Which of the following compounds is most reactive to an aqueous solution of sodium carbonate ?
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The Correct Option is C
Solution and Explanation
Reactivity toward an aqueous base is governed by the acidity of the C-H bond, i.e. the stability of the conjugate base.
Cyclopentadiene (option 3) loses a proton from its \(\mathrm{CH_2}\) to give the cyclopentadienyl anion, which has 6 \(\pi\)-electrons in a planar cyclic conjugated system (aromatic, \(4n+2\) with n = 1). Its \(\mathrm{pK_a}\) is about 16, far lower than those of the other hydrocarbons (the cyclopropenyl-type anion is antiaromatic; cyclobutene and cyclohexadiene give no special stabilisation).
So cyclopentadiene is the most acidic and hence the most reactive toward base.
Answer: (3) cyclopentadiene
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