Radical/radical vs radical/molecule reactions in the formation of PCDD/Fs from (chloro)phenols in incinerators

Pathways from chlorinated phenols as precursors to PCDD/Fs are discussed with focus on the effect of (poly)chlorination on thermochemistry and rate in the displacement of chlorine from a chlorophenol molecule by a (chloro)phenoxy radical (reaction (A) as a key example). Through measurements on the respective methylethers (anisoles) the O–H bond of 2,4,6-TCP turns out to be 5 kcal/mol, and that in PCP 4 kcal/mol, less strong than O–H in phenol itself. On this basis it is concluded that––in contrast with earlier proposals––displacements such as in reaction (A) are at least as slow as reaction (B) of phenoxy radical with chlorobenzene. Compared with condensation of two (chloro)phenoxy radicals, such radical/molecule reactions are therefore an insignificant pathway to dioxins in incinerators.