Question

Arrange the following in the increasing order of basicity:
\[C{{H}_{3}}N{{H}_{2}},{{\left( C{{H}_{3}} \right)}_{2}}NH,{{\left( C{{H}_{3}} \right)}_{3}}N\]

Answer 1

Hint: The criteria on which basicity of a compound is calculated is electronegativity. On the basis of electronegativity we can assign the given compounds in the increasing order of their basicity.

Complete answer:
Basicity of a compound is described as the number of replaceable hydrogen atoms a molecule has or the number of electron pairs it can donate to form a bond. The basicity of amines is due to the lone pair present on the nitrogen atom. According to this we will see how the basicity of amines is described as.
We know that alkyl groups are electron donating functional groups, they tend to give away their electrons easily. Thus, in tertiary amines, there are a lot more alkyl groups causing a positive inductive effect and making it easier for the nitrogen atom to donate its lone pair of electrons and form a bond. So, the basicity will increase as the $+I$ effect from the number of alkyl groups increases.
Thus, in a gaseous state, order of basic strength is in the order:
\[{{3}^{o}}\text{ amine }>{{\text{2}}^{o}}\text{ amine }>{{\text{1}}^{o}}\text{ amine}\]
But, we know that in an aqueous medium, the formation of ions takes place and the $+I$ effect cannot be seen as the ions are separated. The solvation that the primary and secondary amines undergo is much greater than the solvation that the tertiary amine undergoes. This solvation effect increases the density of electrons on the nitrogen atom much more than the $+I$ effect does.
Thus, the order of basicity in an aqueous solvent is:
\[{{2}^{o}}\text{ amine }>{{\text{1}}^{o}}\text{ amine }>{{\text{3}}^{o}}\text{ amine}\]
Therefore, following the aforementioned rule we can assign the compounds on the basis of the increasing basic strength in aqueous solution:
\[{{\left( C{{H}_{3}} \right)}_{3}}N>C{{H}_{3}}N{{H}_{2}}>{{\left( C{{H}_{3}} \right)}_{2}}NH\]
Since, ${{3}^{o}}\text{ amine }>{{\text{2}}^{o}}\text{ amine }>{{\text{1}}^{o}}\text{ amine}$
Therefore, the final answer to the question is:
\[{{\left( C{{H}_{3}} \right)}_{3}}N>C{{H}_{3}}N{{H}_{2}}>{{\left( C{{H}_{3}} \right)}_{2}}NH\]

Note:
Remember that the order of basicity will change depending on the phase of the amine. Solvation, steric hindrance, inductive effect and many other factors contribute to the basicity of amines, please take that into consideration before assigning the order of basicity.