary amine is formed as main product

(ii) When alkyl halide is used in excess quarternary ammonium salt is formed as main product.

Method is not suitable for preparation of aryl amines because aryl amines are relatively less reactive than alkyl halides towards nucleophilic substitution reactions.

(iii) By reduction of nitriles: Nitriles can be reduced to amines using H/ Ni , LiAlH2 or
Na(Hg) / C2H5 OH

(iv) By reduction of amides: Amides are reduced to corresponding amines by LiAlH4

(v) By Gabriel phthalimide synthesis: Gabriel synthesis is used for the preparation of primary amines. When phthalimide is treated with ethanolic potassium hydroxide, it forms potassium salt of phthalimide which on heating further 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 potassium phthalimide.

(vi) By Hoffmann bromamide degradation reaction: Primary amines can be prepared from amides by treatment with Br2 and KOH. Amine contains one carbon atom less than the parent amide

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