Kolbe-Schmitt reaction mechanism

Markovic, Z., Engelbrecht, J. P., Markovic, S. Theoretical study of the Kolbe-Schmitt reaction mechanism. Z Naturforsch., A Phys. Sci. 

Sodium phenoxides can be carboxylated, mostly in the ortho position, by carbon dioxide (the Kolbe-Schmitt reaction). The mechanism is not clearly understood, but... 

The Kolbe-Schmitt reaction[l] has long history related with aspirin and has been a name reaction used for the longest period in an industrial process. While the demand for the manufacturing aromatic hydroxycarboxyhc acids is still successively coming out today with a number of patents, the mechanism of the reaction has remained unsolved. The present nmr spectroscopic studies have proved a [substrate CO2] complex or an intermediate prior to the formation of carboxyhc acids. Another puzzling question about the unstable complex even to moisture, is why the carboxylation of polyhydroxybenzenes, such as resorcinol, should proceed in aqueous solutions. Herein also reported are kinetic studies on the carboxylation of resorcinol in aqueous solutions of alkali hydrogencarbonates. 

The mechanism of the Kolbe-Schmitt reaction was investigated since the late 1800s, but the mechanism of the carboxylation could not be elucidated for more than 100 years. For a long time, the accepted mechanism was that the carbon dioxide initially forms an alkali metal phenoxide-C02 complex, which is then converted to the aromatic carboxylate at elevated temperature. The detailed mechanistic study conducted by Y. Kosugi et al. revealed that this complex is actually not an intermediate in the reaction, since the carefully prepared phenoxide-C02 complex started to decompose to afford phenoxide above 90 °C. They also demonstrated that the carboxylated products were thermally stable even at around 200 °C. The CO2 electrophile attacks the ring directly to afford the corresponding ortho- or para-substituted products. (When the counterion is large (e.g., cesium) the attack of CO2 at the ortho-position is hindered therefore, the para-substituted product is the major product.)... 

Thermodynamic analysis indicates that such reactions are on the whole unfavorable. Furthermore, the complexity and uncertainty of the mechanism by which such reactions may occur makes the choice of proper catalysts largely an empirical selection. However, it must not be overlooked that such reactions as the Kolbe-Schmitt synthesis of salicylic acid from dry sodium phenoxide and carbon dioxide represent processes, the success of which might point the direction to the solution of the problems attending the realization of the type reactions mentioned above. 

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