Kolbe–Schmitt reaction


The Kolbe–Schmitt reaction or Kolbe process is a carboxylation chemical reaction that proceeds by heating sodium phenoxide with carbon dioxide under pressure, then treating the product with sulfuric acid. The final product is an aromatic hydroxy acid which is also known as salicylic acid.
By using potassium hydroxide, 4-hydroxybenzoic acid is accessible, an important precursor for the versatile paraben class of biocides used e.g. in personal care products.
The methodology is also used in the industrial synthesis of 3-hydroxy-2-naphthoic acid. The regiochemistry of the carboxylation in this case is sensitive to temperature.

Reaction mechanism

The Kolbe–Schmitt reaction proceeds via the nucleophile addition of a phenoxide, classically sodium phenoxide, to carbon dioxide to give the salicylate.
The final step is reaction of the salicylate with acid to form the desired salicylic acid.