An organic compound A, molecular formula C6H6O, gives a violet colour with neutral FeCl3 solution. A on treatment with CO2 and NaOH at 400 K under pressure gives B, which on acidification gives a compound C. Compound C reacts with acetyl chloride to give D, which is a popular painkiller.
What is the correct order of acid strength of the following compounds?
I. Phenol
II. p-Cresol
III. m-Nitrophenol
IV. p-Nitrophenol
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → IV > III > I > II
A phenol is acidic because its phenoxide ion is stabilised by delocalisation of the negative charge. A group that stabilises the phenoxide ion further raises the acid strength; a group that destabilises it lowers the acid strength. The lower the pKa, the stronger the acid.
IV. p-Nitrophenol — 1st (strongest), pKa 7.1. –NO2 is electron withdrawing, and at the para position it delocalises the negative charge of the phenoxide ion most effectively.
III. m-Nitrophenol — 2nd, pKa 8.3. –NO2 still withdraws electrons, but at the meta position it cannot take part in the delocalisation of the charge, so the effect is weaker than at para.
I. Phenol — 3rd, pKa 10.0. It has no substituent on the ring.
II. p-Cresol — 4th (weakest), pKa 10.2. –CH3 is electron releasing; it destabilises the phenoxide ion, so cresols are less acidic than phenol.
NCERT (2026-27 edition):
7.4.4 Chemical Reactions – “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 pKa value, the weaker the acid.”
Table 7.3: pKa Values of some Phenols and Ethanol
