A novel transamination reaction in a murexide-like sequence for caffeine detection

Abstract

This communication is a theoretical organic chemistry study on the Hammarsten test for caffeine. He used chlorine water and ammonium hydroxide; a violet colour indicates presence of caffeine. Since a derivative of ammonium purpurate is formed, the assay has been considered a murexide test. However, there are several important variants. The original murexide test for uric acid employs diluted nitric acid; the five-member ring in uric acid molecule is an imidazolone whereas in caffeine it is an imidazole. This difference alters the reaction starting site. Uric acid has no substituents, caffeine presents three methyl groups. The methyl al N-7 is an impediment for purpuric acid formation since a primary amine is required in order to react with a carbonyl group and form a double bond. So, assisted ammonolysis is invoked since ammonium purpurate is formed. This chemical deportment is explained by reaction of the methylamine at N-7 with the very reactive central carbonyl group in alloxan. A concerted mechanism takes place: ammonia displaces the nitrogen of the hemiaminal, a nitrogen-carbon double bond is formed with concomitant separation of hydroxyl ion. The methylimino group at alloxan is hydrated and protonation of the carbinolamine restores alloxan molecule and separation of methylamine.