Which among the following is the strongest acid ?

Solution

Chemistry ma’am · 4 steps

For hydrocarbon acids, remove an H⁺ and check the anion: an aromatic anion makes the strongest acid.

1. Judge the anion. A hydrocarbon behaves as an acid by losing H⁺ from a C–H bond. The more stable the carbanion left behind, the stronger the acid. So compare the anion each compound gives.
2. Aromaticity test. A planar, cyclic, fully conjugated anion with (4n+2) π electrons is aromatic and very stable. With 4n π electrons it is antiaromatic and very unstable.

   Compound	Anion formed	π electrons in ring	Anion stability
   (a) butane	butyl (secondary) carbanion	none	no stabilisation, pKₐ ≈ 50
   (b) toluene	benzyl anion	charge spread by resonance	moderate, pKₐ ≈ 41
   (c) cyclopropene	cyclopropenyl anion	4 (4n, n = 1)	antiaromatic, very unstable
   (d) cyclopentadiene	cyclopentadienyl anion	6 (4n+2, n = 1)	aromatic, pKₐ ≈ 16

3. Why d wins. Removing H⁺ from the CH₂ of cyclopentadiene leaves a lone pair in a p orbital. That lone pair joins the four π electrons of the two double bonds, giving a 6π aromatic ring with the charge shared equally over all five carbons.
4. So. Cyclopentadiene, option (d), is the strongest acid. Its pKₐ of about 16 is close to that of water, far lower than any other hydrocarbon here.

The idea to keep. When a C–H acid is asked, draw the carbanion and count its ring π electrons. Aromatic (4n+2) anion means a strong acid; antiaromatic (4n) anion means a very weak one.

Answer from the NTA final answer key. Solution by PrepWiser.