CHEMICAL PROPERTIES 369 



OH O' 



N- 

 O ^N' "OH O' "N' OH 



Alloxan 

 (keto form) (enol form) (anion) 



but the location of the negative charge and the remaining H atom is indef- 

 inite (see following section). The dissociation of the second proton does 

 not occur readily and pK^, is around 10 or above (Richardson and Cannan, 

 1929). 



Structure 



Alloxan in the solid state can exist in the anhydrous form or as the mono- 

 hydrate or a poly hydrate, all crystalline. It has usually been assumed 

 that alloxan in solution is the monohydrate (Briickmann and Wertheimer, 

 o p 





OH 

 OH 



N"' ^O O^ ^n'^O 



H H 



Alloxan 

 ^11°''^ monohydrate 



1947; Patterson et al, 1949 a; Resnik and Wolff, 1956), as a result of the 

 reactivity of the 5-CO group, but the dihydrate in aqueous solution has 

 also been suggested (Lagercrantz and Yhland, 1963). Although this may 

 be true, there is little or no evidence for either such a hydrated form in 

 solution or for the localization of the water at the 5-CO group. If hydration 

 of this group arises because of adjacent CO groups, it would seem that 

 enolization at the 4- and 6-positions would reduce the reactivity of the 

 5-CO group and the likelihood of hydration. This problem is probably not 

 critical and most of the important reactions of alloxan can be written with 

 either form. 



The question of the keto-enol tautomerism is more important but cannot 

 be answered satisfactorily in the present state of our knowledge. There 

 have been different opinions as to the keto-enol equilibria in the commonly 

 occurring hydroxypyrimidines, but recently more workers have favored 

 the dominance of the keto form. The C — bond distance in the crystalline 

 state is 1.25 A, which is much closer to the C = bond than the phenolic 

 bond; however, in aqueous solution the situation may be different. Com- 

 parison of the ultraviolet absorption spectra of pyrimidines with the cor- 

 responding iV-substituted compounds indicates the keto form to be pre- 

 dominant, and it was concluded that the 2-, 4-, and 6-OH pyrimidines 



