Question: Which compound in each pair is the stronger acid?

The conjugate base stability is a crucial factor in deciding the acidic strength of a particular compound. The acid comprising a conjugate base with high stability is regarded to be more stable.

Acidic strength is controlled by numerous factors, which include the inductive effect and resonance effect. The presence of an electronegative atom can create a stabilized conjugate base, thereby increasing the acidity.

The conjugate base stability is also increased when there is resonance in the molecule.

The compound possessing more stable conjugate base is the strongest acid. The conjugate bases of each pair of compounds can be drawn and compared to understand the strongest acid.

a. The conjugate bases of the compound in question a can be given as:

The first compound in question a

The compound given above is resonance stabilized but is not aromatic.

The second compound in question a

The compound given above contains $6\mathrm{\pi }$ electrons, is aromatic, and has a more stable conjugate base. Hence, the acid of this compound is more acidic.

b. The conjugate bases of the compound in question b can be given as:

The first compound in question b

The given compound is aromatic, has$6\mathrm{\pi }$ electrons, and possesses a more stable conjugate base.

The second compound in question b

The given compound is antiaromatic and has a highly destabilized conjugate base.

Hence the first compound is more acidic than the second compound.