JEE MAIN - Chemistry (2014 (Offline) - No. 12)
Considering the basic strength of amines in aqueous solution, which one has the smallest pKb value?
(CH3)3N
C6H5NH2
(CH3)2NH
CH3NH2
Wyjaśnienie
Arylamines are less basic than alkyl amines and even ammonia. This is due to resonance. In aryl amines the lone pair of electrons on $$N$$ is partly shared with the ring and is thus less available for sharing with a portion.
In alkylamines, the electron releasing alkyl group increases the electron density on nitrogen atom and thus also increases the ability of amine for protonation. Hence more the no. of alkyl groups higher should be the basicity of amine. But a slight discrepancy occurs in case of trimethyl amines due to steric effect. Hence the correct order is $$${\left( {CH{}_3} \right)_2}NH > C{H_3}N{H_2} > \left( {C{H_3}} \right){}_3N > {C_6}{H_5}N{H_2}$$$
In alkylamines, the electron releasing alkyl group increases the electron density on nitrogen atom and thus also increases the ability of amine for protonation. Hence more the no. of alkyl groups higher should be the basicity of amine. But a slight discrepancy occurs in case of trimethyl amines due to steric effect. Hence the correct order is $$${\left( {CH{}_3} \right)_2}NH > C{H_3}N{H_2} > \left( {C{H_3}} \right){}_3N > {C_6}{H_5}N{H_2}$$$