benzofurans and phenoxy-radical

Electrochemically synthesized dihydrobenzofurans were evaluated for their insect antifeedant activities against phytophagous insects. They were prepared through the coupling reactions of various alkenes with a phenoxy cation generated by oxidation near the cathode in the electrolytic reaction. The insect antifeedant activities of these synthetic dihydrobenzofurans were evaluated in the common cutworm (Spodoptera litura) and diamond back moth (Plutella xylostella) with the dual choice leaf disk bioassay method. The insect antifeedant activities of most of the acetophenone-type dihydrobenzofurans were strong, while those of derivatives with a t-butyl group were weaker. The biological activities in insect species differed with the structural features of the compounds.

Topics: Alkenes; Animals; Benzofurans; Biological Assay; Electrochemical Techniques; Electrodes; Electrolysis; Insect Repellents; Moths; Oxidation-Reduction; Phenols; Plants; Spodoptera; Structure-Activity Relationship

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

Quantum chemical calculations were carried out to investigate the homogeneous gas-phase formation of mixed polybrominated/chlorinated dibenzo-p-dioxins/benzofurans (PBCDD/Fs) from the cross-condensation of 2-chlorophenoxy radical (2-CPR) with 2-bromophenoxy radical (2-BPR), 2,4-dichlorophenoxy radical (2,4-DCPR) with 2,4-dibromophenoxy radical (2,4-DBPR), and 2,4,6-trichlorophenoxy radical (2,4,6-TCPR) with 2,4,6-tribromophenoxy radical (2,4,6-TBPR). The geometrical parameters and vibrational frequencies were calculated at the MPWB1K/6-31+G(d,p) level, and single-point energy calculations were performed at the MPWB1K/6-311+G(3df,2p) level of theory. 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 600-1200K. Studies show that the substitution pattern of halogenated phenols not only determines the substitution pattern of the resulting PBCDD/Fs, but also has a significant influence on the formation mechanism of PBCDD/Fs, especially on the coupling of the halogenated phenoxy radicals.

Topics: Benzofurans; Chlorophenols; Computer Simulation; Phenols; Polychlorinated Dibenzodioxins

Water is of great chemical importance due to its ability to form hydrogen bond. Polyhalogenated dibenzo-p-dioxin/benzofurans (PHDD/Fs) are notorious due to their persistence, bioaccumulation and extremely high toxicity. Water is ubiquitous, and a deep knowledge of its influence on the formation mechanism of PHDD/Fs is necessary. This work investigated the influence of water on the homogeneous gas-phase formation of PHDD/Fs from halogenated phenols (HPs) as precursors by using quantum chemical calculations with the aid of the MPWB1K theoretical approach in connection with the 6-31+G(d,p) and 6-311+G(3df,2p) basis sets. The schematic energy profile in the presence of water was constructed and compared with the situation without water. This study reveals for the first time that the introduction of water promotes the formation of halogenated phenoxy radicals (HPRs) from the H abstraction reactions of HPs with atomic H and OH radicals by lowering the reaction energy barriers and opening new low-energy pathways. Another intriguing finding of this work is that the inclusion of a water molecule produces a catalytic effect on the H-shift step involved in the formation of PHDFs and thus their formation potential is enhanced.

Topics: Benzofurans; Dibenzofurans, Polychlorinated; Environmental Pollutants; Hydrocarbons, Brominated; Kinetics; Models, Chemical; Phase Transition; Phenols; Polychlorinated Dibenzodioxins; Water

This contribution investigates mechanistic and kinetic parameters pertinent to formation of mixed dibenzo-p-dioxins and dibenzofurans (PXDD/Fs) from the condensation reactions involving 2-chlorophenoxy (2-CPxy) and 2-bromophenoxy (2-BPxy) radicals. Keto-ether structures act as direct intermediates for the formation of DD, 1-MCDD, 1-MBDD, 1-B,6-CDD and 1-B,9-CDD molecules. Likewise, diketo adducts initiate the formation of 4-MCDF, 4-MBDF and 4-B,6-CDF compounds through interconversion and rearrangement reactions. As formation mechanisms of halogenated dibenzo-p-dioxins and dibenzofurans from precursors of brominated and chlorinated phenols are insensitive to substitution at meta and para sites, our mechanistic and kinetic analysis of reactions involving 2-BPxy and 2-CPxy should also apply to higher halogenated phenoxy radicals.

