Amines exhibit characteristic chemical properties due to the presence of a lone pair of electrons on the nitrogen atom. This lone pair makes amines basic and nucleophilic, enabling them to participate in a variety of reactions.
1. Basic Character of Amines
Amines behave as bases and react with acids to form ammonium salts.
- The nitrogen atom in amines possesses a lone pair of electrons that can accept a proton from an acid.
- Therefore, amines exhibit basic character.
- Amines are weaker bases than hydroxide ions but stronger bases than water.
Reaction:
RNH_2 + HCl \rightarrow RNH_3^{+}Cl^{-} Example:
C_2H_5NH_2 + HCl \rightarrow C_2H_5NH_3Cl
2. Alkylation of Amines
Amines react with alkyl halides to form higher amines. The reaction proceeds through nucleophilic substitution. Further alkylation may lead to tertiary amines and quaternary ammonium salts.
Reaction:
RNH_2 + R'X \rightarrow RNHR' + HX Example:
CH_3NH_2 + CH_3I \rightarrow (CH_3)_2NH + HI
3. Acylation of Amines
Primary and secondary amines react with acid chlorides or acid anhydrides to form amides.
- This reaction is called acylation.
- Tertiary amines do not undergo acylation because they lack a replaceable hydrogen atom attached to nitrogen.
Reaction:
RNH_2 + R'COCl \rightarrow R'CONHR + HCl Example:
C_6H_5NH_2 + CH_3COCl\rightarrowC_6H_5NHCOCH_3 + HCl
4. Carbylamine Reaction
Primary amines react with chloroform and alcoholic KOH to produce foul-smelling isocyanides (carbylamines). Only primary amines give this test. It is used to distinguish primary amines from secondary and tertiary amines.
Reaction:
RNH_2 + CHCl_3 + 3KOH\rightarrowRNC + 3KCl + 3H_2O Example:
C_6H_5NH_2 + CHCl_3 + 3KOH\rightarrowC_6H_5NC + 3KCl + 3H_2O
5. Reaction with Nitrous Acid
Amines react differently with nitrous acid depending on whether they are primary, secondary, or tertiary.
(a) Primary Aliphatic Amines
Nitrogen gas is evolved.
RNH_2 + HNO_2\rightarrowROH + N_2 + H_2O
(b) Primary Aromatic Amines
This reaction forms diazonium salts, which are important intermediates in organic synthesis.
C_6H_5NH_2 + NaNO_2 + 2HCl\rightarrowC_6H_5N_2^{+}Cl^{-} + NaCl + 2H_2O
6. Azo Coupling Reaction
Aromatic diazonium salts react with phenols and aromatic amines to form azo compounds. Azo compounds are brightly coloured and are widely used as dyes.
Reaction:
C_6H_5N_2^{+}Cl^{-}+C_6H_5OH\rightarrowC_6H_5{-}N=N{-}C_6H_4OH