Example 9.4

S Cl + H S + H Cl N,N-Diethylbenzenesulphonamide . 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.

. Reaction with nitrous acid Three classes of amines react differently with nitrous acid which is prepared in situ from a mineral acid and sodium nitrite. (a) Primary aliphatic amines react with nitrous acid to form aliphatic diazonium salts which being unstable, liberate nitrogen gas quantitatively and alcohols. Quantitative evolution of nitrogen is used in estimation of amino acids and proteins.

(b) Aromatic amines react with nitrous acid at low temperatures ( - K) to form diazonium salts, a very important class of compounds used for synthesis of a variety of aromatic compounds discussed in Section . . Secondary and tertiary amines react with nitrous acid in a different manner. .

Reaction with arylsulphonyl chloride Benzenesulphonyl chloride (C H SO Cl), which is also known as Hinsberg’s reagent, reacts with primary and secondary amines to form sulphonamides. (a) The reaction of benzenesulphonyl chloride with primary amine yields N-ethylbenzenesulphonyl amide. The hydrogen attached to nitrogen in sulphonamide is strongly acidic due to the presence of strong electron withdrawing sulphonyl group. Hence, it is soluble in alkali.

(b) In the reaction with secondary amine, N,N-diethyl- benzenesulphonamide is formed. Since N, N-diethylbenzene sulphonamide does not contain any hydrogen atom attached to nitrogen atom, it is not acidic and hence insoluble in alkali. (c) Tertiary amines do not react with benzenesulphonyl chloride. This property of amines reacting with benzenesulphonyl chloride in a different manner is used for the distinction of primary, secondary and tertiary amines and also for the separation of a mixture of amines.

However, these days benzenesulphonyl chloride is replaced by p -toluenesulphonyl chloride. . Electrophilic substitution You have read earlier that aniline is a resonance hybrid of five structures. Where do you find the maximum electron density in these structures?

Ortho- and para- positions to the –NH group become centres of high electron density. Thus –NH group is ortho and para directing and a powerful activating group. (a) Bromination: Aniline reacts with bromine water at room temperature to give a white precipitate of , , -tribromoaniline. The main problem encountered during electrophilic substitution reactions of aromatic amines is that of their very high reactivity.

Substitution tends to occur at ortho- and para- positions. If we have to prepare monosubstituted aniline derivative, how can the activating effect of –NH group be controlled ? This can be done by protecting the -NH group by acetylation with acetic anhydride, then carrying out the desired substitution followed by hydrolysis of the substituted amide to the substituted amine. The lone pair of electrons on nitrogen of acetanilide interacts with oxygen atom due to resonance as shown below: Hence, the lone pair of electrons on nitrogen is less available for donation to benzene ring by resonance.

Therefore, activating effect of –NHCOCH group is less than that of amino group. (b) Nitration: 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 –NH 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. (c) Sulphonation : Aniline reacts with concentrated sulphuric acid to form anilinium hydrogensulphate which on heating with sulphuric acid at -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.