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 solve the problem, we need to identify the reaction that converts phenol to salicylic acid.

1. Understanding the Conversion:
Phenol has the formula C\(_6\)H\(_5\)OH, and salicylic acid is C\(_6\)H\(_4\)(OH)(COOH), a benzene ring with both a hydroxyl (-OH) and a carboxyl (-COOH) group. The conversion involves introducing a carboxyl group to the phenol ring, specifically at the ortho position relative to the hydroxyl group.

2. Evaluating Option C (Reimer-Tiemann Reaction):
The Reimer-Tiemann reaction involves treating phenol with chloroform (CHCl\(_3\)) and a strong base (e.g., KOH) to introduce an aldehyde group (-CHO) ortho to the hydroxyl group, forming salicylaldehyde (C\(_6\)H\(_4\)(OH)(CHO)). This does not directly yield salicylic acid, as the product is an aldehyde, not a carboxylic acid. Further oxidation would be needed, which is not part of the standard reaction. Thus, this option is incorrect.

3. Evaluating Option B (Friedel-Crafts Reaction):
The Friedel-Crafts reaction typically involves alkylation or acylation of an aromatic ring using a halogenated compound and a Lewis acid catalyst (e.g., AlCl\(_3\)). For phenol, the -OH group is strongly activating and ortho-para directing, but Friedel-Crafts acylation (e.g., with CO and HCl) to introduce a carboxyl group directly is not a standard method for salicylic acid synthesis. Moreover, the -OH group can react with the catalyst, complicating the reaction. This option is incorrect.

4. Evaluating Option A (Kolbe Reaction):
The Kolbe-Schmitt reaction (often referred to as the Kolbe reaction in this context) involves heating sodium phenoxide (C\(_6\)H\(_5\)ONa, formed from phenol and NaOH) with carbon dioxide (CO\(_2\)) under pressure at around 125°C, followed by acidification. This introduces a carboxyl group ortho to the phenoxide group, forming salicylic acid:

$ \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 is the standard industrial method for synthesizing salicylic acid from phenol, making this option correct.

5. Evaluating Option D (Coupling Reaction):
A coupling reaction typically refers to reactions like diazo coupling, where a diazonium salt reacts with an aromatic compound (e.g., phenol) to form azo dyes. This does not involve the formation of a carboxyl group or salicylic acid. Thus, this option is incorrect.

Final Answer:
The conversion of phenol to salicylic acid can be accomplished by the Kolbe reaction (option A).