- Photocatalytic degradation of lindane by polyoxometalates: Intermediates and mechanistic aspects
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The photocatalytic degradation of lindane (γ-1,2,3,4,5,6-hexachlorocyclohexane) has been studied in the presence of the polyoxometalate PW12O403- in aqueous solutions. Lindane is fully decomposed to CO2, Cl- and H2O, while a great variety of intermediates has been detected using GC-MS, including aromatic compounds (dichlorophenol, trichlorophenols, tetrachlorophenol, hexachlorobenzene, di- and trichloro-benzenodiol), non-aromatic cyclic compounds (penta-, tetrachlorocyclohexene, heptachlorocyclohexane), aliphatic compounds (tetrachloroethane) and condensation products (polychlorinated biphenyls). The number and nature of the intermediates implies that the mechanism of decomposition of lindane is based on both oxidative and reductive processes. Common intermediates have been reported during photolysis of lindane in the presence of titanium dioxide. A similar overall mechanism of polyoxometalates and TiO2 photocatalysis through the formation of common reactive species is suggested.
- Antonaraki,Triantis,Papaconstantinou,Hiskia
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experimental part
p. 119 - 124
(2010/08/22)
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- Catalysis and inhibition of ester hydrolysis in the presence of resorcinarene hosts functionalized with dimethylamino groups
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Complexation and catalysis of two calixresorcinarene (RES) derivatives with nucleophilic N,N-dimethylamino functions attached to their upper rims in the hydrolysis of carboxylate and sulfonate esters of 4-nitrophenol and 2,4-dinitrophenol have been investigated. Rate constants obey the complexation equation: kobs = kb × Ks + k c[Host]/Ks + [Host] Values of the dissociation constant (Ks) of the complexes are within the range exhibited by other systems such as cyclodextrins-ester complexes. The reactions of sulfonate esters only exhibit inhibition by the macrocyclic hosts. The reactions of the carboxylate esters exhibit catalysis and inhibition depending on the pH of the system. It is proposed that the dimethylamino function in RES3 and RES5 behaves as a nucleophile to form a reactive acylammonium species which subsequently decomposes and regenerates the catalytic amine. In the reaction of substituted phenyl acetates with RES3 the effective charge on the leaving oxygen in the complexed state (+0.88) is slightly more positive than that in the free ester (+0.70). The effective charge on the leaving oxygen in the transition structure is substantially more positive (+0.04 units) than in a model intramolecular reaction of tertiary dimethylamines with aryl esters (-0.53 units). The influence of the host on the reaction in the complex includes an electronic component which is ascribed to solvation of the transition structure of the rate-limiting step by water molecules located within the cavity of the host. It is suggested that this solvation is stronger than that occurring in the transition state for the model intramolecular reaction. Copyright
- Cevasco, Giorgio,Thea, Sergio,Vigo, Daniele,Williams, Andrew,Zaman, Flora
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p. 630 - 636
(2008/02/08)
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- Identification of surrogate compounds for the emission of PCDD/F (I-TEQ value) and evaluation of their on-line realtime detectability in flue gases of waste incineration plants by REMPI-TOFMS mass spectrometry
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Correlations between products of incomplete combustion (PIC), e.g., chloroaromatic compounds, can be used to characterise the emissions from combustion processes, like municipal or hazardous waste incineration. A possible application of such relationships may be the on-line real-time monitoring of a characteristic surrogate, e.g., with Resonance-Enhanced Multiphoton Ionization-Time-of-Flight Mass Spectrometry (REMPI-TOFMS). In this paper, we report the relationships of homologues and individual congeners of chlorinated benzenes (PCBz), dibenzo-p-dioxins (PCDD), dibenzofurans (PCDF) and phenols (PCPh) to the International Toxicity Equivalent (I-TEQ) of the PCDD/F (I-TEQ value) in the flue gas and stack gas of a 22 MW hazardous waste incinerator (HWI). As the REMPI detection sensitivity is decreasing with the increase of the degree of chlorination, this study focuses on the lower chlorinated species of the compounds mentioned above. Lower chlorinated species, e.g., chlorobenzene (MCBz), 1,4-dichlorobenzene, 2,4,6-trichlorodibenzofuran or 2,4-dichlorophenol, were identified as I-TEQ surrogates in the flue gas. In contrast to the higher chlorinated phenols, the lower chlorinated phenols (degree of chlorination 4) were not reliable as surrogates in the stack gas. The identified surrogates are evaluated in terms of their detectability by REMPI-TOFMS laser mass spectrometry. The outcome is that MCBz is the best suited surrogate for (indirect) on-line measuring of the I-TEQ value in the flue gas by REMPI-TOFMS. The correlation coefficient r of the MCBz concentration to the I-TEQ in the flue gas was 0.85.
