Formation of bromo-substituted triclosan during chlorination by chlorine in the presence of trace levels of bromide

The side reactions of triclosan (2,4,4′-trichloro-2′-hydroxydiphenyl ether, TC) and chlorine in the presence of sodium chloride were investigated. In the absence of sodium chloride, three chloro-derivatives of TC, 2′,3,4,4′-tetrachloro-2-hydroxydiphenyl ether (3-Cl-TC), 2′,4,4′,5-tetrachloro-2-hydroxydiphenyl ether (5-Cl-TC), and 2′,3,4,4′,5-pentachloro-2-hydroxydiphenyl ether (3,5-Cl2-TC) were formed, whereas in the presence of sodium chloride, 3-bromo-2′,4,4′-trichloro-2-hydroxydiphenyl ether (3-Br-TC), 5-bromo-2′,4,4′-trichloro-2-hydroxydiphenyl ether (5-Br-TC), (3 or 5)-bromo-2′,4,4′,(5 or 3)-chloro-2-hydroxydiphenyl ether ((3,5)-(BrCl)-TC), and 3,5-dibromo-2′,4,4′-trichloro-2-hydroxydiphenyl ether (3,5-Br2-TC) were additionally formed. Radiochemical neutron activation analysis indicated that 1 g of commercially available sodium chloride contained 73 μg of bromide and the bromide ion was determined to be the source of the side reactions. The rate of decrease of TC due to reaction with chlorine was greatly accelerated by the presence of bromide ion in the system: the rate with only 1×10−5 M bromide ion was three times the rate in the absence of bromide.