Question

Arrange the following in the increasing order of their basic strength.
$\text{C}{{\text{H}}_{3}}\text{N}{{\text{H}}_{2}}\text{ , }{{\left( \text{C}{{\text{H}}_{3}} \right)}_{2}}\text{NH , }{{\text{C}}_{6}}{{\text{H}}_{5}}\text{N}{{\text{H}}_{2}}\text{ , }{{\text{C}}_{6}}{{\text{H}}_{5}}\text{C}{{\text{H}}_{2}}\text{N}{{\text{H}}_{2}}$

Answer 1

Basic strength means the ability to donate electrons to electrophiles (electron deficient species). Check the factors on which the basicity depends and their effect. Here, $\text{C}{{\text{H}}_{3}}\text{N}{{\text{H}}_{2}}$ and ${{\left( \text{C}{{\text{H}}_{3}} \right)}_{2}}\text{NH}$ is aliphatic amines and ${{\text{C}}_{6}}{{\text{H}}_{5}}\text{N}{{\text{H}}_{2}}$ and ${{\text{C}}_{6}}{{\text{H}}_{5}}\text{C}{{\text{H}}_{2}}\text{N}{{\text{H}}_{2}}$ are aromatic amines.

Complete step by step answer:
Let us discuss the factors on which basic strength depends to get the general order of the bases:

S. No.	Factors of basicity	Definitions	Effect on basic strength	Reason of that effect
1.	Inductive effect	It is displacement of sigma electrons due to electronegativity or electropositivity of elements.	Positive inductive effect increases the basicity and Negative inductive effect decreases the basicity	More the positive inductive effect (donation of electrons) or $+\text{I}$ effect which increases the electron density on the molecule. Due to which electron donating ability increases of a molecule, making it basic. Number of $+\text{I}$ groups on a molecule increases, its basicity increases. Whereas, $-\text{I}$ effect decreases the electron density on the molecule making it electron deficient reducing the basicity. Number of $-\text{I}$ groups attached increases, its basic strength decreases. Hence, order of basicity is tertiary$>$ secondary$>$ primary. As methyl groups attached produce $+\text{I}$effect.
2.	Resonance	Resonance is actually the delocalization or movement of $\pi$ electrons.	Decreases the basicity	The delocalization of $\pi$ electrons makes it difficult for nitrogen to share its lone pair. Thus, the basic strength of compound decreases. Hence, aromatic amines are less basic than aliphatic amines.

A. Methyl amine: It has one methyl group attached to it and is aliphatic in nature. It is basic in nature.
B. Dimethyl amine: It has two methyl groups attached to it and is aliphatic in nature. It is highly basic.
C. Aniline: It has no methyl group attached. It is an aromatic amine in which $-\text{N}{{\text{H}}_{2}}$ group is attached to a benzene ring. The lone pair of $-\text{N}{{\text{H}}_{2}}$ group undergoes resonance with the benzene ring making it less basic.

D. Benzyl amine: It is primary amine and lone pair of nitrogen does not undergo resonance with benzene ring due to $\left( -\text{C}{{\text{H}}_{2}}- \right)$ group in between the two. The $-\text{I}$ effect of phenyl group decreases the basicity.

The increasing order of their basic strength is ${{\left( \text{C}{{\text{H}}_{3}} \right)}_{2}}\text{NH}>\text{C}{{\text{H}}_{3}}\text{N}{{\text{H}}_{2}}>{{\text{C}}_{6}}{{\text{H}}_{5}}\text{C}{{\text{H}}_{2}}\text{N}{{\text{H}}_{2}}>{{\text{C}}_{6}}{{\text{H}}_{5}}\text{N}{{\text{H}}_{2}}$.

Note: In aqueous solution, the order of basic strength is ${{\left( \text{C}{{\text{H}}_{3}} \right)}_{2}}\text{NH}>\text{C}{{\text{H}}_{3}}\text{N}{{\text{H}}_{2}}>{{\left( \text{C}{{\text{H}}_{3}} \right)}_{3}}\text{N}$. The order mainly changes due to solvation effect (interaction between solute and solvent, here cations), inductive effect and steric hindrance (crowd of groups).