Arrange the following compounds in increasing order of their acid strength.
(A) Propan-1-ol
(B) Picric Acid
(C) 4-methylphenol
(D) Phenol
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A), (C), (D), (B)
The acid strength of an –OH compound depends on how stable its anion is after it loses a proton. Two factors decide this:
Alcohol or phenol: in a phenoxide ion the negative charge is spread over the benzene ring by resonance, so phenols (pKa about 10) are much stronger acids than alcohols (pKa about 16). The alkyl group of an alkoxide ion pushes electrons towards oxygen (+I effect) and destabilises it further.
Substituents on the ring: an electron withdrawing group (–\( \ce{NO2} \)) at the ortho or para position stabilises the phenoxide ion and increases acidity; each extra –\( \ce{NO2} \) group increases it further. An electron donating group (–\( \ce{CH3} \)) destabilises the phenoxide ion and decreases acidity.
Combining the two factors:
\( \displaystyle \text{alcohol} < \text{phenol with EDG} < \text{phenol} < \text{trinitrophenol} \)
\( \displaystyle \ce{CH3CH2CH2OH} < \ce{CH3C6H4OH} < \ce{C6H5OH} < \ce{(NO2)3C6H2OH} \)
Matching the given compounds:
(A) Propan-1-ol: \( \ce{CH3CH2CH2OH} \) (alcohol)
(C) 4-methylphenol: \( \ce{CH3C6H4OH} \) (phenol with an electron donating group, pKa 10.2)
(D) Phenol: \( \ce{C6H5OH} \) (pKa 10.0)
(B) Picric acid: 2,4,6-trinitrophenol (three electron withdrawing groups, pKa about 0.4)
Shortcut: Propan-1-ol (A) is the weakest acid and picric acid (B) the strongest. Only Option (2) starts with (A) and ends with (B).
NCERT (2026-27 edition):
Acidity of Phenols – …
