Which of the following hydrocarbons is the most acidic?

Acyclopentane
Bcyclopenta-1,3-diene
Cbenzene
Dcyclohexane

Answer: B

Loss of H+ from the

CH X_{2}

of cyclopenta-1,3-diene gives the cyclopentadienyl anion, which is planar and has 6 pi electrons, i.e. aromatic. The other conjugate bases have no such stabilisation, so cyclopentadiene (pKa about 16) is by far the most acidic.

Practice General Organic Chemistry I: Electronic Effects More on Applications: stability of carbocations, carbanions, free radicals

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