Question

Find the least basic among the following
$
  A.{\text{ }}C{H_3}N{H_2} \\
  B.{\text{ }}N{H_3} \\
  C.{\text{ }}{C_2}{H_5}N{H_2} \\
  D.{\text{ }}PhN{H_2} \\
 $

Answer 1

Hint: We know that the basicity of the substance increases as the number of +I (inductive effect) groups increases while the basicity decreases as the number of (-I) groups increases. Before that we must know that the Inductive effect is an effect on the transmission of unequal bonding electron sharing through a chain of atoms in a molecule, resulting in a permanent dipole in a bond.

Complete step by step answer:
As we know that the alkyl groups are electron-donating groups and they have a positive inductive effect. Thus the alkyl groups in the methylamine $C{H_3}N{H_2}$ and ${C_2}{H_5}N{H_2}$ increases the electron density of the nitrogen atom in the amine group makes them more basic substances. Hence option (A) and (C) are incorrect.
We know that ammonia is a weak base as it has a lone pair of electrons in its nitrogen but it is stronger base than phenylamine and weaker base than methyl and ethyl amine. Hence Option (C) is incorrect.
Now coming to the option D,
The phenyl group is an electron-withdrawing group (-I) and the lone pair of nitrogen at amine group delocalizes within the phenyl ring which reduces the electron density and makes the substance least basic.
Hence option (D) is correct.

Additional Note:
Inductive effect:
Inductive effect means the polarization of the sigma bond due to electron attracting and electron releasing effect of an adjacent atom.
\[C{H_3}^{\delta + + } - C{H_2}^{\delta + } - C{l^{\delta - }}\]
We have two types of inductive effect namely positive inductive effect (+I) and negative inductive effect (-I).
Positive inductive effect:
An atom or a group which attracts electrons towards itself less than a hydrogen atom is known as an electron-donating group and they are said to have a positive inductive effect (+I). Some of the groups with +I effects are ${\text{ - C}}{{\text{H}}_{\text{3}}}{\text{, - }}{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{5}}}{\text{,}}{\left( {{\text{ - C}}{{\text{H}}_{\text{3}}}} \right)_{\text{3}}}{\text{etc}}{\text{.}}$
Negative inductive effect:
An atom or a group which attracts electrons towards itself more strongly than a hydrogen atom is known as an electron attracting group and they are said to have a negative inductive effect (-I). Some of the groups with -I effects are ${\text{ - N}}{{\text{O}}_{\text{2}}}{\text{, - Cl, - Br, - I,etc}}{\text{.}}$

Note:
We must know that the inductive effect is different from the resonance effect. In the Resonance effect, pi electrons are involved and in the inductive effect, sigma electrons are involved.