Consider the following statements about Kolbe’s reaction and the Reimer-Tiemann reaction.
(A) In Kolbe’s reaction, the phenoxide ion undergoes electrophilic substitution with carbon dioxide.
(B) In Kolbe’s reaction, carbon dioxide acts as a strong electrophile.
(C) In the Reimer-Tiemann reaction, a –CHO group is introduced at the para position of the benzene ring of phenol.
(D) In the Reimer-Tiemann reaction, the intermediate substituted benzal chloride is hydrolysed in the presence of alkali to produce salicylaldehyde.
Which of the statements given above are incorrect? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (B) and (C) only
In both reactions phenol is treated with sodium hydroxide, and the new group enters the ortho position of the ring:
\( \displaystyle \text{Kolbe:}\ \ \ce{C6H5OH} \xrightarrow{\ce{NaOH}} \ce{C6H5ONa} \xrightarrow[\text{(ii) } \ce{H+}]{\text{(i) } \ce{CO2}} \ce{HOC6H4COOH}\ \text{(salicylic acid)} \)
\( \displaystyle \text{Reimer-Tiemann:}\ \ \ce{C6H5OH} \xrightarrow[\text{(ii) } \ce{H+}]{\text{(i) } \ce{CHCl3} + \text{aq. } \ce{NaOH}} \ce{HOC6H4CHO}\ \text{(salicylaldehyde)} \)
(A) In Kolbe’s reaction, the phenoxide ion undergoes electrophilic substitution with carbon dioxide. — Correct statement. Phenoxide ion, formed from phenol and NaOH, is the species that reacts with CO2; ortho-hydroxybenzoic acid is the main product.
(B) In Kolbe’s reaction, carbon dioxide acts as a strong electrophile. — Incorrect statement. Carbon dioxide is a weak electrophile. The reaction works because the phenoxide ion is even more reactive than phenol towards electrophilic substitution.
(C) In the Reimer-Tiemann reaction, a –CHO group is introduced at the para position of the benzene ring of phenol. — Incorrect statement. Chloroform and sodium hydroxide introduce the –CHO group at the ortho position, which is why the product is salicylaldehyde (2-hydroxybenzaldehyde).
(D) In the Reimer-Tiemann reaction, the intermediate substituted benzal chloride is hydrolysed in the presence of alkali to produce salicylaldehyde. — Correct statement. The –CHCl2 group of the substituted benzal chloride is hydrolysed by the alkali to the –CHO group.
NCERT (2026-27 edition):
7.4.4 Chemical Reactions – “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.”
“On treating phenol with chloroform in the presence of sodium hydroxide, a –CHO group is introduced at ortho position of benzene ring. This reaction is known as Reimer - Tiemann reaction.”
“The intermediate substituted benzal chloride is hydrolysed in the presence of alkali to produce salicylaldehyde.”