Question

The stability order of the following carbocations is :

• A. $\left(i\right)>\left(ii\right)>\left(iii\right)$
• B. $\left(iii\right)>\left(i\right)>\left(ii\right)$
• C. $\left(ii\right)>\left(i\right)>\left(iii\right)$
• D. $\left(i\right)>\left(iii\right)>\left(ii\right)$

Answer 1

The correct option is C $\left(ii\right)>\left(i\right)>\left(iii\right)$

In structure (i):
There is no resonance taking place, it is only stabilized by hyperconjugation due to presence of $\alpha$-hydrogens.
In structure (ii) :
Even though it has less $\alpha$-hydrogens than structure (i), this structure is stabilized by resonance. So, it is more stable than structure (ii) because resonance effect is more dominating than hyperconjugation.
In structure (iii) :
This structure is planar, is cyclic, has conjugation and has $4\pi e^{-}$ thus making it anti aromatic. Since anti-aromatic structures are the least stable. So, correct order of stability is : $\left(ii\right)>\left(i\right)>\left(iii\right)$