The Dibenzofuran System

The comprehension of the destruction of dihenzofurans is important because these compounds are emitted in the environment and degrade very slowly. For this purpose, this study investigated the dihenzofuranyl + O2 reaction system to provide an elementary mechanism describing the decomposition of this association. The determination of the thermochemical parameters, the transition state structures and the kinetic parameters on important reaction paths of the dibenzofuranyl + O2 reaction system are reported in this work. 

The reaction paths describing the formation and subsequent reactions of the active radical (phenyl) via abstraction by radical species (e.g. OH or Cl, H, or O) are important to include in mechanisms because sufficient thermal energy is available to overcome the endothermicity which is around 10 kcal mof  

As a consequence of the activity of these radicals, abstraction reactions can occur even at moderate temperatures in several downstream zones of an incinerator. 

This study estimates thermodynamic properties of intermediates and products in the decomposition of the dibenzofuranyl + O2 reaction system. Molecular properties for species are calculated by Density Functional Theory by use of the Gaussian 98 program suites. The reactions are first analyzed by the construction of a potential energy diagram of the system. 

Isodesmic reaction analysis is used to calculate the enthalpies of the intermediates and the well depths of adducts. The new Group Additivity parameters developed in a previous chapter, aided to evaluate the enthalpies of formation of large molecules. 

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The active principles of the drug are present in the resin which contains about 30 derivatives of 2-(2-isopropyl-5-methylphenyl)-5-pentylresorcinol, known as cannabinoids. Considerable confusion exists in the nomenclature, depending on whether they are numbered as substituted monoterpenes or dibenzopyrans. To avoid confusion the dibenzofuran system is used in this volume. 

Neubert, R., G. Golor, R. Stahlmann, H. Helge, and D. Neubert. 1992. Polyhalogenated dibenzo-p-dioxins and dibenzofurans and the immune system. 4. Effects of multiple-dose treatment with 2,3,7,8-tetrachloro-dibenzo-p-dioxin (TCDD) on peripheral lymphocyte subpopulations of a non-human primate (Callithrix jacchus). Arch. Toxicol. 66 250-259. 

Two chapters of this volume deal with closely related tricyclic heteromatic systems. The last comprehensive review of the dibenzofurans appeared in 1951 the subject is now updated by Sargent and Stransky. Carbazoles have also not been comprehensively reviewed recently this has been done in this volume by Joule. 

Some chemicals can affect other parts of the immune system. For example, polychlorinated hydrocarbons such as dioxins, dibenzofurans, and polychlorinated biphenyls (PCBs) damage the thymus, which is a lymphoid organ, producing mature T lymphocytes from the precursor cells, which are produced in the bone marrow. The result is depletion of the T cells in the thymus. 

The TT-electron excess of the five-membered rings is accompanied by a high rr-donor character. The best measure of rr-donation is the value of first ionization potential, IP, which for all aromatic heterocycles with one heteroatom of pyrrole type reflects the energy of highest occupied rr-orbital. IP, values decrease in the sequence pyrrole > indole > carbazole furan > benzo[/ ]furan > dibenzofuran thiophene > benzo[/ ]thiophene (Section 2.3.3.9, Tables 21 and 23). Thus, the more extensive the rr-system, the stronger is its electron donor ability. Furan and thiophene possess almost equal rr-donation, which is considerably lower than that of pyrrole. 

Nitrobiphenyl can also be converted to dibenzofurane. The nitrocompound, after isomerization to a nitrite, loses nitrogen oxide 27>. The remaining oxygen radical attacks the phenyl ring and cyclizes to the furane system. 

The non-aromatic portion of the second monomer unit in peceyline and peceylanine is also identical (13C spectra), and is based on vincorine epoxide. Since peceyline contains no methoxy-groups other than those in the ester functions, and its H aromatic resonances are consistent with an unsymmetrical dibenzofuran system, a possible structure for peceyline is (186). 

Systems considered are the parent heterocycles with one heteroatom (furan, thiophene, selenophene, and pyrrole), their monoaza and polyaza derivatives, and the monobenzo derivatives of the above systems. Dibenzo derivatives such as dibenzofuran and carbazole are not taken into account, since they have no position in the five-mem bered ring susceptible to electrophilic attack. Likewise, no consideration is given to more complicated systems, formed by the fusion of two heterocyclic rings such as thienothiophenes or pyrrolopyridines, for which, in any case, no quantitative work is available. 

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