Question

Choose the correct option regarding the given compounds:

• A. Compound (a) is the most stable
• B. Compound (c) is the most stable
• C. Compound (c) is the least stable
• D. Compound (b) is more stable than (a)

Answer 1

The correct option is B Compound (c) is the most stable

For compound (a)
Cyclopentadiene:
Here one of the carbon atoms is $s{p}^3$ hybridised. So, it is not planar. It is non-aromatic.
For compound (b)
Cyclopentadienyl cation:
All the carbon atoms are $s{p}^2$ hybridised, and hence the structure is planar. It has conjugation and number of $\pi e^{-}=4$
It follows $\left(4n\right)\pi e^{-}$ rule.
It is anti-aromatic.
For compound (c)
Cyclopentadienyl anion:
It is planar.
Has conjugation
Follows Huckel's rule $(4n+2)e^{-}$ , since it has $6\pi e^{-}$
It is Aromatic.
Stability order :
Aromatic>Non-aromatic>Anti-aromatic

Hence, option (b) is correct.