JEE Chemistry Amines Notes Set 04

Amines: Classification and Structure

Amines are derivatives of ammonia in which one, two or three hydrogens are replaced by alkyl or aryl groups. They are called primary, secondary or tertiary according to the number of carbon groups on nitrogen. The nitrogen is sp3 hybridised and the molecule is pyramidal with a lone pair, which explains the basic and nucleophilic character. The general formula of a saturated aliphatic amine is \(\ce{C_nH_{2n+3}N}\).

Preparation of Aliphatic Amines

Physical Properties

1. Methylamine, dimethylamine, trimethylamine and ethylamine are gases or very volatile liquids with a fishy smell.
2. Primary and secondary amines form intermolecular hydrogen bonds (weaker than those of alcohols), so their boiling points are above those of alkanes but below those of alcohols of similar mass.
3. Boiling point order for isomers: primary > secondary > tertiary, because tertiary amines have no N-H.
4. Lower amines dissolve in water by hydrogen bonding; solubility falls as the alkyl part becomes larger. Amine solubility for isomers follows 1° > 2° > 3°.

Chemical Properties of Aliphatic Amines

1. Basic character: amines accept a proton to give alkylammonium salts, \(\ce{RNH2 + HCl -> RNH3+Cl-}\). Their solution turns red litmus blue.
2. Alkylation: further reaction with alkyl halides finally gives quaternary ammonium salts.
3. Acylation: 1 and 2° amines react with acid chlorides or anhydrides to give N-substituted amides; tertiary amines do not, because they have no N-H.
4. Reaction with nitrous acid: 1° gives alcohol with evolution of N2; 2° gives yellow oily N-nitrosoamine; 3° gives a soluble ammonium nitrite salt.
5. Carbylamine reaction: only primary amines (aliphatic or aromatic) with \(\ce{CHCl3}\) and alcoholic KOH give a foul-smelling isocyanide through dichlorocarbene.
6. Hinsberg's test: benzenesulphonyl chloride gives with a primary amine a sulphonamide soluble in alkali, with a secondary amine an insoluble sulphonamide, and with a tertiary amine no reaction.
7. Condensation with aldehydes or ketones: primary amines give Schiff's bases (imines, also called anils).
8. Hoffmann mustard oil test: primary amine heated with \(\ce{CS2}\) and \(\ce{HgCl2}\) gives an alkyl isothiocyanate with a pungent smell.

Separation of Amines

A mixture of 1, 2 and 3° amines can be separated by fractional distillation (used in industry), by Hinsberg's method with benzenesulphonyl chloride, or by Hofmann's method with diethyl oxalate, which gives a solid product with primary amines, a liquid with secondary amines and no reaction with tertiary amines.

Points to Remember

1. Nitrosoamines are carcinogenic.
2. An amine with a chiral nitrogen cannot be resolved because of rapid pyramidal inversion.
3. Schiff's bases from primary amines and carbonyl compounds contain the C=N bond.
4. Acidic character of amines is shown by reaction with sodium metal to give H2; amides show it with NaNH2.