Determination of relative formation rates of dibenzofurans via gas-phase condensation of phenols

The gas-phase formation of dibenzofuran (DF), chlorinated dibenzofurans (CDFs), dibenzo-p-dioxin (DD) and chlorinated dibenzo- p-dioxins (CDDs) from phenol and chlorinated phenol precursors, which are present in the exhaust stream of combustion processes, has been studied in the laboratory. Experimental results confirm that DF and CDF formation occurs via carbon-carbon coupling of phenoxy radicals at unsubstituted ortho sites; hence only those precursors with at least one unchlorinated ortho site will form DF/CDF. The formation of dibenzo-p-dioxin congeners, which are formed from these precursors with substituted ortho sites, was found to be much less important in the gas-phase. Experiments also show that the number and pattern of additional chlorine substituents affect the yield of DF/CDF significantly, but in a complex manner. Both electronic and steric effects due to chlorine substitution appear to be important in the relative rates of formation of DF/CDF congeners from phenol precursors. Unchlorinated phenol appears to be the most potent DF/CDF precursor, followed by 3-chlorophenol and 3,4-dichlorophenol. In contrast, 2-chlorophenol is a weak CDF precursor.; To further investigate the steric and electronic effects of DF/CDF formation, computational chemistry was used to model the phenoxy radical coupling mechanism, which is believed to be rate limiting. Ab initio electronic structure methods were employed to determine the optimized structures and to calculate the electronic and free energies of the reactants, product, and transition states formed in this step. Results indicate that molecular modeling can provide deeper insight into the details of this reaction. Relative reaction yields were calculated and found to be in reasonable agreement with observed data. This work has contributed to the development of a method that may be able to predict the distribution of DF/CDF congeners in the gas phase.