Formation of Citric Acid Anhydrides

As mentioned above, behaviour of citric acid in chemical reactions is similar to that expected from other hydroxycarboxylic acids. However, the tertiary sterically hindered hydroxyl group does not undergo all the common reactions being mainly important only in complex formation reactions. The natural and synthetic derivatives of citric acid and different reactivities of the carboxylic groups were reviewed by Milewska [478] (see also [479]) and therefore only the most important chemical reactions involving citric acid are shortly discussed here. 

Dehydration of organic acids leads to corresponding anhydrides, but as stated by Repta and Higuchi [480, 481], the mentioned in the literature dehydrated citric acid is actually anhydrous crystalline form of citric acid or aconitic acid, but not a true anhydride of citric acid. They were able to synthesize and isolate the mono-molecular unsymmetrical anhydride of citric acid. The conversion of citric acid to its anhydride is performed by interacting of the solid acid with an excess of acetic anhydride in glacial acetic acid at 36-38 °C. The melting point of prepared white crystalline solid anhydride is at 121-123 °C. The identification of thise compound was performed by elementary analysis, potentiometric titration, cryoscopic and NMR measurements. From three possible anhydride stractures (A) - symmetrical anhydride, (B) - unsymmetrical anltydride and (C) - intermolecular anhydride. 

Actually, an interest in the anhydride of citric acid synthesis started when Repta et al. [488] and Robinson et al. [489] analyzed behaviour of acetic anhydride and 

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