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JEE Main levelSingle correct optionDifficulty 2 of 3g1173

Which of the following carbocations is the most stable?

  1. Acycloprop-2-en-1-ylium (cyclopropenyl cation)
  2. Bcyclopenta-2,4-dien-1-ylium (cyclopentadienyl cation)
  3. Cmethyl cation
  4. Dcyclopropyl cation

Answer: A

The cyclopropenyl cation is planar, cyclic and fully conjugated with 2 pi electrons (4n+2, n = 0), so it is aromatic. The cyclopentadienyl cation has 4 pi electrons (antiaromatic); the cyclopropyl and methyl cations have no special stabilisation. Cyclopropenyl is therefore the most stable.
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