Basicity of Amines - Practice Questions & MCQ

Amines like ammonia are basic in nature. The basic nature is due to the presence of an unshared pair of electrons on nitrogen atom. This lone pair of electrons is available for the formation of a new bond with a proton or Lewis acids.

Amines are weak bases as they combine partially with the water to form hydroxyl ions.

$\mathrm{R}-\mathrm{NH}_2+\mathrm{H}_2 \mathrm{O} \rightleftharpoons \mathrm{R}-\mathrm{NH}_3^{+}+\mathrm{OH}^{-}$

Alkylamines are stronger bases than ammonia. This can be explained in terms of electron releasing inductive effect of alkyl group. As a result, the electron density on the nitrogen atom increases and thus, they can donate the lone pair of electrons more easily than ammonia.

The electron releasing effect is maximum in tertiary amines and minimum in primary amines. Aromatic amines have lesser electron density as the lone pairs over nitrogen are delocalised with the benzene ring.

$3^{\circ}$ Amine $>2^{\circ}$ Amine $>1^{\circ}$ Amine $>\mathrm{NH}_3>$ Arylamines

However, to state that the basic nature would follow the electron releasing effect would be an oversimplification and a number of other factors like solvation and steric factors also have to be taken into account.

Thus, the actual order of basic strength of amines essentially is an experimentally derived order and is usually found to follow the following order:

$2^{\circ}$ Amine $>1^{\circ}$ Amine $>3^{\circ}$ Amine $>\mathrm{NH}_3$ when $\mathrm{R}=\mathrm{CH}_3$ and 

$2^{\circ}$ Amine $>3^{\circ}$ Amine $>1^{\circ}$ Amine $>\mathrm{NH}_3$ when $\mathrm{R}=\mathrm{C}_2 \mathrm{H}_5$

It is also to be noted that the basic strength of Alkyl amines is more than that of Ammonia which is more basic than Aryl amines.