Arrange the following compounds in increasing order of their acid strength:
(A) Propan-1-ol
(B) 3-nitrophenol
(C) 3,5-Dinitrophenol
(D) Phenol
(E) 4-Methylphenol
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → A < E < D < B < C
Phenols are more acidic than alcohols because the phenoxide ion is stabilised by resonance (the negative charge is delocalised over the ring), while the alkoxide ion is not. Among phenols, an electron withdrawing group (–\( \ce{NO2} \)) spreads the negative charge further and increases acidity; an electron releasing group (–\( \ce{CH3} \)) does the opposite.
A. Propan-1-ol — first (weakest acid): an alcohol; its alkoxide ion has no resonance (pKa about 16).
E. 4-Methylphenol — \( \ce{CH3} \) releases electrons and destabilises the phenoxide ion; second: less acidic than phenol (pKa 10.2).
D. Phenol — third: the parent phenol, pKa 10.0.
B. 3-Nitrophenol — one –\( \ce{NO2} \) withdrawing electrons by its −I effect; fourth: more acidic than phenol (pKa 8.3).
C. 3,5-Dinitrophenol — two –\( \ce{NO2} \) groups add their electron withdrawing effects; last (strongest acid of the five).