Kolbe–Schmitt reaction

The synthesis of ortho -hydroxybenzoic acid and derivatives thereof, alkali metal phenolates prepared by heating with carbon dioxide at high pressure is known as the Kolbe-Schmitt reaction or the Kolbe synthesis. The reaction was discovered in 1860 by AWH Kolbe as a method for the synthesis of salicylic acid, which he carried out the implementation of the above components at 180-200 ° C. This synthesis was improved in 1885 by Rudolf Schmitt.

Mechanism

For the mechanism of this reaction, a complex of sodium phenolate and carbon dioxide is postulated that explained together with the stability of the formed intermediate chelate complex and the high ortho- selectivity of this reaction. By complexing the Kohlenstoffdioxidmolekül becomes polarized, whereby the electrophilic character is enhanced, and it is fixed in a position which allows only the ortho- attack of the ring. We speak here also of the chelate effect. To form an OH group at Carbonylensauerstoffatom it comes to re-aromatization. Now, the negatively charged oxygen atom attacks the sulfuric acid. It caused this sodium hydrogen sulfate and ortho -hydroxybenzoic acid.

The corresponding para -hydroxybenzoic acid and their derivatives can be synthesized from the respective Kaliumphenolaten.

The Kolbe -Schmitt reaction is limited to phenols and substituted phenols (including higher annelated phenols such as naphthols ). This reaction is still used for the industrial synthesis of salicylic acid from sodium, which is why the Kolbe -Schmitt reaction is often referred to simply as salicylic acid synthesis.