Amines Notes
Methods of Preparation of Amines
Amines are an organic compounds that are formed when one or more hydrogen atoms in an ammonia molecule are replaced with an alkyl group. Amines can be prepared by a number of methods, some of which are discussed below-
Reduction of Nitro Compounds
Amines can be prepared by reduction of nitro compounds with hydrogen gas in the presence of a metal catalyst, such as nickel, palladium, or platinum. The reduction can also be carried out with metals in an acidic medium.
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.
Gabriel 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.
Ammonolysis of Alkyl Halides
When alkyl halide is heated with an alcoholic solution of excess ammonia, it undergoes a nucleophilic substitution reaction in which the halogen atom is replaced by an amino (–NH2) group to form primary amine. This process of breaking of the C – X bond by ammonia is known as ammonolysis. 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:
Reduction of Amides
Amines can be prepared by reducing amides. Reaction involves the nucleophilic addition of a hydride ion to the amide carbonyl group.
Reduction of Nitriles
Primary amines can be prepared by reducing nitriles by lithium aluminum hydride.
Chemical Reactions of Amines
Carbylamine Reaction
Aliphatic and aromatic primary amines on heating with chloroform and ethanolic potassium hydroxide form isocyanides or carbylamines which are foul smelling substances. Secondary and tertiary amines do not show this reaction. This reaction is known as carbylamine reaction or isocyanide test and is used as a test for primary amines.
R-NH2 + CHCl3 + 3KOH → R-NC + 3KCl + 3H2O
Electrophilic substitution Reaction of Aniline
–NH2 group is ortho and para directing and a powerful activating group.
Bromination of Aniline
Aniline reacts with bromine water at room temperature to give a white precipitate of 2,4,6-tribromoaniline.
Nitration of Aniline
Direct nitration of aniline yields tarry oxidation products in addition to the nitro derivatives. Moreover, in the strongly acidic medium, aniline is protonated to form the anilinium ion which is meta directing. That is why besides the ortho and para derivatives, significant amount of meta derivative is also formed.
However, by protecting the –NH2 group by acetylation reaction with acetic anhydride, the nitration reaction can be controlled and the p-nitro derivative can be obtained as the major product.
Sulphonation of Aniline
Aniline reacts with concentrated sulphuric acid to form anilinium hydrogensulphate which on heating with sulphuric acid at 453-473K produces p-aminobenzene sulphonic acid, commonly known as sulphanilic acid, as the major product.
Aniline does not undergo Friedel-Crafts reaction (alkylation and acetylation) due to salt formation with aluminium chloride, the Lewis acid, which is used as a catalyst. Due to this, nitrogen of aniline acquires positive charge and hence acts as a strong deactivating group for further reaction.
