Question: Rank the following compounds in the order of increasing acidity.

A conjugate acid-base pair in which the two substances differ only by the presence of a proton is known as acids and bases according to the Lowry-Bronsted concept.

By adding a proton to a base, a conjugate acid is formed, and by the removal of a proton from an acid, a conjugate base is formed.

The strength of an acid or base and the strength of its conjugate base or conjugate acid are interrelated. The relation is that as the acid strength increases, the conjugate base becomes weaker. The acid is weaker, the strength of the conjugate base is strong.

The compounds with the most stable conjugate base are the most acidic.

• For the first one, there is no delocalization due to resonance; so, it has the most unstable base and, hence, least acidic acid.
• For the second one, there are two resonance structures; so, the acid has an intermediate acidity.
• For the third one, an aromatic conjugate base that is most stable is present; so, the acid will be most acidic.