PrepRung
Sign inSign up free

JEE Main 2022 · Chemistry · General Organic Chemistry and Isomerism · medium

Ly arranges the compounds in decreasing order of acidity of the most acidic hydrogen in each? I: cyclopenta-1,3-diene (C5H6), II: propyne, III…

Single correct option

Which of the following correctly arranges the compounds in decreasing order of acidity of the most acidic hydrogen in each? I: cyclopenta-1,3-diene (C5H6\mathrm{C_5H_6}), II: propyne, III: ethene, IV: ethane
  1. A. I >> II >> III >> IV
  2. B. II >> I >> III >> IV
  3. C. I >> III >> II >> IV
  4. D. II >> III >> I >> IV
Show answer and solution

Answer: (A) I >> II >> III >> IV

Cyclopentadiene loses its CH2\mathrm{CH_2} proton to give the cyclopentadienyl anion, which has 6 π6\ \pi electrons in a planar cyclic conjugated system (aromatic), so its pKa≈16\mathrm{p}K_a\approx16 is by far the lowest. Propyne's sp\mathrm{sp} carbon (50% s-character) gives a stable carbanion (pKa≈25\mathrm{p}K_a\approx25), ethene's sp2\mathrm{sp^2} C-H (≈44\approx44) is weaker, and ethane's sp3\mathrm{sp^3} C-H (≈50\approx50) the weakest. Order: I >> II >> III >> IV.
Report a mistake

Original question written in the JEE Main 2022 pattern and checked twice for the correct answer. Not an official question paper. How we write and check questions · Corrections policy · Report a mistake