Mechanism and thermal rate constants for complete series reactions of bromochlorophenols with H

Abstract

Bromochlorophenoxy radicals (BCPRs) are the principal precursors for the formation of mixed polybrominated and chlorinated dibenzo-p-dioxins and dibenzofurans (PBCDD/Fs). In this study, the formation of BCPRs from a complete series of reactions of 96 bromochlorophenol (BCP) congeners with H atoms was analyzed using quantum chemical methods at the MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+G(d,p) level. The rate constants of each reaction were obtained using canonical variational transition-state (CVT) theory along with a small-curvature tunneling (SCT) contribution over a wide temperature range of 600–1200 K. In addition, abstraction reactions of bromophenols (BPs) and chlorophenols (CPs) by H are compared with reactions of BCPs with H. The computational results indicate that the relative reactivity of the O–H bonds in the BCPs appears to be assisted and dominated by the total degree and pattern of halogenation, but it does not appear to be determined by the distribution of the bromine and chlorine atoms. In addition, halogen substitution at the ortho position increases the stability of the BCPs and reduces the reactivity of the O–H bonds. This work is beneficial in that it provides reference data for future experimental research into the formation of PBCDD/Fs.