Question

Which of the following orders is correct regarding the basicity of aliphatic amines?
A.${\text{C}}{{\text{H}}_{\text{3}}}{\text{N}}{{\text{H}}_{\text{2}}}$> ${\left( {{\text{C}}{{\text{H}}_{\text{3}}}} \right)_{\text{2}}}{\text{NH}}$> ${\left( {{\text{C}}{{\text{H}}_{\text{3}}}} \right)_{\text{3}}}{\text{N}}$
B.${\text{C}}{{\text{H}}_{\text{3}}}{\text{N}}{{\text{H}}_{\text{2}}}$<${\left( {{\text{C}}{{\text{H}}_{\text{3}}}} \right)_{\text{2}}}{\text{NH}}$<${\left( {{\text{C}}{{\text{H}}_{\text{3}}}} \right)_{\text{3}}}{\text{N}}$
C.${\text{C}}{{\text{H}}_{\text{3}}}{\text{N}}{{\text{H}}_{\text{2}}}$>${\left( {{\text{C}}{{\text{H}}_{\text{3}}}} \right)_{\text{2}}}{\text{NH}}$<${\left( {{\text{C}}{{\text{H}}_{\text{3}}}} \right)_{\text{3}}}{\text{N}}$
D.${\text{C}}{{\text{H}}_{\text{3}}}{\text{N}}{{\text{H}}_{\text{2}}}$<${\left( {{\text{C}}{{\text{H}}_{\text{3}}}} \right)_{\text{2}}}{\text{NH}}$>${\left( {{\text{C}}{{\text{H}}_{\text{3}}}} \right)_{\text{3}}}{\text{N}}$

Answer 1

The basicity of aliphatic amines depends on the ability of the nitrogen atom to donate its lone pair of electrons. There are several factors that affect this property. More the hydrogen attached to nitrogen, less is the basicity.

Complete step-by-step answer:
The alkyls groups exert $\left( { + {\text{I}}} \right)$ effect and the effect increases with an increase in the number of carbon atoms, so the inductive effect of the propyl group is higher than the ethyl group which is higher than the methyl group.
In the given amines are the methyl amine, N, N- dimethyl amine, and N, N, N- trimethylamine. As the inductive is higher for the methyl group than the hydrogen atom hence the presence of three methyl groups in N, N, N- trimethyl amine causes higher electron density on the nitrogen atom and it has higher ability to donate electrons. Hence the order of basicity of the amines is:
${\text{C}}{{\text{H}}_{\text{3}}}{\text{N}}{{\text{H}}_{\text{2}}}$<${\left( {{\text{C}}{{\text{H}}_{\text{3}}}} \right)_{\text{2}}}{\text{NH}}$<${\left( {{\text{C}}{{\text{H}}_{\text{3}}}} \right)_{\text{3}}}{\text{N}}$
But due to the presence of three methyl groups, there arises a lot of steric hindrance in the trimethyl amine molecule, due to which the lone pair of electrons goes out of the plane of the molecule and is not available to be donated. So the correct order is:
${\text{C}}{{\text{H}}_{\text{3}}}{\text{N}}{{\text{H}}_{\text{2}}}$<${\left( {{\text{C}}{{\text{H}}_{\text{3}}}} \right)_{\text{2}}}{\text{NH}}$>${\left( {{\text{C}}{{\text{H}}_{\text{3}}}} \right)_{\text{3}}}{\text{N}}$

The correct answer is option D.

Note: The inductive effect can be defined in organic chemistry as the phenomenon that arises in molecules due to unequal sharing of electron pairs in a chain of atoms leading to a permanent dipole in a bond. If an electronegative atom is joined to the chain of carbon atoms in the organic compound, such as the halogen molecules, then the carbon atoms acquires a partial positive charge while the electronegative atom acquires a partial negative charge. This is called the negative inductive effect or the $\left( { - {\text{I}}} \right)$ effect. On the other hand, the presence of atoms that are more electropositive than the carbon atom exert a $\left( { + {\text{I}}} \right)$ effect or the positive inductive effect. These effects are permanent effects that last throughout the molecule but its effect decreases from the first carbon atom to the last one.