Aromatic Kolbe-Schmitt synthesis

Formation of Aromatic Carboxylic Acids The Kolbe-Schmitt Synthesis... 

Synthesis of salicylic acid by the Kolbe-Schmitt process (see above) is an example for the direct carboxylation of aromatics.34 At the same time, it is also an example for the limitations of such reactions. Formal insertion of C02 into a C-H bond can usually be achieved with compounds containing active hydrogens (such as phenolates), and perhaps more importantly, the product carboxylic acids have to be stabilized by salt formation. Liberation of the free acid from such carboxylates requires a mineral acid and produces 1 equiv of inorganic salt (waste) (Scheme 6.1). 

Aromatic hydroxycarboxylic acids, especially salicylic acid, have a wide range of applications, for example, as valuable raw materials and intermediates in the production of pharmaceutical chemicals. Originally, salicylic acid was synthesized by the Kolbe-Schmitt reaction [57], which consists of two steps (1) the synthesis and purification of alkali metal phenoxides and (2) carboxylation (Scheme 4.4). Another possible synthetic method is via the attack of a trichloromethyl cation (generated by a copper catalyst from carbon tetrachloride) on the phenoxide anion, followed by hydrolysis of the C—Cl bonds with concentrated sodium hydroxide, because it is fairly difficult to replace an aromatic hydrogen with carboxyl functionality [58]. 

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