Topics: Benzofurans; Dioxins; Environmental Pollutants; Halogenation; Hydrocarbons, Halogenated; Kinetics; Models, Chemical; Molecular Structure; Phase Transition; Phenols

The bromophenoxy radicals (BPRs) are key intermediate species involved in the formation of polybrominated dibenzo-p-dioxin/dibenzofurans (PBDD/Fs). In this work, the formation of BPRs from the complete series reactions of 19 bromophenol (BP) congeners with H and OH radicals were investigated theoretically by using the density functional theory (DFT) method and the direct dynamics method. The geometries and frequencies of the reactants, transition states, and products 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. The rate constants were evaluated by the canonical variational transition-state (CVT) theory with the small curvature tunneling (SCT) contribution over a wide temperature range of 600-1200K. The present study indicates that the reactivity of the O-H bonds in BPs as well as the formation potential of BPRs from BPs is strongly related to the bromine substitution pattern. The obtained results can be used for future estimates of PBDD/F emissions quantity based on the well estimated PCDD/F inventory.

Topics: Benzofurans; Bromine; Dioxins; Hydrocarbons, Brominated; Hydrogen; Hydroxyl Radical; Kinetics; Phenols; Temperature

Antioxidants could be promising agents for management of oxidative stress-related diseases. New biologically active compounds, belonging to a rare class of natural lignans with antiangiogenic, antitumoral and DNA intercalating properties, have been recently synthesized. These compounds are benzo[kl]xanthene lignans (1,2) and dihydrobenzofuran neolignans (3,4). The radical scavenging and chain-breaking antioxidant activities of compounds 1-4 were studied by applying different methods: radical scavenging activity by DPPH rapid test, chain-breaking antioxidant activity and quantum chemical calculations. All studied compounds were found to be active as DPPH scavengers but reaction time with DPPH and compounds' concentrations influenced deeply the evaluation. The highest values of radical scavenging activity (%RSAmax) and largest rate constants for reaction with DPPH were obtained for compounds 2 and 3. Comparison of %RSAmax with that of standard antioxidants DL-α-tocopherol (TOH), caffeic acid (CA) and butylated hydroxyl toluene (BHT) give the following new order of %RSA max: TOH (61.1%) > CA (58.6%) > 3 (36.3%) > 2 (28.1%) > 4 (6.7%) > 1 (3.6%) = BHT (3.6%). Chain-breaking antioxidant activities of individual compounds (0.1-1.0 mM) and of their equimolar binary mixtures (0.1 mM) with TOH were determined from the kinetic curves of lipid autoxidation at 80 °C. On the basis of a comparable kinetic analysis with standard antioxidants a new order of the antioxidant efficiency (i.e., protection factor, PF) of compounds 1-4 were obtained: 2 (7.2) ≥ TOH (7.0) ≥ CA (6.7) > 1 (3.1) > 3 (2.2) > ferulic acid FA (1.5) > 4 (0.6); and of the antioxidant reactivity (i.e. inhibition degree, ID): 2 (44.0) >> TOH (18.7) >> CA (9.3) >> 1 (8.4) > 3 (2.8) > FA (1.0) > 4 (0.9). The important role of the catecholic structure in these compounds, which is responsible for the high chain-breaking antioxidant activity, is discussed and a reaction mechanism is proposed. Higher oxidation stability of the lipid substrate was found in the presence of equimolar binary mixtures 2 + TOH, 3 + TOH and 4 + TOH. However, an actual synergism was only obtained for the binary mixtures with compounds 3 and 4. The geometries of compounds and all possible phenoxyl radicals were optimized using density functional theory. For description of the scavenging activity bond dissociation enthalpies (BDE), HOMO energies and spin densities were employed. The best correlation between theoretical and experimental d

