Dibenzo dioxin calculation

The reaction of iV-(2,4-dinitrophenyl)amino acids with base in aqueous dioxane has been shown to give benzimidazole iV-oxides (7). The rate-determining step is likely to be formation of an iV-alkylidene-2-nitrosoaniline intermediate (6), which is followed by rapid cyclization and decarboxylation.19 The loss of carbon dioxide from perbenzoate anions has been investigated by mass spectrometry and electronic structure calculations. The results, including isotopic labelling experiments, support a mechanism involving initial intramolecular nucleophilic attack at either the ortho- or ipso-ring positions. They also indicate that epoxides may be intermediates en route to the phenoxide products.20 There has also been a theoretical study of the formation of trichlorinated dibenzo-/ -dioxins by reaction of 2,4,5-trichlorophenolate ions with 2,4-dichlorophenol.21... 

Molecular orbital calculations of 2,3,7,8-TCDD and dibenzo- -dioxin indicate that excitation of either molecule results in promotion of an electron into an anti-bonding orbital with electron density primarily located on the 2,3,7,8 positions, with only a small percentage on the 1,4,6,9 positions (37). The primary photoproducts of 2,3,7,8-TCDD in organic solvent has been suggested to be the 2,3,7-trichloro Isomer and the dlchloro isomers (38). These photoproducts also appear to photodegrade faster than 2,3,7,8-TCDD. No other photoproducts have been identified from the photolysis of... 

Ah-receptor-mediated toxicity is particularly associated with the highly toxic compound 2,3,7,8-tetrachloro-dibenzo-p-dioxin (TCDD), commonly referred to as dioxin. TCDD, and the concept of toxicity equivalency factors (TEFs) based on TCDDs, will be dealt with in Chapter 7. The main point to make at this juncture is that the toxicity of each individual coplanar congener in a mixture can be expressed in terms of a toxic equivalent calculated relative to the toxicity of dioxin. Summation of the toxic equivalents of the individual coplanar PCBs gives a measure of the toxicity of the whole mixture, as expressed through the Ah receptor mechanism. 

For polychlorinated biphenyls (PCBs), rate constants were highly dependent on the number of chlorine atoms, and calculated atmospheric lifetimes varied from 2 d for 3-chlorobiphenyl to 34 d for 236-25 pentachlorobiphenyl (Anderson and Hites 1996). It was estimated that loss by hydroxy-lation in the atmosphere was a primary process for the removal of PCBs from the environment. It was later shown that the products were chlorinated benzoic acids produced by initial reaction with a hydroxyl radical at the 1-position followed by transannular dioxygenation at the 2- and 5-positions followed by ring fission (Brubaker and Hites 1998). Reactions of hydroxyl radicals with polychlorinated dibenzo[l,4]dioxins and dibenzofurans also play an important role for their removal from the atmosphere (Brubaker and Hites 1997). The gas phase and the particulate phase are in equilibrium, and the results show that gas-phase reactions with hydroxyl radicals are important for the... 

Koester, CJ. and Hites, R.A. Calculated physical properties of polychlorinated dibenzo-p-dioxins and dibenzofurans, Chemosphere, 17(12) 2355-2362, 1988. 

Govers HAJ, Luijk R, Evers EHG (1990), Chemosphere 20 287-294.. .Calculation of heat of vaporization, molar volume and solubility parameter of polychlorinated dibenzo-p-dioxins Govers HAJ, van der Wielen FWM, Olie K (1995), J. Chromat. A 715 267-278.. .Derivation of solubility parameters of chlorinated dibenzofurans and dibenzo-p-dioxins from gas chromatographic retention parameters via SOFA"... 

The 2,3,7,8-TCDD toxicity equivalence of PCDDs/PCDFs present in the sample is calculated according to the method recommended by the Chlorinated Dioxins Workgroup (CDWG) of the EPA and the Centers for Disease Control (CDC). This method assigns a 2,3,7,8-TCDD toxicity equivalency factor (TEF) to each of the 17 2,3,7,8-substituted PCDDs/PCDFs shown in Table 11 ("Update of Toxicity Equivalency Factors (TEFs) for Estimating Risks Associated with Exposures to Mixtures of Chlorinated Dibenzo-p-Dioxins and Dibenzofurans (CDDs/CDFs)" March 1989 (EPA 625/3-89/016)) The 2,3,7,8-TCDD toxicity equivalence of the PCDDs/PCDFs present in the sample is calculated by summing the product of the TEF and the concentration for each of the compounds listed in Table 2. 

Modified neglect of diatomic overlap (MNDO) calculations have been shown to accurately predict the isomer distribution of polychlorinated dibenzo-1,4-dioxins and polychlorinated dibenzofurans in industrial combustion processes, an indication of thermodynamic control in these processes <2002MI1287>. 

Koto T, Toyota K, Sato K, Shiomi D, Takui T. An ab initio calculation of UV spectra of polychlorinated dibenzo-p-dioxins SAC-CI study. Angew Chem Int Ed submitted. 

The findings of the chlorinated dibenzo-p-dioxin and dibenzofuran isomers in persons residing in contaminated areas and both measured and calculated levels of these compounds in breast milk in the United States presents a possible cause for concern regarding health effects in the nursing infant. Further study is indicated to determine if there is any real clinical risk at the levels found. 

Figure 1 Effects of Chlorella diet on fecal excretion amounts of polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF) congeners in contaminated rice oil-fed rats. Values are means SD, n=4. Vhlues of acceleration index of fecal excretion amounts of PCDD and PCDF congeners due to Chlorella diet were calculated using the following equation Acceleration index of fecal excretion amounts of PCDD...

Thermal destruction tests on PCBs indicate that essentially complete destruction occurs in well designed incinerator systems. The potential for formation of polychlorinated dibenzofurans (PCDFs) and dibenzo-p-dioxins (PCDDs) during thermal destruction of PCBs can be examined by thermochemical equilibrium calculations. The calculations predict that, under oxidizing conditions, formation of PCDFs and PCDDs is not thermodynamically favored. However, under pyrolytic conditions (absence or near absence of oxygen), as may arise in an inadequately designed or operated incinerator, thermochemical equilibrium calculations indicate that trace amounts of possible precursors to PCDFs and PCDDs can form [192]. 

TABLE 1 gives the reported values or ranges of the physical-chemical properties of chlorobenzenes (CBs), polychlorinated biphenyls (PCBs) and polychlorinated dibenzo-p-dioxins (PCDDs). Fugacity ratios were obtained from a single estimated entropy of fusion of 56 J mol °K (Yalkowsky 1979), molar volumes were calculated by the Le Bas method, an additive group contribution method (Reid et al. 1977). Total surface area (TSA) values were obtained from Yalkowsky et al. (1979 a,b). Solubilities, vapour pressures and octanol/water partition coefficients (Andren et al. 1986 Shiu and Mackay 1986 Bobra et al. 1985) are also tabulated. Henry s law constants were calculated as PSl/C and the octanol solubility Q as C Kq, . 

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