Rate constants of reactions of bromine with phenols in aqueous solution

The kinetics of bromination of six ortho- and para-substituted phenols was investigated between pH 5 and pH 12 in aqueous solution. Kinetics was followed with a continuous-flow reactor previously validated by studying the fast reaction between chlorine and ammonia. The overall reaction rate between bromine and phenols is controlled by the reaction of HOBr with the phenoxide ion between pH 6 and pH 10. The reaction of HOBr with the undissociated phenols and the reaction of BrO- with the phenoxide ions become only significant for pH<6 and pH>10, respectively. The second-order rate constants for the reaction of HOBr with phenoxide ions vary between 1.4(+-0.1)×10^3 and 2.1(+-0.5)×10^8 M^-1 s^-1 for 2,4,6trichlorophenol and 4-methylphenol, respectively. Hammett-type correlation was obtained for the reaction of HOBr with the phenoxide ions (log(k)=8.0-3.33×(sigma)(sigma)) and was compared with Hammett-type correlations of HOCl and HOI. The reaction rate of bromine with phenol-like organic compounds was estimated to be about 10^3-fold higher than with chlorine and 10^3-fold lower than with ozone in drinking water treatment conditions.