Kinetics of hydrolysis of chloroform and bromoform in aqueous solutions

The base-catalyzed hydrolysis of chloroform (CF) and bromoform (BF) was examined in water-methanol solutions. The progress of reaction was determined by following the decrease in [OH-] and the increase in [halide] ions as function of time. The effects of temperature, [OH-] and the water: methanol ratio on the rates of reaction were tested in some detail. The hydrolysis of CF was first order with respect to [OH-] and the overall reaction followed second order kinetics. In 1:1 water/methanol solutions the reaction had an activation energy of 18.84 kcal mole(-1). The rate decreased with the decrease of [OH-] and with the increase in alcohol content of the solution. Bromoform hydrolyzed in 1:5 water: alcohol solutions according to two-, second order kinetic parallel reactions of different rates. The initial faster rate was ascribed to a free radical mechanism with an activation energy of 13.22 kcal mole(-1). The second, slower reaction had an activation energy comparable to that of CF suggesting the same hydrolysis mechanism. Under identical conditions BF hydrolyzed twice as fast as CF. The mechanisms accounting for the base-catalyzed hydrolysis of haloforms are reviewed in short. The one involving the primary, reversible extraction of the proton, followed by a slow S(N)1 removal of a halogen anion was favoured. The intermediate formation of dihalocarbene was supported by experiments with additions of KBr or KI to the reaction medium. These additives slowed the overall hydrolysis and produced mixed trihalomethanes. The feasibility of applying the knowledge gained from this study to the purification of THM-contaminated potable water is under investigation.