Oxidation of caffeine by phosphate radical anion in aqueous solution underanoxic conditions

The photooxidation of caffeine in presence of peroxydiphosphate (PDP) in aq ueous solution at natural pH (similar to7.5) has been carried out in a quan tum yield reactor using a high-pressure mercury lamp. The reactions were fo llowed spectrophotometrically by measuring the absorbance of caffeine at la mbda (max) (272 nm). The rates of reaction were calculated under different experimental conditions. The quantum yields were calculated from the rates of oxidation of caffeine and the intensity of light at 254 nm which was mea sured by using peroxydisulphate solution as a standard chemical actinometer . The reaction rates of oxidation of caffeine by PDP increase with increase in [PDP] as well as with increase in light intensity, while they are indep endent of [caffeine]. The quantum yields of oxidation of caffeine by PDP al e independent of [PDP] as well as light intensity. However, quantum yields of oxidation of caffeine by PDP increase with increase in caffeine concentr ation. On the basis of these experimental results and product analysis, 3 p robable mechanism has been suggested in which PDP is activated to phosphate radical anions (PO4.2-) by direct photolysis of PDP and also by the sensit izing effect of caffeine. The phosphate radical anions thus produced react with caffeine by electron transfer reaction, resulting in the formation of caffeine radical cation, which deprotonates in a fast step to produce C8-OH adduct radicals. These radicals might react with PDP to give final product 1,3,7-trimethyluric acid and PO4.2- radicals, the latter propagates the ch ain reaction.