On the chemistry of the Archetti test for caffeine and uric acid

The Archetti colour test is based on the reduction of ferricyanide ions to ferrocyanide, giving Prussian blue. However, the oxidation sequence of reactions that take in the organic molecule has not been described. Some electroanalytical experiments on the oxidation of caffeine have been done but the results have not been adequately interpreted. Besides, they are incomplete since only the redox reactions can be detected, but not the isomerization and degradation path. In the present communication the complete oxidation route of caffeine is given, as well as the electron flow in each step. It involves hydration of the imino group and oxidation to 1,3,7-trimethyl-uric acid, formation of radical ion at C-9, interaction with the C─C double bond, aziridinone formation, carbon monoxide extrusion, carbonium ion neutralization, imine acidolysis, isomerization of carbinolamine and hydrolysis to the end products.