The Kolbe–Schmitt reaction is a carboxylation reaction in which phenoxide ions react with carbon dioxide to form hydroxybenzoic acids. The reaction provides a useful way to add a carboxyl group directly to an activated aromatic ring. ^[1–3]

The reaction is named after two German chemists, Hermann Kolbe and Rudolf Schmitt, who described it in the mid-1800s. 

Example

The most common example of this reaction is making salicylic acid from sodium phenoxide and carbon dioxide. It can be shown in two parts. ^[1–3]

C₆H₅ONa + CO₂ → o-HOC₆H₄COONa

o-HOC₆H₄COONa + HCl → o-HOC₆H₄COOH + NaCl

Salicylic acid is used to prepare aspirin and related salicylate derivatives. 

Mechanism

Step 1: Formation of Phenoxide Ion ^[4]

Phenol reacts with sodium hydroxide and loses the hydrogen ion from its –OH group. This forms sodium phenoxide, which contains the phenoxide ion, C₆H₅O⁻. ^[1–4]

C₆H₅OH + NaOH → C₆H₅ONa + H₂O

In the phenoxide ion, the negative charge is delocalized into the benzene ring. This increases electron density at the ortho and para positions, making the ring more reactive toward carbon dioxide than phenol itself.

Step 2: Reaction With Carbon Dioxide

Under elevated temperature and pressure, the electron-rich phenoxide ring reacts with carbon dioxide. In the classical sodium phenoxide reaction, carboxylation occurs mainly at the ortho position.

In simplified form, the reaction can be represented as:

C₆H₅ONa + CO₂ → o-HOC₆H₄COONa

This product is sodium salicylate. During the reaction, the carbon dioxide molecule becomes attached to the aromatic ring as a carboxylate group, and aromaticity is restored after proton transfers.

Step 3: Acidification

Sodium salicylate is a salt, so it must be treated with acid to form the final product. During acidification, the carboxylate group is protonated to produce salicylic acid.

o-HOC₆H₄COONa + HCl → o-HOC₆H₄COOH + NaCl

Thus, the overall sequence is:

phenol → sodium phenoxide → sodium salicylate → salicylic acid

The role of Kolbe–Schmitt equation in preparing salicylic acid makes it especially significant in pharmaceutical chemistry. Its broader use in forming hydroxybenzoic acid derivatives gives it continued value in organic synthesis.