Read the passage carefully and answer the questions.
Alcohols and phenols are formed when a hydrogen atom in a hydrocarbon, aliphatic and aromatic respectively is replaced by - OH group. These are further classified on the basis of number of hydroxy groups and according to the hybridization of the carbon atom \( sp^2 \) or \( sp^3 \) to which the -OH group is attached. They find applications in industry as well as in day-to-day life. One of the most commercially obtained alcohol also known as wood spirit is obtained by destructive distillation of wood. Both alcohols and phenols are acidic in nature. The acidity of alcohols and phenols vary according to the nature of substituent(s) attached to them. Alcohols and phenols react with carboxylic acids, acid chlorides and acid anhydrides to form esters. Introduction of acetyl group in alcohols and phenols is called acetylation. Phenol undergo electrophilic aromatic substitution reaction because the -OH activates the benzene ring. It undergoes nitration, halogenation and sulphonation reactions.
Identify the strongest acidic compound
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → p-Nitrophenol
A phenol is acidic because it can lose \( \ce{H+} \) from its –OH group to form a phenoxide ion, whose negative charge is spread over the benzene ring by resonance:
\( \displaystyle \ce{C6H5OH + H2O <=> C6H5O^- + H3O^+} \)
Any substituent that stabilises the phenoxide ion makes the phenol a stronger acid.
In p-nitrophenol the nitro group is para to the –OH group. It withdraws electrons by both its −I and −R effects, and the negative charge of the phenoxide ion is delocalised right onto the oxygen atoms of the nitro group. This makes p-nitrophenol the strongest acid of the four, as the pKa values show (a smaller pKa means a stronger acid):
\( \displaystyle \text{p-nitrophenol}\ (7.1) \gt \text{phenol}\ (10.0) \gt \text{p-cresol},\ \text{p-methoxyphenol}\ (\text{about } 10.2) \)
Why the other options are incorrect
Option (1) Phenol — Incorrect. It has no substituent to help the ring spread the charge, so it is a much weaker acid (pKa 10.0) than p-nitrophenol.
Option (3) p-Cresol — Incorrect. The methyl group releases electrons (+I effect and hyperconjugation), which destabilises the phenoxide ion; p-cresol is even slightly weaker than phenol.
Option (4) p-Methoxyphenol — Incorrect. The oxygen of –\( \ce{OCH3} \) donates its lone pair into the ring (+R effect), which outweighs its −I effect at the para position; it is also slightly weaker than phenol.
NCERT (2026-27 edition):
Reactions Involving Cleavage of O–H Bond – …
