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JEE Advanced levelSingle correct optionDifficulty 3 of 3g1096

Which of the following planar cyclic ions is the least stable?

Acyclopenta-2,4-dien-1-ylium, $C X_{5} H X_{5} X^{+}$
Bcyclopropenyl cation, $C X_{3} H X_{3} X^{+}$
Ccyclopentadienide anion, $C X_{5} H X_{5} X^{-}$
Dcycloheptatrienyl cation, $C X_{7} H X_{7} X^{+}$

Answer: A

The cyclopropenyl cation (2

π

electrons), the tropylium cation (6) and the cyclopentadienide anion (6) all satisfy the

4 n + 2

rule and are aromatic. The cyclopentadienyl cation has 4

π

electrons (

4 n

), so it is antiaromatic and strongly destabilised.

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