Arrange the following in decreasing order of their acidic strength?
(A) Phenol
(B) o-Nitrophenol
(C) m-Nitrophenol
(D) p-Nitrophenol
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (D), (B), (C), (A)
Phenol is acidic because the phenoxide ion formed on losing \( \ce{H^{+}} \) is stabilised by resonance. A substituent changes the acidity by changing the stability of this ion:
Electron withdrawing groups (such as –\( \ce{NO2} \)) spread the negative charge further, stabilise the phenoxide ion and increase acidity.
Position matters: at the ortho and para positions, –\( \ce{NO2} \) withdraws electrons by both the −R (resonance) and −I (inductive) effects, and the negative charge can move onto the nitro group. At the meta position, only the weaker −I effect acts.
Ortho vs para: o-nitrophenol is slightly weaker than p-nitrophenol, because intramolecular hydrogen bonding between –OH and –\( \ce{NO2} \) holds the proton more firmly.
A lower pKa means a stronger acid. In decreasing order of acidic strength:
(D) p-nitrophenol > (B) o-nitrophenol > (C) m-nitrophenol > (A) phenol
Shortcut: p-Nitrophenol (D), with −R and −I effects and no intramolecular hydrogen bond, is the most acidic. Only Option (3) starts with (D).
NCERT (2026-27 edition):
Acidity of Phenols – …
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| 29-05 S1 26-05 S1 | – | – | – | – |