Which among the following is the strongest acid ?
Show Hint
- \(\mathrm{CH_3CH_2CH_2CH_3}\)
The Correct Option is D
Solution and Explanation
Option (1) butane (pKa about 50) gives an unstabilised alkyl carbanion. Option (2) toluene (pKa about 41) gives a benzyl carbanion, stabilised by resonance. Option (3) cyclopropene (pKa about 46) gives a strained cyclic carbanion with no special stabilisation and is weakly acidic.
Option (4) cyclopentadiene loses H+ from its CH2 group to give the cyclopentadienyl anion, which has 6 \(\pi\) electrons and is aromatic (Huckel rule, pKa about 16).
The most stable conjugate base belongs to cyclopentadiene, so it is the strongest acid.
Answer: (4) cyclopentadiene
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