Theoretical study of the peroxidation of chlorophenols in gas phase and aqueous solutions

Theoretical study of the reactions between singlet oxygen and chlorophenols is an important aspect in understanding the reaction mechanism of the dye-sensitized photodegradation. With the intention of finding certain predictors to be used for the determination of the most probable reaction path and estimating the dye-sensitized photodegradation rates of chlorophenols, the reactions of 1O2 with six chlorophenols (CPs), including 2-chlorophenol (2-CP), 3-CP, 4-CP, 2,4-dichlorophenol (2,4-DCP), 2,4,6-trichlorophenol (2,4,6-TCP) and pentachlorophenol (PCP), were investigated by using the density functional theory. Results suggested that 1,3-addition to a double bond connected to a hydrogen-carrying group, resulting in the formation of allylic hydroperoxides, and 1,4-addition to chlorophenols to form of hydroperoxide ketones are thermodynamically more likely to take place. Furthermore the reaction barrier of the former one is lower than that of later one, which tends to conclude that 1,3-addition to a double bond connected to a hydrogen-carrying group to form allylic hydroperoxides is the most likely route both in gas phase and aqueous solution. Those reactions are thermodynamically more likely to take place in presence of water, but the reaction barriers increased. Also it was observed that with the increase of chlorine substitutions, the reactions become less exergonic and kinetically less favorable due to the increase in reaction barriers.