Which among the following is the strongest acid?

Step 1: Acid Strength Principle

Acid strength of C-H bonds:

• Stronger acid = more stable conjugate base
• Aromatic stabilization of conjugate base = dramatically increases acidity

Step 2: Evaluate Each Compound

(a) Butane: $\ce{sp^3}$ C-H, pKa ~50. Very weak acid.

(b) Cyclopropene: $\ce{sp^2}$ C-H (vinyl type), pKa ~43. Moderate.

(c) Cyclopentadiene: The $\ce{sp^3}$ $\ce{CH2}$ group has pKa ~16 (comparable to water!). When the H is removed, the cyclopentadienyl anion forms, which is aromatic (6 pi electrons). This aromatic stabilization makes cyclopentadiene extremely acidic for a hydrocarbon. pKa ~16.

(d) Toluene: Benzylic $\ce{CH3}$ pKa ~43.

Cyclopentadiene is the strongest acid because its conjugate base (cyclopentadienyl anion) is aromatic.

Key Points:

• Aromatic conjugate base = dramatically lower pKa (stronger acid)
• Cyclopentadiene pKa ~16: loss of H from CH2 gives aromatic $\ce{C5H5^-}$
• This is why cyclopentadiene can react with NaH, NaOH, etc. (very reactive CH2)