Alcohols are the compounds containing hydroxyl group attached to the alkyl group, whereas phenols are hydroxyl derivatives of aromatic hydrocarbons in which hydroxyl group is directly attached to carbon atom of aromatic ring. Alcohols are versatile compounds and can behave both as nucleophiles as well as electrophiles.
Phenols, too are reactive compounds and are known to undergo reactions on : phenolic group, benzene ring as well as various other name reactions like Reimer-Tiemann reaction, Kolbe's reaction etc.
The major product of the given set is:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → salicylic acid (2-hydroxybenzoic acid)
Phenol undergoes two named ortho-substitutions that are easy to mix up. In Kolbe’s reaction the phenoxide ion reacts with CO2 and gives an acid, salicylic acid; in the Reimer-Tiemann reaction phenol reacts with CHCl3/NaOH and gives an aldehyde, salicylaldehyde. Here the reagent is \( \ce{CO2} \), so this is Kolbe’s reaction. Phenol is first turned into sodium phenoxide, which is attacked by \( \ce{CO2} \), a weak electrophile, mainly at the ortho position; acidification frees the acid:
\( \displaystyle \ce{C6H5OH + NaOH -> C6H5ONa + H2O} \)
\( \displaystyle \ce{C6H5ONa + CO2} \xrightarrow{\text{400 K, 4-7 atm}} \ce{HOC6H4COONa} \)
\( \displaystyle \ce{HOC6H4COONa} \xrightarrow{\ce{H3O+}} \ce{HOC6H4COOH} \)
For comparison, the Reimer-Tiemann reaction:
\( \displaystyle \ce{C6H5OH + CHCl3 + 4NaOH -> NaOC6H4CHO + 3NaCl + 3H2O} \xrightarrow{\ce{H+}} \ce{HOC6H4CHO} \)
So the major product is ortho-hydroxybenzoic acid (salicylic acid).
Why the other options are incorrect
Option (1) 4-hydroxybenzoic acid — Incorrect. The para isomer is only a minor product under these conditions; the ortho acid is the main product.
Option (2) 3-hydroxybenzoic acid — Incorrect. The –\( \ce{O^{-}} \) group directs to ortho and para positions, so meta substitution does not occur.
Option (4) benzoic acid — Incorrect. The –OH group is not removed in the reaction; benzoic acid has no –OH on the ring.
NCERT (2026-27 edition):
Kolbe’s Reaction – “Phenoxide ion generated by treating phenol with sodium hydroxide is even more reactive than phenol towards electrophilic aromatic substitution. Hence, it undergoes electrophilic substitution with carbon dioxide, a weak electrophile. Ortho hydroxybenzoic acid is formed as the main reaction product.”