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The photooxidation of caffeine in presence of peroxydiphosphate (PDP) in aqueous solution at natural pH (∼7.5) has been carried
out in a quantum yield reactor using a high-pressure mercury lamp. The reactions were followed spectrophotometrically by measuring
the absorbance of caffeine at λmax (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 measured 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 independent of [caffeine]. The quantum yields of oxidation
of caffeine by PDP are independent of [PDP] as well as light intensity. However, quantum yields of oxidation of caffeine by
PDP increase with increase in caffeine concentration. On the basis of these experimental results and product analysis, a probable
mechanism has been suggested in which PDP is activated to phosphate radical anions (PO4
·2−) by direct photolysis of PDP and also by the sensitizing 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 chain reaction. 相似文献
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