In the following compounds:
I. Phenol
II. p-Cresol
III. m-Nitrophenol
IV. p-Nitrophenol
The order of acidity is:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → IV > III > I > II
Phenols are acidic because the phenoxide ion left after losing \( \ce{H^{+}} \) is stabilised by delocalising its negative charge into the ring. Groups that stabilise the phenoxide ion increase acidity; groups that destabilise it decrease acidity.
IV p-Nitrophenol (pKa 7.1) — the –\( \ce{NO2} \) group at the para position withdraws electrons by both −R and −I effects; the negative charge can be delocalised onto the nitro oxygens. Most acidic.
III m-Nitrophenol (pKa 8.3) — at the meta position the –\( \ce{NO2} \) group acts only by the −I effect (no resonance with the \( \ce{O^{-}} \)), so it is less acidic than the para isomer but more acidic than phenol.
I Phenol (pKa 10.0) — no substituent.
II p-Cresol (pKa 10.2) — the –\( \ce{CH3} \) group releases electrons (+I, hyperconjugation), destabilising the phenoxide ion. Least acidic.
NCERT (2026-27 edition):
Acidity of Phenols – Table 7.3: pKa Values of some Phenols and Ethanol
| Compound | Formula | pKa |
|---|---|---|
| o-Nitrophenol | o–O2N–C6H4–OH | 7.2 |
| m-Nitrophenol | m–O2N–C6H4–OH | 8.3 |
| p-Nitrophenol | p–O2N–C6H4–OH | 7.1 |
| Phenol | C6H5–OH | 10.0 |
| p-Cresol | p–CH3–C6H4–OH | 10.2 |
| Ethanol | C2H5OH | 15.9 |