Arrange the following compounds in the decreasing order of their acidic strength.
A. p-Cresol
B. Ethanol
C. 2,4,6-trinitrophenol
D. o-Nitrophenol
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → C, D, A, B
The acidic strength of these compounds is decided by how well the anion left after losing \( \ce{H^{+}} \) is stabilised:
Phenols vs alcohols: the phenoxide ion is stabilised by resonance with the ring, while the ethoxide ion is not (and its ethyl group pushes electrons onto oxygen). So phenols are much more acidic than alcohols.
Electron withdrawing groups such as –\( \ce{NO2} \) at the ortho/para positions spread the negative charge further and increase acidity; more such groups mean higher acidity.
Electron donating groups such as –\( \ce{CH3} \) destabilise the phenoxide ion and decrease acidity.
Combining these effects:
\( \displaystyle \text{trinitrophenol} > \text{nitrophenol} > \text{methylphenol} > \text{alcohol} \)
Matching the given compounds (pKa values; lower pKa means stronger acid):
(C) 2,4,6-trinitrophenol: picric acid, pKa about 0.4
(D) o-Nitrophenol: pKa 7.2
(A) p-Cresol: 4-methylphenol, pKa 10.2
(B) Ethanol: pKa 15.9
Shortcut: Picric acid (C) is the strongest and ethanol (B) is the weakest. Only Option (4) starts with (C) and ends with (B).
NCERT (2026-27 edition):
Acidity of Phenols – …
