Arrange the following compounds in increasing order of their acid str…

The correct answer is Option (3) → (D) , (A) , (B) , (C)

Phenol is acidic because the phenoxide ion left after the loss of H⁺ is stabilised by delocalisation of its negative charge over the benzene ring:

    \( \displaystyle \ce{C6H5OH + H2O <=> C6H5O^- + H3O^+} \)

A substituent on the ring changes the acid strength by changing the stability of this ion:

• Electron withdrawing groups such as –NO₂ pull the negative charge further away from oxygen, stabilise the phenoxide ion and increase the acid strength. At the ortho and para positions the nitro group acts 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 –CH₃ push electron density into the ring, destabilise the phenoxide ion and decrease the acid strength.

A smaller pK_a means a stronger acid.

• (D) p-Cresol — first (weakest acid). The –CH₃ group is electron releasing, so the phenoxide ion is less stable than that of phenol (pK_a 10.2).

• (A) Phenol — second. No substituent; its phenoxide ion is stabilised by the ring alone (pK_a 10.0).

• (B) m-Nitrophenol — third. At the meta position the –NO₂ group cannot take the negative charge by resonance; it withdraws electrons only by its −I effect, so the acid strength rises less (pK_a 8.3).

• (C) p-Nitrophenol — last (strongest acid). At the para position the –NO₂ group withdraws electrons by resonance and −I effect; the negative charge spreads onto the oxygen atoms of the nitro group, so the phenoxide ion is the most stable (pK_a 7.1).

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. The greater the pK_a value, the weaker the acid.”

Table 7.3: pK_a Values of some Phenols and Ethanol

Note: The same/similar question has been asked in different years by NTA as per the details given below: