Influence des ions bromures et iodures sur la chloration chimique de lʹuracileEffect of the bromide and iodide ions on the chemical uracil chlorination

In regards of the protonations of uracil and halogenouracils (Al Iskandarani et al., 1995), the experimental conditions of analysis was determined by High Performance Liquid Chromatography (Fig. 1) and UV spectrometry (Fig. 2). The chlorination of a 10−2 M uracil solution was followed at pH 6 and at 20–22°C using increasing concentrations in sodium hypochlorite. After 60 min, the chlorination yields a mixture containing a large quantity of 5-chlorouracil when the molar concentration ratio R = [H2UH]/[CIO−] is equal to the unity. After this maximum ( 80% [H2UH]), the 5-chlorouracil disappears for reaching total decomposition when R = 6 (Fig. 3). A similar behaviour is noted when the chlorination is carried out in presence of a 10−3 M sodium chloride solution. In presence of added sodium iodide or sodium bromide, the chlorination leads to produce in addition to 5-chlorouracil, important quantities of 5-iodouracil ( 90% [H2UH] at R = 2) (Fig. 4) and 5-bromouracil ( 50% [H2UH] at R = 3) (Fig. 5), respectively. 5-Iodouracil, 5-bromouracil and 5-chlorouracil completely disappear to form byproducts when R = 20 and R = 14, respectively. These observations show that the addition of iodide ions slackens the disappearing of the uracil, stabilizing the concentration of the 5-chlorouracil at about 55% of the initial uracil concentration at R = 10. This effect is accentuated in presence of bromide ions which limit the concentration in 5-chlorouracil to nearly 10% [H2UH] at R = 6. A similar behaviour at different levels is observed, when the initial uracil solution contains an equimolecular mixture of iodide and bromide ions (Fig. 6). Moreover, the competitive formation of 5-halogenouracils produces maximum concentrations at R = 2.5 and R = 7 for 5-iodouracil and 5-bromouracil, respectively. The level of the 5-chlorouracil concentration remains low ( 8% [H2UH]) at R = 14. The complete disappearance of all products is reached at about R = 20. The halogenouracils are probably formed by addition of hypohalite groups XO− to the C-C double bond of uracil. It seems clear that existence of hypobromite and/or hypoiodite ions was an oxidation result of bromide and/or iodide ions by sodium hypochlorite.