On the interaction of uric acid with ammonium molybdate in acidic medium

This communication is a theoretical organic chemistry study on the interaction of uric acid with ammonium molybdate in acidic medium (Gigli test for uric acid). Colour tests give rapid, visible, results. However, it is interesting know what is happening in the test tube at molecular level. In the present paper we describe the reaction route of the above mentioned assay. Each step is fully commented and the electron flow is also given. The key stages are: addition of molybdic acid to the C-C double bond, acidolysis of the organometallic ester with reduction to molybdenum (IV) and epoxide formation (oxidation step). Ring opening of the oxirane and reaction with water. A glycol is formed followed by opening of the five-member ring giving a carbonyl and an ureido chain. Finally, elimination of urea gives rise to 2,4,5,6-tetraoxo-hexahydropyrimidine (alloxan) which is hydrated. The blue colour observed (molybdenum blue) is a mixture of several oxides.