Koch reaction, catalysts mechanism

The decarbonylation of aromatic aldehydes with sulfuric acid" is the reverse of the Gatterman-Koch reaction (11-16). It has been carried out with trialkyl- and trialkoxybenzaldehydes. The reaction takes place by the ordinary arenium ion mechanism the attacking species is H and the leaving group is HCO, which can lose a proton to give CO or combine with OH from the water solvent to give formic acid." Aromatic aldehydes have also been decarbonylated with basic catalysts." When basic catalysts are used, the mechanism is probably similar to the SeI process of 11-38. See also 14-39. 

The simplest acyl chloride, formyl chloride, H-C-Cl, is unstable, decomposing to HCl and CO upon attempted preparation. ThCTefore, direct Friedel-Crafts formylation of benzene is impossible. An alternative process, the Gattermann-Koch reaction, enables the introduction of the formyl group, -CHO, into the benzene ring by treatment with CO under pressure, in the presence of HCl and Lewis acid catalysts. For example, methylbenzene (toluene) can be formylated at the para position in this way in 51% yield. The electrophile in this process was observed directly for the first time in 1997 by treating CO with HF-SbFs under high pressure C NMR 8 139.5 ppm IR V = 2110 cm . What is the structure of this species and the mechanism of its reaction with methylbenzene Explain the spectral data. (Hints Draw the Lewis structure of CO and proceed by considering the species that may arise in the presence of acid. The comparative spectral data for free CO are C NMR 8 = 181.3 ppm IR P = 2143 cm . )... 

H. Koch [593] formulated the mechanism of the carboxylic acid synthesis found by him via initial formation of a carbonium ion from the starting material and the acid catalyst with subsequent addition of carbon monoxide to the carbonium ion to give an acylium cation in the first reaction step (1) followed by the reaction of the acylium cation with water or alcohol in the second step (2). 

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