Theoretical study of the dechlorination reaction pathways of octachlorodibenzo-p-dioxin

The dechlorination reaction pathways of 1,2,3,4,6,7,8,9-octachlorodibenzo-p-dioxin (OCDD) by the hydrogen atom are investigated by the density-functional theory B3PW91 method. The dechlorination reactions have large exothermicity and small activation energies. The activation energies (≈5 kcal/mol) of the σ-complex formation due to the hydrogen addition are lower than those (≈9 kcal/mol) of the direct chlorine abstraction. It is suggested that the σ-complex plays an important role in the reactions, although it has scarcely been shown in previous studies of the dechlorination of dioxins. The σ-complex formation is favored at low temperatures and the chlorine abstraction is favored at high temperatures. Furthermore, it is found that the lateral positions have a marginal preference over the longitudinal positions. The dechlorination of OCDD by the hydrogen atom is thus not likely to result in a dominant formation of the laterally substituted toxic congeners.