The exact order of acidity of the compounds p-nitrophenol, acetic acid, acetylene and ethanol is

• A

  p-nitrophenol $ < $ acetic acid $ < $ acetylene $ < $ ethanol
• B

  acetic acid $ < \mathrm{p}-$ nitrophenol $ < $ acetylene $ < $ ethanol
• C

  acetylene $ < \mathrm{p}$-nitrophenol $ < $ ethanol $ < $ acetic acid
• Correct answer
  D

  acetylene $ < $ ethanol $ < $ p-nitrophenol $ < $ acetic acid

Solution

Acetic acid is most acidic as its conjugte base has equivalent Resonating structure followed by para-nitrophenol. Ethanol is less acidic then para-nitrophenol as conjugate base of p-nitrophenol is resonance stablized but not ethoxide $\left(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{O}\right)$ Also electronegativity of carbon is lesser than oxygen, hence acetylene is less acidic.

Hence, the correct answer is option: 'D'.