Dibenzo dioxin radicals

Brubaker WW, RA Hites (1997) Polychlorinated dibenzo-/)-dioxins and dibenzofurans gas-phase hydroxyl radical reactions and related atmospheric removal. Environ Sci Technol 31 1805-1810. 

Atkinson, R. (1987a) Estimation of OH radical reaction rate constants and atmospheric life times for polychlorobiphenyls, dibenzo-/>-dioxins and dibenzofurans. Environ. Sci. Technol. 21, 305-307. 

It has been argued466 that the solution photolysis rates of polychlorinated dibenzo-/ -dioxins may be explained by the preferential photodissociation of chlorine atoms from a lateral vs a non-lateral position to yield the corresponding aryl radical and/or aryl cation-aryl carbene intermediate. 

S Halogenated Compounds Many halogenated compounds, either because of their extensive use or their persistence are of environmental interest. The smaller, halogenated aliphatics have been used extensively and are common ground water contaminants," while the persistence of the PCBs and chlorinated dibenzo dioxins is well documented. Hydroxyl radicals are the more important oxidants in the vapor phase and the hydrocarbon structure determines the nature of the reaction. Second-order rate constants for some of these compounds are summarized in Table 6.27. 

Taylor, P.H., T. Yamada, and A. Neuforth (2005), Kinetics of OH radical reaction with dibenzo-/>-dioxin and selected chlorinated dibenzo-/ -dioxins, Chemosphere, 58, 243-252. 

This color transformation has been observed in dibenzo-p-dioxin (Structure I) and in its bromo, chloro, nitro, methyl, and ethyl derivatives in addition, the observed electron spin resonance (ESR) signals indicated the presence of paramagnetic species (2, 3). This phenomenon has been attributed to the formation of cation radicals in acid solution. 

Studies were made on a series of chlorinated dibenzo-p-dioxin cation radicals in trifluoromethanesulfonic acid (TFMS acid). TFMS acid was... 

The 1-chloro- and 2-chlorodibenzo-p-dioxins, which readily dissolved in TFMS acid, formed cation radicals without UV irradiation or the addition of oxidizing agents. With the exception of broader resonance lines. Figure 3 shows that the five-line pattern observed with 1-chloro-dibenzo-p-dioxin is similar to that of the unsubstituted dibenzo-p-dioxin. Apparently, protons at the 2, 3, 7, and 8 positions became less equivalent... 

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... 

Brubacker, W. W. Jr., and R. A. Hites, Polychlorinated dibenzo-p-dioxins and dibenzofurans Gas phase hydroxyl radical reactions and related atmospheric removal , Environ. Sci. Technol., 31, 1805-1810 (1997). 

The preparation of dibenzo[6,e ][ 1,4]dioxin cation radical (66) has been achieved by oxidation of the heterocycle in ethyl acetate-lithium perchlorate at a platinum anode, using a controlled potential of 1.2 volts vs. Ag-AgC104. The blue solid collected at the anode contained between 85-90% of (66) as the perchlorate (74JHC139). The purple cation radical perchlorate of phenoxathiin, (67), is obtained in high purity by oxidation of phenoxathiin in benzene with 70% perchloric acid-acetic acid (75JOC2756). Similar perchloric acid oxidation of thianthrene affords the dark reddish brown perchlorate of (68) (69JOC3368) and the heterocycle can also be oxidized on a preparative scale with antimony pentachloride (62JCS4963). 

The gas-phase reactions of the N03 radical with biphenyl, 2,2 -dichlorobiphenyl, dibenzo-p-dioxin, 1-chlorodibenzo-p-dioxinanddibenzofuran have been studied at room temperature.92,93,95,98,99 While no reactions of the NOs radical with biphenyl, 2,2 -dichlorobiphenyl or dibenzofuran were observed,92,93,95,98,99 dibenzo-p-dioxin and 1-chlorodibenzo-p-dioxin were observed to react, with the measured reaction rate constants increasing with increasing NOz concentration.95,99 As discussed in detail by Atkinson92 and Kwok et al.,99 the reactions of the N03 radical with the PCBs, PCDDs and PCDFs appear to proceed by initial addition of the N03 radical to form an N03 adduct, which either thermally decomposes back to reactants or reacts with 02 and/or NOz (Scheme 1). 

The rate constants presented in Table 2 suggest that the N03 radical reactions with the gas-phase PCBs and PCDFs have upper limits to the rate constants for any reaction of fcabs < 10-15 cm3 molecule-1 s-1 (N02 concentrations in the troposphere are sufficiently low that the contribution of any N02-dependent reaction is encompassed within the upper limit to the bimolecular reaction rate constant fcabs). While dibenzo-p-dioxin and 1-chlorodibenzo-p-dioxin react with the N03 radical by the N02-dependent mechanism shown in Scheme 1 and equation (13), the N02 concentrations in the troposphere are sufficiently low that the effective bimolecular rate constants, fca(fcc[N02] + fcd[02])/fcb, are below the upper limits to the rate constants fcabs. 

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