Question

Amongst the following the strongest base in aqueous medium is?
A.${\left( {C{H_3}} \right)_2}NH$
B.$C{H_3}N{H_2}$
C.${C_6}{H_5}NHC{H_3}$
D.$NCC{H_2}N{H_2}$

Answer 1

Hint: Nitrogen-containing compounds are commonly known as amines. Amines are aliphatic as well as aromatic in nature. In the amines, there is an electronegativity difference between the nitrogen and hydrogen atoms which makes them very reactive in nature.

Complete answer:
Amines have nitrogen as a central metal atom which has a lone pair of electrons over it. Due to this lone pair of electron amines are basic in nature.
amines have a tendency to react with water molecules and get converted into its protonated form, this shows that electron donating capabilities of amines are much higher than water. Therefore, amines are more basic than water.
The strength of basicity of amines depends upon the availability of lone pair electron and resonance.
$ \to $ Comparison of basicity of aliphatic amines- alkyl group present in aliphatic amine is electron donating in nature therefore, shows $\left( { + {\rm I}} \right)$ inductive effect. Due to $\left( { + {\rm I}} \right)$, electron density on nitrogen atoms increases and thereby strength of basicity also increases. Therefore, with the increase of the alkyl group $\left( { + {\rm I}} \right)$ effect increases and basicity increases. Order of basicity of amine in general case-${3^ \circ } > {2^ \circ } > {1^ \circ }$
This trend is followed by amines in gaseous state but in liquid or aqueous phase tertiary amines show unexpected less basic strength than other amines. Therefore, the basic strength of amine in aqueous state is- ${2^ \circ } > {1^ \circ } > {3^ \circ }$.
$ \to $ comparison of basicity of aliphatic and aromatic amines- in aliphatic amines lone pairs of electrons are located over the nitrogen atom and do not participate in resonance. This alkyl group shows $\left( { + {\rm I}} \right)$ effect to increase the electron density. However, in case of aromatic amine lone pairs of electrons participate in resonance and therefore their availability decreases for sharing and no alkyl group is present to show $\left( { + {\rm I}} \right)$ effect. Hence, aromatic amines are less basic than aliphatic amines.
Now from the option mentioned above we clearly see that: -
${\left( {C{H_3}} \right)_2}NH$ is secondary amine, $C{H_3}N{H_2}$ is primary amine, ${C_6}{H_5}NHC{H_3}$ is aromatic amine, and $NCC{H_2}N{H_2}$ is primary aliphatic amine.
So, the order of basicity of amine in aqueous medium will be-
${\left( {C{H_3}} \right)_2}NH > C{H_3}N{H_2} > {C_6}{H_5}NHC{H_3} > NCC{H_2}N{H_2}$
$ \Rightarrow $ The strongest base in aqueous medium is secondary amine ${\left( {C{H_3}} \right)_2}NH$.

Therefore option $\left( A \right)$ is the correct option.

Note:
$NCC{H_2}N{H_2}$ shows the minimum basicity because it contains electron attracting$\left( { - I} \right)$ cyanide group in a molecule which decreases the electron density over the nitrogen atom. In aliphatic amine electron pair are present in $\left( {s{p^3}} \right)$ orbitals while in case of aromatic amines lone pair electrons are present in $\left( {s{p^2}} \right)$ orbitals. So due to lesser space availability in $\left( {s{p^2}} \right)$ orbital aromatic amines are less basic.