Question

The order of basic strength of ${C_6}{H_5}N{H_2}$ $\left( 1 \right)$, ${C_2}{H_5}N{H_2}$ $\left( 2 \right)$, ${\left( {{C_2}{H_5}} \right)_2}NH$ $\left( 3 \right)$, and $N{H_3}$ $\left( 4 \right)$ is:
(A) $1 < 4 < 2 < 3$
(B) $1 < 2 < 3 < 4$
(C) $4 < 3 < 2 < 1$
(D) $3 < 2 < 4 < 1$

Answer 1

Bases are the substances which release $O{H^ - }$ ions when dissolved in water. Amines are compounds which contain nitrogen atoms with a lone pair. Basically amines are derivatives of ammonia. Chemical formula of ammonia is $N{H_3}$.

Complete step by step answer:
All the three classes of amines ( primary, secondary and tertiary ) act as Lewis bases because of the presence of a lone pair of electrons on the nitrogen atom. The basic strength is different in different amines.
Aromatic amines are weaker than ammonia and aliphatic amines as the lone pair comes in conjugation with the ring. In case of aliphatic amines, the alkyl group is the $+ I$ group, so it increases the electron density on nitrogen and hence it becomes more basic.
Now the order of aliphatic amines in gaseous phase is
${3^ \circ } > {2^ \circ } > {1^ \circ }$
That is, tertiary amines are most basic as they have three $+ I$ groups around them, so electron density increases on nitrogen atoms and it becomes most basic. Whereas ammonia is least basic since it is not surrounded by any $+ I$ group.
So, according to the above discussion, the order of basicity of the given compounds is
${C_6}{H_5}N{H_2} < N{H_3} < {C_2}{H_5}N{H_2} < {({C_2}{H_5})_2}NH$

Hence the correct option is A, $1 < 4 < 2 < 3$.

Note: The order of basicity of amines in aqueous solution is different than in gaseous phase, the order is
${2^ \circ } > {3^ \circ } > {1^ \circ }$
The reason behind this is that in aqueous solution not only inductive effect but solvation effect ( interaction of cation with water molecules ) is also a deciding factor. So, greater the size less is the solution. Also steric hindrance also affects the order of basicity.