Topics: alpha-Tocopherol; Antioxidants; Benzofurans; Biphenyl Compounds; Butylated Hydroxytoluene; Caffeic Acids; Free Radical Scavengers; Hot Temperature; Kinetics; Lignans; Lipids; Oxidation-Reduction; Phenols; Picrates; Quantum Theory; Solutions; Structure-Activity Relationship; Thermodynamics; Xanthenes

It is important to understand the role of phenol in the dioxin formations because it is present in the high amount in municipal waste incinerators (MWIs). The formation mechanism of dioxins from the cross-condensation of PhRs with 2-CPRs and 2,4,6-TCPRs was investigated by using hybrid density functional theory (DFT) and compared with the dioxin formation mechanism from the self-condensation of single chlorophenol precursors. The geometrical parameters were optimized at the MPWB1K level with the 6-31+G(d,p) basis set without symmetry constraints. Single-point energy calculations were carried out at the MPWB1K/6-311+G(3df,2p) level of theory. The rate constants were deduced by using canonical variational transition-state (CVT) theory with small curvature tunneling (SCT) contribution over the temperature range of 600-1200 K. The Arrhenius formulas were reported for the first time. Results show that phenol is responsible for the formation of dioxin congeners. This work, together with results already published from our group, provides a comprehensive investigation of the homogeneous gas-phase formation of dioxins from (chloro)phenol precursors and should help to clarify the formation mechanism of dioxins in real waste combustion and to develop more effective control strategies.

Topics: Benzofurans; Chlorophenols; Dibenzofurans, Polychlorinated; Kinetics; Phenols; Polychlorinated Dibenzodioxins; Thermodynamics

This study investigates the kinetic parameters of the formation of the chlorophenoxy radical from the 2-chlorophenol molecule, a key precursor to polychlorinated dibenzo-p-dioxins and dibenzofurans (PCCD/F), in unimolecular and bimolecular reactions in the gas phase. The study develops the reaction potential energy surface for the unimolecular decomposition of 2-chlorophenol. The migration of the phenolic hydrogen to the ortho-C bearing the hydrogen atom produces 2-chlorocyclohexa-2,4-dienone through an activation barrier of 73.6 kcal/mol (0 K). This route holds more importance than the direct fission of Cl or the phenolic H. Reaction rate constants for the bimolecular reactions, 2-chlorophenol + X --> X-H + 2-chlorophenoxy (X = H, OH, Cl, O2) are calculated and compared with the available experimental kinetics for the analogous reactions of X with phenol. OH reaction with 2-chlorophenol produces 2-chlorophenoxy by direct abstraction rather than through addition and subsequent water elimination. The results of the present study will find applications in the construction of detailed kinetic models describing the formation of PCDD/F in the gas phase.

Topics: Benzofurans; Chlorine; Chlorophenols; Free Radicals; Gases; Hydrogen; Hydroxides; Kinetics; Oxygen; Phenols; Polychlorinated Dibenzodioxins; Quantum Theory

Polychlorinated naphthalenes (PCNs) formed along with dibenzo-p-dioxin and dibenzofuran products in the slow combustion of dichlorophenols (DCPs) at 600 degrees C were identified. Each DCP reactant produced a unique set of PCN products. Major PCN congeners observed in the experiments were consistent with products predicted from a mechanism involving an intermediate formed by ortho-ortho carbon coupling of phenoxy radicals; polychlorinated dibenzofurans (PCDFs) are formed from the same intermediate. Tautomerization of the intermediate and H2O elimination produces PCDFs; alternatively, CO elimination to form dihydrofulvalene and fusion produces naphthalenes. Only trace amounts of tetrachloronaphthalene congeners were formed, suggesting that the preferred PCN formation pathways from chlorinated phenols involve loss of chlorine. 3,4-DCP produced the largest yields of PCDF and PCN products with two or more chlorine substituents. 2,6-DCP did not produce tri- or tetra-chlorinated PCDF or PCN congeners. It did produce 1,8-DCN, however, which could not be explained.

Topics: Air Pollutants; Benzofurans; Chlorophenols; Dibenzofurans, Polychlorinated; Environmental Monitoring; Hydrocarbons, Chlorinated; Isomerism; Models, Chemical; Naphthalenes; Oxidation-Reduction; Phenols; Temperature