Q.66. - Organic Chemistry

Question

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

Step-by-Step Solution

Answer

Answer

 

The aromaticity of compound A makes it more stable than compound B.

Step 1: Stability of compounds

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.

Step 2: Aromatic and 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.

Step 3: Stability of compound A and compound B

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