Question

The most stable and the least stable resonating structures are respectively

• A. I and II
• B. I and III
• C. II and III
• D. III and II

Answer 1

The correct option is C II and III

In structure I, neither cycloheptatriene nor cyclopentadiene alone is aromatic.

In structure II, both the rings are aromatic because both seven (6 $\pi$-electrons) as well as five membered (6 $\pi$-electrons) rings are aromatic. Hence II is the most stable structure.

In structure III, both the rings are anti-aromatic because both seven (8 $\pi$-electrons) as well as five membered (4 $\pi$-electrons) rings are anti-aromatic. Hence III is the least stable structure.