Question

In an acidic medium, which one behaves as the strongest base?
(1) Nitrobenzene
(2) Aniline
(3) Phenol

Answer 1

Basicity in general is defined as the ability of the molecule or compound to donate $O{H^ - }$ion or accept a proton ${H^ + }$. Higher the rate of accepting a proton, more basic will be the compound or molecule. The basicity is defined by the functional group present in the molecule.

Complete answer: Aniline is an aromatic amine. In the case of Phenol, the functional group is alcohol and in Nitrobenzene the functional group is a nitro group. Now, we will analyse the ability of these functional groups to accept a proton and behave as a base.
Now, In Phenol the hydroxyl group ($- OH$) attached to the $s{p^2}$ hybridised carbon atom of the benzene ring acts as an electron-withdrawing group. This is due to the presence of more negative oxygen atoms present in the molecule that attracts the shared pair of electrons towards itself and increases the polarity of the $- OH$ bond. This is due to the presence of more electronegative $s{p^2}$ hybridised carbon atoms. As a result of this, the hydrogen from the $- OH$ bonds leaves resulting in the formation of phenoxide ions, making phenol acidic. Hence phenol will not show basic character.
Now, in case of Nitrobenzene due to the presence of an electron-withdrawing group like $- N{O_2}$ the acidity of the molecule increase as they increase the positive charge on the benzene ring, hence they show acidic character more rather than basic character.
In case of aniline, the lone pair of electrons on nitrogen is available for donation and it is a less electronegative element than oxygen. Hence it will show basic character.
Therefore, we can say that, in an acidic medium out of Aniline, Phenol, and Nitrobenzene, Aniline is the strongest Base.

Hence the correct option is (2),aniline.

Note: The basicity of a compound is directly dependent on the presence of electron-withdrawing groups and directly depends on the electron-donating groups i.e. availability to donate lone pairs of electrons as they deactivate the ring.