Kolbe-Schmitt reaction

Schematic of the Kolbe-Schmitt reaction. Reagents: phenol, base, carbon dioxide (CO2), acid work-up. Product: o-hydroxy benzoic acid, p-hydroxy benzoic acid.

The Kolbe-Schmitt reaction is an organic reaction used to convert a phenol to a hydroxy benzoic acid using carbon dioxide gas, a base, and acid work-up. The mechanism begins with deprotonation of the phenol by the base to form a phenoxide. The phenoxide rearranges to form a carbonyl group while the aromatic ring attacks the CO2 molecule. The attack can happen from either the ortho or para position, which explains the two possible products, and results in the loss of aromaticity of the ring. Another deprotonation by the base regenerates aromaticity and produces the phenoxide again. Protonation of the phenoxide and the carboxy-late anions yield the final ortho- and/or para-hydroxy benzoic acids.[1][2]

Mechanism

Mechanism of the Kolbe-Schmitt reaction. Two possible pathways lead to product.

References:

1.
Kolbe, H. Ann. Chem. Pharm. 1860, 113, 125–127.
2.
Schmitt, R. J. Prakt. Chem. 1885, 31, 397–411.