benzofurans and 2-bromophenol

This study investigates reaction pathways for the formation of pre-PXDD/F intermediates via a radical/molecule mechanism. Thermodynamic and kinetic parameters for the combination reactions of 2-bromophenol (2-BP) and 2-chlorophenol (2-CP) precursors with key radical species including the phenoxy radicals, the phenyl radicals and the phenoxyl diradicals were calculated in detail. The couplings of phenoxy radicals with 2-B(C)P tend to produce pre-PXDD intermediates of halogenated o-phenoxyphenol. The combinations of phenyl and phenoxyl diradicals with 2-B(C)P produce two types of structures, i.e., dihydroxybiphenyl and o-phenoxyphenyl, which exclusively act as prestructures of PXDFs. These condensation reactions, especially those involving the phenyl C atom sites in phenyl and phenoxyl diradicals, are proven to be both thermodynamically and kinetically favorable and are nearly comparable with the corresponding steps involved in the radical/radical reactions. Most importantly, reactions of phenyl and phenoxyl diradicals with halogenated phenols solely lead to the formation of PXDFs, which to some extent provides a plausible explanation for the high PXDF-to-PXDD ratios in the real environment. Therefore, our study reveals the pivotal role of the radical/molecule mechanism in homogeneous gas-phase PXDD/F formation, especially in PXDF formation. The present results fill in a knowledge gap that has hitherto existed regarding dioxin formation and improve our understanding of PXDD/F formation characteristics in the environment.

Topics: Benzofurans; Chlorophenols; Dibenzofurans; Dioxins; Halogenation; Kinetics; Models, Chemical; Models, Theoretical; Phenols; Polychlorinated Dibenzodioxins; Thermodynamics

This study investigated the homogeneous gas-phase formation of polybrominated dibenzo-p-dioxin/dibenzofurans (PBDD/Fs) from 2-BP, 2,4-DBP, and 2,4,6-TBP as precursors. First, density functional theory (DFT) calculations were carried out for the formation mechanism. The geometries and frequencies of the stationary points were calculated at the MPWB1K/6-31+G(d,p) level, and the energetic parameters were further refined by the MPWB1K/6-311+G(3df,2p) method. Then, the formation mechanism of PBDD/Fs was compared and contrasted with the PCDD/F formation mechanism from 2-CP, 2,4-DCP, and 2,4,6-TCP as precursors. Finally, the rate constants of the crucial elementary reactions were evaluated by the canonical variational transition-state (CVT) theory with the small curvature tunneling (SCT) correction over a wide temperature range of 600-1200 K. Present results indicate that only BPs with bromine at the ortho position are capable of forming PBDDs. The study, together with works already published from our group, clearly shows an increased propensity for the dioxin formations from BPs over the analogous CPs. Multibromine substitutions suppress the PBDD/F formations.

Topics: Air Pollutants; Benzofurans; Bromine Compounds; Dioxins; Kinetics; Phenols

A one-pot synthesis of benzofurans which utilizes a palladium-catalyzed enolate arylation is described. The process demonstrates broad substrate scope and provides differentially substituted benzofurans in moderate to excellent yields. The utility of the method is further demonstrated by the synthesis of the natural product eupomatenoid 6 in three steps.

Topics: Benzofurans; Catalysis; Ketones; Palladium; Phenols

A series of dibenzofurans were efficiently and conveniently synthesized via one-pot consecutive C(sp(2))-O bond formation reaction (SNAr) in the presence of anhydrous K(2)CO(3), followed by C(sp(2))-C(sp(2)) bond formation reaction (intramolecular palladium-catalyzed aryl-aryl coupling reaction) between aryl halides and ortho-bromophenols. The desired dibenzofurans were obtained in 32-99% isolated yields.

Topics: Benzofurans; Catalysis; Chromates; Chromatography, Thin Layer; Hydrocarbons, Halogenated; Indicators and Reagents; Magnetic Resonance Spectroscopy; Palladium; Phenols; Potassium Compounds; Spectrometry, Mass, Electrospray Ionization

