Question

Rank the following compounds in the decreasing order of their stability:

• A. $\left(a\right)>\left(b\right)>\left(c\right)$
• B. $\left(b\right)>\left(c\right)>\left(a\right)$
• C. $\left(c\right)>\left(a\right)>\left(b\right)$
• D. $\left(a\right)>\left(c\right)>\left(b\right)$

Answer 1

The correct option is C $\left(c\right)>\left(a\right)>\left(b\right)$

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