In Kolbe’s reaction, phenol is first treated with sodium hydroxide, a…

The correct answer is Option (4) → The phenoxide ion is even more reactive than phenol towards electrophilic substitution, so it reacts with carbon dioxide, a weak electrophile.

In Kolbe’s reaction carbon dioxide is the electrophile, but a weak one. Phenol itself is not reactive enough for it. Sodium hydroxide turns phenol into the phenoxide ion, whose ring is even more electron rich, and this ion is attacked by CO₂ at the ortho position:

    \( \displaystyle \ce{C6H5OH} \xrightarrow{\ce{NaOH}} \ce{C6H5O^-Na+} \xrightarrow[\text{(ii) }\ce{H+}]{\text{(i) }\ce{CO2}} \text{o-}\ce{HOC6H4COOH} \)

Why the other options are incorrect

Option (1) Carbon dioxide is a strong electrophile, so it reacts more readily with an anion such as the phenoxide ion. — Incorrect. Carbon dioxide is a weak electrophile; that is why the more reactive phenoxide ion is needed.

Option (2) The phenoxide ion is less reactive than phenol, so the substitution stops after one –COOH group enters the ring. — Incorrect. The phenoxide ion is more reactive than phenol towards electrophilic substitution, not less.

Option (3) Sodium hydroxide converts carbon dioxide into the carbonate ion, which then attacks phenol at the ortho position. — Incorrect. Sodium hydroxide is used to form the phenoxide ion from phenol. The negatively charged carbonate ion is not an electrophile; the ring is attacked by CO₂ itself.

NCERT (2026-27 edition):
7.4.4 Chemical Reactions – “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.”