Abstract
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.
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