Kolbe-Schmidt reaction

What is Kolbe-Schmidt reaction?

In 1860, Kolbe first reported a chemical reaction that was subsequently studied by Schmidt in 1886. The reaction involves the conversion of carbon dioxide and either sodium or potassium phenolate into ortho- or para-hydroxyl benzoic acid. This reaction is commonly referred to as the Kolbe-Schmidt reaction, the Kolbe-Schmidt carboxylation, or the Kolbe-Schmidt synthesis.

Kolbe-Schmidt reaction - general reaction scheme - the Kolbe-Schmidt carboxylation - the Kolbe-Schmidt synthesis
Kolbe-Schmidt reaction (with sodium yields ortho-hydroxyl benzoic acid)

Interestingly, heating a mixture of phenol, potassium carbonate, and carbon dioxide CO2 can also result in the formation of hydroxyl benzoic acid.

Sodium phenolate leads to the formation of ortho-hydroxyl benzoic acid, while potassium phenolate often results in para-hydroxyl benzoic acid.

Kolbe-Schmidt reaction - general reaction scheme - the Kolbe-Schmidt carboxylation - the Kolbe-Schmidt synthesis
Kolbe-Schmidt reaction (with potassium yields para-hydroxyl benzoic acid)

The precise mechanism of this reaction is unclear, but it is hypothesized that sodium phenolate exists as contact ion pairs, while potassium phenolate forms dissociated ion pairs. This difference in ion pairing may account for the differing positions of the hydroxyl group in the resulting product.

Although this reaction is generally feasible only with highly activated phenols containing electron-donating groups, some phenols with electron-withdrawing groups have also been shown to undergo the reaction. Additionally, the reaction has been observed with 3-hydroxy pyridine and hydroxyl-2(1H)-pyridinone.

References

Kolbe, H. (1860), Ueber Synthese der Salicylsäure. Justus Liebigs Ann. Chem., 113: 125-127. https://doi.org/10.1002/jlac.18601130120

Shares