Arrange the following compounds in increasing order of their acid strength:
(A) 4-methylphenol
(B) 3-nitrophenol
(C) 2,4,6-trinitrophenol
(D) Phenol
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A), (D), (B), (C)
Phenol is acidic because the phenoxide ion left after losing \( \ce{H^{+}} \) is stabilised by delocalisation of the negative charge over the benzene ring. A substituent changes the acidity by changing this stability:
Electron withdrawing groups (such as –\( \ce{NO2} \)) spread the negative charge further, stabilise the phenoxide ion and increase acidity. The effect is strongest at the ortho and para positions (resonance and inductive effect), weaker at meta (inductive effect only), and it adds up when there are several such groups.
Electron releasing groups (such as –\( \ce{CH3} \)) increase the negative charge on oxygen, destabilise the phenoxide ion and decrease acidity.
Combining the two effects (lower pKa means stronger acid):
alkyl phenol < phenol < one –\( \ce{NO2} \) (meta) < three –\( \ce{NO2} \) (ortho and para)
4-methylphenol < phenol < 3-nitrophenol < 2,4,6-trinitrophenol
Matching the given compounds:
(A) 4-methylphenol: electron releasing –\( \ce{CH3} \), pKa 10.2
(D) Phenol: pKa 10.0
(B) 3-nitrophenol: one –\( \ce{NO2} \) at meta, pKa 8.3
(C) 2,4,6-trinitrophenol (picric acid): three –\( \ce{NO2} \) groups at ortho and para
NCERT (2026-27 edition):
Acidity of Phenols – …
