Introduction
The Kolbe Reaction, also known as the Kolbe–Schmitt reaction, is an important organic reaction used to prepare aromatic hydroxy acids. In this reaction, a phenoxide ion reacts with carbon dioxide to form a carboxylated product, usually at the ortho position.
One of the most common applications is the preparation of salicylic acid from sodium phenoxide and carbon dioxide under pressure at around 120°C.
Reaction Overview
When phenol is converted into a phenoxide ion (usually using sodium hydroxide), it becomes much more reactive. This activated aromatic ring undergoes electrophilic attack by carbon dioxide, resulting in the formation of hydroxybenzoic acids.
Mechanism of Kolbe Reaction
The mechanism involves the following steps:
- Formation of phenoxide ion: Phenol reacts with a base to form phenoxide, which is more reactive than phenol itself.
- Electrophilic attack by CO₂: Carbon dioxide acts as a weak electrophile and attacks the activated aromatic ring at the ortho or para position.
- Formation of carboxylated intermediate: The intermediate then rearranges and stabilises to form a hydroxybenzoate salt.
- Acidification: Acid treatment converts the salt into the final hydroxy acid.
Temperature Influence on Product
- At 120°C, the ortho product (salicylic acid) is formed predominantly.
- At 230°C, the reaction shifts to form the para-hydroxybenzoic acid, which is the more thermodynamically stable product.
- Potassium salicylate can rearrange from ortho to para position at higher temperatures.
Why Phenoxide Is More Reactive
Phenoxide carries a negative charge and strong electron-donating resonance structures. This makes the ring much more activated towards electrophiles, even weak ones like CO₂.
Applications
The Kolbe–Schmitt reaction has major industrial importance:
- Manufacture of salicylic acid – the starting material for aspirin.
- Used to synthesise various aromatic carboxylic acids.
- Helpful in producing intermediates for dyes, pharmaceuticals, and fragrances.
Key Points to Remember
- Phenoxide ion reacts with CO₂, not phenol.
- Ortho product dominates at lower temperature.
- Para product dominates at high temperature.
- The reaction involves electrophilic aromatic substitution by carbon dioxide.
Detailed Notes:
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