The homogeneous, gas-phase oxidative thermal degradation of a 50:50 mixture of 2-bromophenol and 2-chlorophenol was studied in a 1 cm i.d., fused silica flow reactor at a concentration of 88 ppm, with a reaction time of 2.0 s, over a temperature range of 300 to 1000 degrees C. Observed products in order of decreasing yield included the following: dibenzo-p-dioxin (DD), 4-bromo-6-chlorodibenzofuran (4-B,6-CDF), phenol, 4,6-dibromodibenzofuran (4,6-DBDF), 2,6-dibromophenol, 4,6-dichlorodibenzofuran (4,6-DCDF), 2-bromo-4-chlorophenol, 2,4-dibromophenol, 2-chloro-4-bromophenol, 4-monobromodibenzofuran (4-MBDF), 4-monochlorodibenzofuran (4-MCDF), dibenzofuran (DF), 1-monobromodibenzo-p-dioxin (1-MBDD), 1-monochlorodibenzo-p-dioxin (1-MCDD), 2,4,6-tribromophenol, naphthalene, chloronaphthalene, bromonaphthalene, chlorobenzene, bromobenzene, and benzene. The results are compared and contrasted with previous results reported for the oxidations of pure 2-chlorophenol and 2-bromophenol as well as results for the pyrolysis of the mixture of 2-chlorophenol and 2-bromophenol. 4,6-DBDF and 4,6-DCDF were observed in higher yields than under pyrolytic conditions but considerably less than the yields observed for the individual oxidation of 2-chlorophenol and 2-bromophenol. The effect on chlorine and bromine on the concentration of hydroxyl radical is shown to control the dioxin-to-furan ratio.

Topics: Benzofurans; Chlorophenols; Dioxins; Environment; Gases; Hydroxyl Radical; Models, Chemical; Naphthalenes; Oxygen; Phenols; Polychlorinated Dibenzodioxins; Temperature

We studied the surface-mediated pyrolytic thermal degradation of 2-bromophenol, a model brominated hydrocarbon that may form brominated dioxins in combustion and thermal processes, on silica-supported copper oxide in a 1 mm i.d., fused silica flow reactor at a constant concentration of 88 ppm over a temperature range of 250-550 degrees C. Observed products included dibenzo-p-dioxin (DD), 1-monobromodibenzo-p-dioxin (1-MBDD), dibromo-dibenzo-p-dioxin (DBDD),tribromodibenzo-p-dioxin (TrBDD), 4-monobromodibenzofuran (4-MBDF), dibenzofuran (DF), 2,4,6-tribromophenol, 2,4- and 2,6-dibromophenol, and polybrominated benzenes. These results are compared and contrasted with previous work on surface-catalyzed pyrolysis of 2-chlorophenol. Polybrominated dibenzofurans (PBDFs) are formed by the Langmuir-Hinshelwood mechanism, and the formation of polybrominated dibenzo-p-dioxins (PBDDs) is through an Eley-Rideal mechanism. Yields of PBDDs are at least 16x greater for 2-bromophenol than for analogous PCDDs from 2-chlorophenol. Higher yields of polybrominated phenols and polybrominated benzenes are also observed. This can be attributed to the relative ease of bromination over chlorination and the higher concentration of bromine atoms in the 2-bromophenol system versus chlorine atoms for the 2-chlorophenol system.

Topics: Benzofurans; Copper; Dioxins; Hot Temperature; Incineration; Phenols; Polychlorinated Dibenzodioxins; Silicon Dioxide; Soil Pollutants

The homogeneous, gas-phase pyrolytic thermal degradation of a 50:50 mixture of 2-bromophenol and 2-chlorophenol was studied in a 1 cm i.d., fused silica flow reactor at a total concentration of 88 ppm, reaction time of 2.0 s, and temperatures from 300 to 1000 degrees C. Observed products included (in decreasing yield) naphthalene, dibenzo-p-dioxin (DD), phenol, dibenzofuran (DF), bromobenzene, chloronaphthalene, 4-bromo-6-chlorodibenzofuran (4-B,6-CDF), bromonaphthalene, benzene, 4,6-dichlorodibenzofuran (4,6-DCDF), chlorobenzene, 4-monobromodibenzofuran (4-MBDF), 4-monochlorodibenzofuran (4-MCDF), 1-mono-bromodibenzo-p-dioxin (1-MBDD), 2-chloro,4-bromophenol, 2,4-dibromophenol, and 2-bromo-4-chlorophenol. Unlike the case for the pyrolysis of pure 2-chlorophenol, 4,6-DCDF was observed, but the analogous 4,6-DBDF remained undetected similar to the individual results with 2-MBP. This indicates that the presence of bromine increases the concentration of chlorine atoms available for the formation of 4,6-DCDF. Due to bromine atoms acting as better leaving groups than chlorine atoms, the yield of DD was increased over that observed for the pyrolysis of 2-chlorophenol. The addition of bromine to a chlorinated hydrocarbon system results in an increase in the total yield of PCDD/Fs as well as PBDD/Fs and mixed PBCDD/Fs due to the ease of bromine elimination reactions as well as an increase of the chlorine atom concentration.

Topics: Benzofurans; Chlorophenols; Dioxins; Hot Temperature; Hydrocarbons, Brominated; Kinetics; Models, Chemical; Phenols