Question

The order of acidity of following acids is:

(A)$ii > iii > iv > i$
(B) $iii > ii > i > iv$
(C) $iv > ii > iii > i$
(D) $iv > iii > ii > i$

Answer 1

Acidity of any compound increases when an electron withdrawing group is attached to it. Whereas, the acidity of the compound decreases when an electron donating group is attached to it.

Complete step by step answer:
The electron withdrawing groups help to increase the acidity of benzoic acid by delocalizing the charge of the carboxylate ion. They deactivate the benzene ring for electrophilic attack and make the benzoic acid more acidic. On the other hand, the electron density toward the negative charge of carboxylate ions. They make the benzene ring active to the electrophilic attack and the benzoic acids less acidic.
As we know that, $N{O_2}$ is a good electron withdrawing group thus it increases the acidity of the benzoic acid by removing the electron density form carboxylate ion.
This effect is maximum when the nitro group is attached at ortho-position. This is because of the increased steric hindrance which forces the carboxyl group to twist out of the plane of the benzene ring. This inhibits the resonance of the carboxyl group with phenyl ring which in turn increases the acidity of the carboxyl group that was otherwise reduced due to destabilizing across conjugation.
However, in para-nitrobenzene, both inductive and resonance effects of the nitro group withdraw the electron density. And in meta-nitrobenzene, only the inductive effect is responsible to remove the electron density.
Hence, the correct order of acidity of given acids is: $iv > iii > ii > i$.
Hence, the correct option is (D).

Note:
More will be the electronegativity of atoms, more will be the tendency to withdraw electrons and increase acidity. Less electronegativity depicts more tendency to donate to decrease acidity.