Theoretical study of the reaction of 2,4-dichlorophenol with 1O2

The dye sensitized photodegradation of chlorophenols has attracted a great deal of attentions recently because of its high efficiency, low cost and the convenience in utilizing solar energy directly. Much experimental evidences suggest that singlet oxygen, a very active oxygen species formed in the photosensitized process, is responsible for the photooxidation of the chlorophenols. However, the theoretical aspects of the detailed mechanism of the reaction between chlorophenols and singlet oxygen have been less frequently addressed. 2,4-dichlorophenol is used as a model chlorophenol in this work and four types of reactions between 2,4-dichlorophenol and singlet oxygen were investigated at the density functional theory level (functional B3LYP): (1)1,3-addition to a double bond connected to a hydrogen-carrying group, resulting in the formation of allylic hydroperoxides; (2)1,4-cycloaddition to a system containing at least two conjugated double bonds, resulting in the formation of the 1,4-peroxides; (3)[π2+π2] 1,2-cycloaddition to an isolated double bond, resulting in the formation of 1,2-peroxides; (4)1,4-addition to chlorophenols with the formation of hydroperoxide ketones. The type (1) and type (4) reactions are the most exergonic ones, (−17.5 to −20.0 kcal mol−1) and type (1) also has the lowest reaction barrier, therefore is predicted to be the most likely route.