Question

The conversion of phenol to salicylic acid can be accomplished by

• A. Kolbe reaction
• B. Friedel-Crafts reaction
• C. Reimer-Tiemann reaction
• D. Coupling reaction

Hint:

The Reimer-Tiemann reaction involves the chlorination of phenol followed by hydrolysis, which yields salicylic acid, a precursor of aspirin.

Answer 1

The Correct Option is A

To synthesize salicylic acid from phenol, we must identify the correct reaction. Phenol (C\(_6\)H\(_5\)OH) is converted to salicylic acid (C\(_6\)H\(_4\)(OH)(COOH)) by introducing a carboxyl group (-COOH) ortho to the existing hydroxyl group (-OH) on the benzene ring.

1. Reimer-Tiemann Reaction: This reaction treats phenol with chloroform and a strong base, yielding salicylaldehyde (C\(_6\)H\(_4\)(OH)(CHO)). This produces an aldehyde, not a carboxylic acid, so it is incorrect.

2. Friedel-Crafts Reaction: While capable of acylating aromatic rings, direct introduction of a carboxyl group to phenol using Friedel-Crafts methods is not standard. The phenolic hydroxyl group can also interfere with Lewis acid catalysts. Therefore, this is incorrect.

3. Kolbe Reaction: The Kolbe-Schmitt reaction involves reacting sodium phenoxide (C\(_6\)H\(_5\)ONa) with carbon dioxide (CO\(_2\)) under heat and pressure, followed by acidification. This reaction precisely adds a carboxyl group at the ortho position, forming salicylic acid. This is the standard industrial synthesis.

$ \text{C}_6\text{H}_5\text{ONa} + \text{CO}_2 \xrightarrow{\text{heat, pressure}} \text{C}_6\text{H}_4(\text{OH})(\text{COONa}) \xrightarrow{\text{H}^+} \text{C}_6\text{H}_4(\text{OH})(\text{COOH}) $
This option is correct.

4. Coupling Reaction: Coupling reactions, such as diazo coupling, are used to form azo dyes and do not produce salicylic acid or involve the addition of a carboxyl group. This option is incorrect.

Conclusion: The Kolbe reaction (option A) is the correct method for converting phenol to salicylic acid.