Arrange the following phenols in the decreasing order of their acidic strength:
(A) Phenol
(B) o-Cresol
(C) o-Nitrophenol
(D) 2,4-Dinitrophenol
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (D), (C), (A), (B)
A phenol is acidic because it loses \( \ce{H+} \) to give the phenoxide ion, which is stabilised by resonance. Anything that stabilises the phenoxide ion more makes the phenol a stronger acid, and anything that destabilises it makes the phenol weaker.
Electron-withdrawing groups such as –\( \ce{NO2} \) spread the negative charge of the phenoxide ion and increase acidity. At the ortho and para positions the nitro group withdraws by resonance as well as by its −I effect, and each extra nitro group adds to the effect.
Electron-releasing groups such as –\( \ce{CH3} \) push electron density into the ring, intensify the negative charge on oxygen and decrease acidity.
Applying this to the four phenols:
(A) Phenol, \( \ce{C6H5OH} \): no substituent (pKa 10.0).
(B) o-Cresol, \( \ce{CH3-C6H4-OH} \): an electron-releasing methyl group, so slightly weaker than phenol (pKa 10.2).
(C) o-Nitrophenol, \( \ce{O2N-C6H4-OH} \): one electron-withdrawing nitro group at the ortho position (pKa 7.2).
(D) 2,4-Dinitrophenol: two nitro groups, at the ortho and para positions, both withdrawing by resonance: the strongest acid of the four (pKa about 4).
In decreasing order of acidic strength:
\( \displaystyle \text{2,4-dinitrophenol} \gt \text{o-nitrophenol} \gt \text{phenol} \gt \text{o-cresol} \)
Shortcut: The dinitrophenol must come first and the cresol last. Only Option (2) starts with (D) and ends with (B).
NCERT (2026-27 edition):
Reactions Involving Cleavage of O–H Bond – “… In substituted phenols, the presence of electron withdrawing groups such as nitro group, enhances the acidic strength of phenol. This effect is more pronounced when such a group is present at ortho and para positions. It is due to the effective delocalisation of negative charge in phenoxide ion when substituent is at ortho or para position. On the other hand, electron releasing groups, such as alkyl groups, in general, do not favour the formation of phenoxide ion resulting in decrease in acid strength. Cresols, for example, are less acidic than phenol.”