Question: Explain why compound A is much more stable than compound B.

Numerous factors can influence the stability of a particular compound, and one among them is aromaticity.Aromatic compounds possess greater stability than anti-aromatic compounds.

The count of pi electrons in aromatic compounds can be found using Huckle’s rule, and it comprises $\mathbf{4}\mathit{n}\mathbf{+}\mathbf{2}\mathbf{\pi }$ electrons. The count of pi electrons in anti-aromatic compounds is given by $\mathbf{4}\mathit{n}\mathbf{\pi }$electrons.

The second resonance structure of compound A indicates that the ring is completely conjugated and contains $6\mathrm{\pi }$electrons leading them to become aromatic and a stable compound. The second resonance structure of compound A can be given as:

Second resonance structure of compound A

The charge-separated resonance structure of compound B provides the ring with $4\mathrm{\pi }$electrons. Hence, the compound B is anti-aromatic and unstable. The second resonance structure of compound B can be given as:

Second resonance structure of compound B