On the chemistry of the Weidel test for xanthine

In this communication the reaction route of the Weidel test for xanthine is provided.In that assay the compound is dissolved in warm chlorine water, evaporated on the water bath and treated under bell-jar with NH3.A dark pink or purple develops, changed to violet by sodium or potassium hydroxide.After hydration of the imidazole ring to a carbinoldiamino group, there is protonation at N-9 and ring opening.Acid catalysed hydration of the C-C double bond yields a hemiaminal that loses ammonia, giving a carbonyl group at C-4 in the pyrimidine ring.Hydrolysis of the formamido group in the chain affords uramil which is oxidised in part to alloxan with protonated hypochlorous acid.Combination of the last two barbituric acid derivatives form purpuric acid whose ammonium salt is purple.