Formation of dibenzodioxins and dibenzofurans in homogenous gas-phase reactions of phenols

Products, Rates, and Mechanism of the Gas-Phase Condensation of Phenoxy Radicals between 500-840 K

“…[3] Once it was known that A is a precursor of DF, the slow combustion of A in a large excess of benzene (1:200) was conducted, as a step towards more realistic conditions. Under the conditions of experiments 3, the conversion of A was 99.93% and the only important organic products were phenol and DF; other products with two benzene rings, if present, comprise less than 1% on DF.…”

“…[1] DF formation in the homogeneous gas-phase pyrolysis and slow combustion of phenol has been ascribed to the dimerisation of two phenoxy radicals as the first step. [2,3] More recent kinetic and product studies [4,5] showed that DF formation from two phenoxy radicals can occur with an overall rate constant of 10 8  -1 s -1 and above, not so much less than k for the first step, which is approximately 10 10  -1 s -1 . A simple rationale is given in Scheme 1.…”

“…A simple rationale is given in Scheme 1. [3,6] While dimer I will revert to phenoxy radicals at elevated temperatures, enolisation to A (2,2'-dihydroxybiphenyl) can ultimately lead the way to DF. A has been found in trace amounts (next to DF) in the pyrolysis of phenol [4] and from the hydrogenolysis of anisole.…”

“…A has been found in trace amounts (next to DF) in the pyrolysis of phenol [4] and from the hydrogenolysis of anisole. [5] Also, at 500°C authentic A can be rapidly converted into DF with high yield, [3] presumably by a radical (chain) process. The formation of dibenzop-dioxin (DD) was mentioned in none of these cases; significant proportions of (chlorinated) DDs arise, however, when ortho-chlorine substituents are introduced.…”

Formation of tetrahalogenated dibenzo-p-dioxins (TXDDs) by pyrolysis of a mixture of 2,4,6-trichlorophenol and 2,4,6-tribromophenol

“…The main pyrolytic products from 2,4,6-TCP were identified as the 1,3,6,8-and 1,3,7,9-TCDD isomers. As reported previously [26][27][28], 1,3,6,8-TCDD isomer is produced only by direct condensation; whereas, the Smiles rearrangement is capable of producing both the 1,3,6,8-and 1,3,7,9-TCDD isomers. Similarly, the pyrolysis of 2,4,6-TBP can lead to the formation of the 1,3,6,8-and 1,3,7,9-TBDD isomers.…”