Amines
4.0 Basic Nature of Amines
4.0 Basic Nature of Amines
The presence of unshared pair of electrons over the nitrogen atom and difference in electronegativity between nitrogen and hydrogen atoms makes amines reactive.
Amines, being basic in nature, react with acids to form salts. Amine salts on treatment with a base like $NaOH$, gives back the parent amine.
Note: Amine salts are soluble in water but insoluble in organic solvents like ether. This reaction is the basis for the separation of amines from the non basic organic compounds insoluble in water.
The reaction of amines with mineral acids to form ammonium salts shows that these are basic in nature. Amines have an unshared pair of electrons on nitrogen atom due to which they behave as Lewis base.
The structure plays an important role in deciding the basicity of amines.
Basic character of an amine depends upon the ease of formation of the cation by accepting a proton from the acid. The more stable the cation is relative to the amine, more basic is the amine.
- The order of basicity of amines in the gaseous phase follows the expected order: $$tertiary\ amine\ >\ secondary\ amine\ >\ primary\ amine\ >\ NH_3$$
- In the aqueous state, the substituted ammonium cations get stabilized not only by electron releasing effect of the alkyl group i.e., $(+I)$ effect but also by solvation with water molecules. The greater the size of the ion, lesser will be the solvation and the less stabilised is the ion. Greater is the stability of the substituted ammonium cation, stronger should be the corresponding amine as a base. Thus, the order of basicity of aliphatic amines should be: $$primary\ >\ secondary\ >\ tertiary$$ which is opposite to the inductive effect based order.
- When the alkyl group is small, let us say $–CH_3$ group, there is no steric hindrance to Hydrogen bonding. In case the alkyl group is bigger than $CH_3$ group, let us say $–C_2H_5$ there will be steric hinderance to Hydrogen bonding. Therefore, the change of nature of the alkyl group plays an important role in changing the order of basic strength.
- Greater the number of resonating structures, greater is the stability. Let us say, five resonating structures of aniline is more stable than anilinium ion. Hence, the basic nature of aniline or other aromatic amines would be less than that of ammonia.
In case of substituted aniline, it is observed that electron releasing groups like $–OCH_3$, $–CH_3$ increase basic strength whereas electron withdrawing groups like $–NO_2$, $–SO_3H$, $–COOH$ and $–X$ decrease it.
