Mechanism of free radical oxidation of caffeine in aqueous solution

Abstract

The oxidation of caffeine with persulfate and hydroxyl radicals in aqueous solution has been studied by EPR spectroscopy and HPLC analysis. In both cases the formation of 1,3,7-trimethyluric acid is observed as the main final product. A C8–OH radical adduct is postulated as the intermediate after reaction with OH˙, and leads to the final product after further oxidation. This radical is too short-lived to be observed by EPR. After oxidation of caffeine with SO₄^˙- its radical cation is detected by EPR. This radical reacts with water to produce the above mentioned C8–OH radical adduct after deprotonation. In the presence of dibasic phosphate a spectrum attributed to the C8-phosphate radical adduct is observed. This radical results from the nucleophilic attack of the buffer on the radical cation of caffeine. Hydrolysis and oxidation of the phosphate radical intermediate results in the formation of 1,3,7-trimethyluric acid.