NEET Chemistry Notes Organic Compounds Containing Nitrogen – Properties of Amines

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 K_b or smaller the value of pK_b , 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.
  

Acylation

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.

Carbylamine Reaction

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:

•  Bromination

The main problem encountered during electrophilic substitution reactions of aromatic amines is that these have very high reactivity.

• Selective Bromination
  
• Nitration

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.

• Sulphonation

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.

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