Treatment of sodium salt of phenol with carbon dioxide under pressure brings about substitution of the carboxyl group, -COOH, for hydrogen of the ring. This provides a path of conversion of phenol into hydroxy carboxylic acids. The acids formed are industrially very important compounds and form many useful compounds.
Why do we use sodium phenoxide rather than phenol for this reaction?

Explanation

To answer the question of why sodium phenoxide is used rather than phenol for the reaction with carbon dioxide under pressure, we can break down the reasoning into several steps:

### Step-by-Step Solution:

1. **Understanding the Reaction**: The reaction involves the treatment of sodium phenoxide (the sodium salt of phenol) with carbon dioxide (CO2) under pressure, leading to the substitution of a hydrogen atom on the aromatic ring with a carboxyl group (-COOH). This results in the formation of hydroxycarboxylic acids.

2. **Nature of Sodium Phenoxide**: Sodium phenoxide is the deprotonated form of phenol, where the hydroxyl group (-OH) has lost a hydrogen ion (H+) and is now negatively charged (O-Na+). This negative charge makes sodium phenoxide a stronger nucleophile compared to neutral phenol.

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