Kolbe Reaction: Synthesis of Salicylic Acid
The Kolbe Reaction (also known as the Kolbe-Schmitt reaction) is an industrial method for the synthesis of Salicylic Acid (2-Hydroxybenzoic acid) by heating sodium phenoxide with carbon dioxide under pressure.
1. General Reaction
Phenol is first treated with Sodium Hydroxide to form Sodium Phenoxide. This undergoes electrophilic substitution with $CO_2$ followed by acidification to yield the product.
Conditions:
- Reagents: NaOH, $CO_2$
- Temperature: ~400 K ($125^\circ C$)
- Pressure: 4-7 atm
2. Detailed Mechanism
The reaction proceeds via Electrophilic Aromatic Substitution (EAS).
Step 1: Activation of the Ring
Phenol reacts with NaOH to form the Phenoxide ion. The phenoxide ion is much more reactive towards electrophilic substitution than phenol because the negative charge on oxygen increases electron density in the benzene ring (especially at ortho and para positions).
Step 2: Electrophilic Attack
Carbon Dioxide ($CO_2$) acts as a weak electrophile. The electron-rich ortho position of the phenoxide ion attacks the electrophilic carbon of $CO_2$.
Step 3: Rearrangement and Acidification
The intermediate undergoes tautomerization (proton transfer) to restore aromaticity, followed by acidification to form Salicylic Acid.
3. Why Ortho Product is Major?
The reaction predominantly yields the Ortho isomer due to the stabilization of the transition state by the coordination of the Sodium ion ($Na^+$) between the phenoxide oxygen and the carboxylate oxygen (Chelation effect).
4. Applications: Synthesis of Aspirin
Salicylic acid produced by the Kolbe reaction is the starting material for the manufacture of Aspirin (Acetylsalicylic acid), a widely used analgesic and anti-inflammatory drug.
Kolbe Reaction Quiz
Test your knowledge on Phenol & Salicylic Acid chemistry. 10 MCQs with explanations.
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