Arrange the following compounds in increasing order of their acidity:
(A) Phenol
(B) 4-methylphenol
(C) 3-nitrophenol
(D) 4-nitrophenol
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (B), (A), (C), (D)
Phenol is acidic because the phenoxide ion left after the loss of \( \ce{H^{+}} \) is stabilised by delocalisation of its negative charge over the ring. A substituent changes the acidity by changing this stability:
Electron withdrawing groups such as –\( \ce{NO2} \) pull the negative charge further away from oxygen and increase the acidity. At the ortho and para positions the nitro group takes part in the delocalisation by resonance as well as by its −I effect; at the meta position it acts only through the weaker −I effect.
Electron releasing groups such as –\( \ce{CH3} \) push electron density towards the ring, destabilise the phenoxide ion and decrease the acidity; cresols are weaker acids than phenol.
\( \displaystyle \ce{C6H5OH + H2O <=> C6H5O- + H3O+} \)
(A) Phenol: pKa 10.0, the reference.
(B) 4-methylphenol (p-cresol): pKa 10.2, electron releasing \( \ce{CH3} \), the weakest acid.
(C) 3-nitrophenol: pKa 8.3, −I effect only.
(D) 4-nitrophenol: pKa 7.1, −I and resonance, the strongest acid.
A larger pKa means a weaker acid. In increasing order of acidity:
\( \displaystyle \text{4-methylphenol} \lt \text{phenol} \lt \text{3-nitrophenol} \lt \text{4-nitrophenol} \)
So the order is (B), (A), (C), (D).
Shortcut: The cresol (B) is the weakest acid and 4-nitrophenol (D) the strongest; only Option (1) starts with (B) and ends with (D).
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.”
