Question

Arrange the following in their increasing order of ${\text{p}}{{\text{K}}_{\text{b}}}$ values
${{\text{C}}_6}{{\text{H}}_5}{\text{N}}{{\text{H}}_2}{\text{,}}{{\text{C}}_2}{{\text{H}}_5}{\text{N}}{{\text{H}}_2}{\text{,}}{{\text{C}}_6}{{\text{H}}_5}{\text{NHC}}{{\text{H}}_3}$

Answer 1

The given compounds are altogether called amines. They include carbon, hydrogen and nitrogen. Amines are similar to ammonia. ${\text{p}}{{\text{K}}_{\text{b}}}$ values are related to basicity. They are inversely related. Basicity is explained with the help of Lewis concept.

Complete step by step solution:
Amines are the organic derivatives of ammonia in which one or more alkyl or aryl groups are attached to the nitrogen atom. They are classified on the basis of the number of alkyl or aryl groups attached to the nitrogen atom. Primary amines have one alkyl or aryl group, secondary amines have two alkyl or aryl, tertiary amines have three alkyl or aryl groups and quaternary amines have four alkyl or aryl groups.
From the given compounds, ${{\text{C}}_6}{{\text{H}}_5}{\text{N}}{{\text{H}}_2}$ and ${{\text{C}}_2}{{\text{H}}_5}{\text{N}}{{\text{H}}_2}$ are primary amines. ${{\text{C}}_6}{{\text{H}}_5}{\text{N}}{{\text{H}}_2}$ is an aryl amine. ${{\text{C}}_6}{{\text{H}}_5}{\text{NHC}}{{\text{H}}_3}$ is a secondary amine.
As we know that the amines act as Lewis bases which can donate the electron pair. There is a lone pair of electrons in the nitrogen atom which act as nucleophiles and bonds with any electrophile. According to Lowry-Bronsted theory, it is considered as a base since it can accept protons. Smaller values of ${\text{p}}{{\text{K}}_{\text{b}}}$denotes that the corresponding group is a strong base.
Basicity is influenced by several factors like substitution of alkyl groups and substitution by electron withdrawing groups. Presence of more number of alkyl groups makes the amine more basic. When there is an electron withdrawing group attached to nitrogen, then its basicity is decreased.
Thus secondary amines are more basic than primary amines. Also, aryl amines are generally more basic than alkyl amines because lone pairs of electrons are delocalized.
Now combining all these information, we get the order of ${\text{p}}{{\text{K}}_{\text{b}}}$ values as:
${{\text{C}}_2}{{\text{H}}_5}{\text{N}}{{\text{H}}_2}{\text{ < }}{{\text{C}}_6}{{\text{H}}_5}{\text{NHC}}{{\text{H}}_3}{\text{ < }}{{\text{C}}_6}{{\text{H}}_5}{\text{N}}{{\text{H}}_2}$

Note:
Amines take up protons from water which gives ammonium and hydroxide ion. This equilibrium constant is denoted as ${{\text{K}}_{\text{b}}}$. It is also known as base-dissociation constant. ${\text{p}}{{\text{K}}_{\text{b}}}$ value is the negative logarithm of ${{\text{K}}_{\text{b}}}$ value.