NEET Chemistry Notes Organic Compounds Containing Nitrogen – Properties of Amines
Properties of Amines
Physical Properties of Amines
- The lower aliphatic amines are gases with fishy odour. Aniline and other aryl amines are usually colourless but get coloured on storage due to atmospheric oxidation.
- Boiling points and solubility of isomeric amines decrease with branching, i.e.
- Amines have lower boiling point than corresponding alcohols
Chemical Properties of Amines
Amines are reactive and behave as nucleophiles due to the presence of one unshared pair of electron.
Basic Nature of Amines
- Amines are basic in nature, so react with acids and form salt.
- Larger the value of Kb or smaller the value of pKb , stronger is the base.
- The order of basic strength in case of methyl substituted amines and ethyl substituted amines in aqueous solution is as follows :
Electron donating substituent such as —CH3,—OCH3 increases the basicity while electron withdrawing group decreases the basicity.
- More the s-character of the hybrid orbital containing lone pair of electrons, lesser is the basic character.
In this reaction, 1° amines give N-substituted amide while 2° amines give N,N-disubstituted amides.
Aniline does not undergo Friedel-Crafts reaction (alkylation and acetylation) due to salt formation with AlCl3 (anhy), a Lewis acid, which is used as a catalyst.
Secondary (R—NH—R) and tertiary amines (H3 N) do not show this reaction, hence it is used as a test for primary amines.
Reaction with Nitrous Acid
Aliphatic primary amines when react with HNOz, give alcohol.
Electrophilic Substitution Reaction
This is an organic reaction in which an atom that is attached to an aromatic system is replaced by an electrophile. Some of the most important electrophile aromatic substitution, i.e. bromination, nitration, sulphonation etc., are given below:
The main problem encountered during electrophilic substitution reactions of aromatic amines is that these have very high reactivity.
- Selective Bromination
Benzene can be treated with a nitric acid to give nitrobenzene. The overall reaction is given below:
However, by protecting the —NH2 group through acetylation with acetic anhydride, the nitration can be controlled and p-nitro aniline is obtained as a major product.
Benzene can be treated with a very concentrated sulphuric acid to give benzene sulphuric acid in a reversible reaction. The overall reaction is given below:
- Hinsberg’s Test
Distinction between primary, secondary and tertiary amines is made by C 6H 5S02Cl, which is known as Hingberg’s reagent.
Primary amines give an insoluble mass which is soluble in alkali
Secondary amines give an insoluble substance, which remains unaffected by alkali.
Tertiary amines do not react with benzene sulphonyl chloride because of the absence of H-atoms.