ALLOXAN, or Mesoxalyl Urea, C4H2N2O4 or
CO 
|
NH—CO NH—CO |
 CO.
|
an oxidation product of uric acid, being obtained from it by the action of cold nitric acid, C6H4N4O3 + H2O + O = C4H2N2O4 + CO(NH2)2. It crystallizes from water in colourless rhombic prisms, containing four molecules of water of crystallization, and possesses a very acid reaction. It serves as the starting-point for the preparation of many related substances. Zinc and hydrochloric acid in the cold convert it into alloxantin (q.v.), hydroxylamine gives nitroso-barbituric acid, C4H2N2O3:NOH, baryta water gives alloxanic acid, C4H4N2O5, hot dilute nitric acid oxidizes it to parabanic acid (q.v.), hot potassium hydroxide solution hydrolyses it to urea and mesoxalic acid (q.v.) and zinc and hot hydrochloric acid convert it into dialuric acid, C4H4N2O4. M. Nencki has shown that alloxan combines with thiourea in alcoholic solution, in the presence of sulphur dioxide to form pseudothiouric acid, C5H6N4SO3. Methyl and dimethylalloxans are also known, the former being obtained on oxidation of methyl uric acid, and the latter on oxidation of caffeine (q.v.).