Reinvestigation of the mechanism of bromination of uracil and its N-methyl derivatives

Abstract

Uracils bearing substituents at N₁ react rapidly with bromine in aqueous acidic solutions to give “HOBr” addition products, which undergo slow acid-catalysed dehydration to 5-bromo-uracils (4). Uracils bearing only hydrogen at N¹ react rapidly with bromine to give sequentially (4) and then the 5,5-dibromo-derivatives (6). This rapid reaction is suppressed, however, in strong acid, where N¹ of the intermediate (7) may be protonated. Mechanisms to accommodate these observations are suggested, including one involving a trasient N-bromo species (2; R¹= Br).