An insight on the mechanism of the Hoppe-Seyler test for xanthine

Being xanthine an important biomolecule, it has attracted the attention of scholars from different points of view. In the analytical field some tests for identification have been developed. Hoppe-Seyler detected xanthine using calcium chlorohypochlorite and sodium hydroxide since a green ring, changing to brown, is formed. It is a challenge to the theoretical chemist disentangle what is happening at molecular level during the assay. In this communication emphasis is placed in a reactive tri-polar intermediate that arises in a reaction sequence. That is, a carboxylate and a newly formed zwitterion. Reaction with water can eliminate the last dipole (a negatively charged nitrogen and a carbonium ion). However, there is other pair of ions that are connected: the carboxylate and the carbonium ion at β-position. This situation produces immediate decarboxylation, eliminating two electrical charges in a single internal way, and forming a double bond. Neutralization of the negatively charged nitrogen atom gives an ureido. This reaction sequence produces the principal reaction product, imidazo-imidazolone. The observed colours are due to halochromism.