Name Reactions of Amines
(a) Gabriel phthalimide synthesis: This reaction is used for the preparation of aliphatic primary amines. In this reaction, phthalimide is first of all treated with ethanolic KOH to form potassium phthalimide. Potassium phthalimide on treatment with alkyl halide gives N-alkyl phthalimide, which on hydrolysis with dilute hydrochloric acid gives a primary amine as the product.
(b) Hoffmann bromamide reaction: When a primary acid amide is heated with an aqueous or ethanolic solution of NaOH or KOH and bromine (i.e., NaOBr or KOBr), it gives a primary amine with one carbon atom less.
(c) Sandmeyer’s reaction: The Cl– , Br– and CN– nucleophiles can easily be introduced in the benzene in the presence of Cu (I) ion. This reaction is called Sandmeyer’s reaction.
(d) Gatterman’s reaction: Chlorine or bromine can be introduced in benzene ring by treating the diazonium salt solution with corresponding halogen acid in the presence of copper powder.
(e) Carbylamine reaction (Isocyanide test): Aliphatic and aromatic primary amines when heated with chloroform and alcoholic solution of KOH give isocyanides (carbylamines) which have extremely unpleasant smell.
(e) Coupling reaction: The reaction of diazonium salts with phenols and aromatic amines to form azo compounds of the general formula, Ar—N—N—Ar is called coupling reaction. The mechanism is basically that of electrophilic substitution where the diazonium ion is electrophile. In this reaction, the nitrogen atoms of the diazo group are retained in the product. The coupling with phenols takes place in mildly alkaline medium while with amines, it occurs under faintly acidic conditions. For example,
Coupling generally occurs at the p-position, w.r.t., the hydroxyl or the amino group, if free, otherwise it takes place at the o-position.
TEST FOR AMINES
(a) Hinsberg’s test: In this test, the amine is first treated with Hinsberg’s reagent (benzenesulphonyl chloride) and then shaken with aqueous KOH solution when the three amines behave in different ways.
(i) A 1° amine gives a clear solution which on acidification gives an insoluble N-alkyl benzene sulphonamide.
(ii) A 2° amine gives an insoluble N, N-dialkyl benzene sulphonamide which remains unaffected on addition of acid.
(iii) A 3° amine does not react at all.
(b) Isocyanide test (Carbylamine test): Primary amines (aliphatic as well as aromatic) react with chloroform in the presence of alcoholic KOH to form foul smelling carbylamine.
Secondary and tertiary amines (aliphatic as well as aromatic) do not give this test.
(c) Azo dye test: It involves the reaction of any aromatic primary amine with HNO2 (NaNO2 + dil. HCl) at 273–278 K followed by treatment with an alkaline solution of 2-naphthol, where a brilliant yellow, orange or red coloured dye is obtained.
Aliphatic primary amines under these conditions give a brisk evolution of N2 gas with the formation of primary alcohols, i.e., the solution remains clear.
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