- Blumenstock,Zimmermann,Schramm,Kettrup
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p. 507 - 518
(2007/10/03)
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- Complexation catalysis: effective charge development in the aminolysis of phenyl esters in chlorobenzene catalysed by crown ethers
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Kinetics of the butylaminolysis of substituted phenyl acetates in chlorobenzene in the presence of a variety of crown ethers obey the following rate law.Rate = kb2 + kc The individual rate constants fit Broensted-type relationships: log kb = -0.75 pKa + 4.21, log kc = -0.58 pKa + 3.41, log kc = -0.61 pKa + 3.18, where pKa refers to the ionization of the phenol in aqueous solution.The Broensted β1g values for kb and kc are calibrated with the value of βeq recently determined for acetyl transfer between phenolate ions in chlorobenzene.The sensitivity, β1g, of kc is consistent with the rate-limiting formation of a crown ether-zwitterion adduct with subsequent fast (non-rate-limiting) ArO-C bond fission.The Broensted data for kb when calibrated by βeq is consistent with rate-limiting proton transfer from zwitterion to base. 18-Crown-6 enables proton transfer to occur between phenol and butylamine in chlorobenzene according to the equation: BuNH2 + ArOH + Crown Bu-NH3+*Crown + ArO-. The equilibrium constant (K) for the above reaction with a series of substituted phenols has a Broensted selectivity (β) of 2.1 compared with that for the ionization of phenols in water.
- Maude, Antony B.,Williams, Andrew
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p. 691 - 696
(2007/10/02)
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- Rate and Equilibrium Studies of the Reaction of Oxyanions with 2-Phenyloxazol-5(4H)-one
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Equilibrium constants for the reaction of phenoxide ions with 2-phenyloxazol-5(4H)-one at 25 deg C and 1M ionic strength obey a Broensted relationship (log kOH/kArO = log K' = -173pKArOH - 15.81) and are not subject to steric effects from ortho-substituents.Both forward and reverse rate parameters exhibit steric effects, and the Broensted equations for meta- and para-substituted species are log kOH = -0.81 pKArOH + 9.75, and log kArO = 0.95pKArOH - 6.40.There is no break in the Broensted line in the region of pKArOH 5-11, consistent with a single transition-state.An upward deviation exists for oxyanions with low basicity (pKXOH 5); one of these oxyanions, betaine, has a solvent deuterium oxide isotope effect for its reaction with the oxazolone greater than 2, consistent with a general base mechanism for attack for these species.The results for nucleophilic attack of phenolate anions are in agreement with a concerted displacement at the carbonyl group.
- Chrystiuk, Edwin,Jusoh, Adelina,Santafianos, Dino,Williams, Andrew
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p. 163 - 168
(2007/10/02)
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- Photochemistry of halogenated benzene derivatives. Part VI. Photoreactions of tetra- and pentachlorophenols in water-acetonitrile mixtures
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Laboratory photochemical studies of aqueous acetonitrile solution of some polychlorinated phenols (PCPs) such as 2,3,4,5-tetrachlorophenol (2,3,4,5-Cl4-Pn)(1), 2,3,4,6-Cl4-Pn(2), 2,3,5,6-Cl4-Pn(3), and pentachlorophenol (Cl5-Pn)(4) at λ>285 nm have been carried out for 6 and 24 h exposure times.All the investigated PCPs underwent reductive dechlorination.This process was dependent not only upon the position of OH group but also upon the relative positions of the Cl substituents on the benzene ring.The Cl4-Pn 2 (and 3) and Cl5-Pn (4) also yielded photoproducts of molecular formulae C8H4Cl3NO(M+.=235) and C8H3Cl4NO(M+.=269), respectively.Furthermore, phenol 3 is unique amongst the investigated PCPs; in addition to the above mentioned photoproducts, it yielded hexa-, hepta-, and octachlorodihydroxybiphenyl(s) as well as heptachlorohydroxydiphenyl ether.
- Choudhry, Ghulam Ghaus,Wielen, Frans W. M. van der,Webster, G. R. Barrie,Hutzinger, Otto
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p. 469 - 475
(2007/10/02)
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- Reactivity Dependence on the Crystalline State. Reaction of Gaseous Chlorine on Solid Phenols
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Reaction of gaseous chlorine on powders of 3,5-dichlorophenol (A), 2,6-dimethylphenol (B), 1 : 1 molecular compound between A and B (C) and the mixture of equimolar amounts of powders of A and B (D) is studied under different experimental conditions.When the molecules are engaged in the compound C, the reactivity of the least reactive molecules (A) decreases while the reactivity of the most reactive molecules (B) increases.An attempt is made to interpret the results from the crystalline structures of A, B and C.
- Vicens, Jacques,Perrin, Robert,Bavoux, Claude
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- Catalytic Hydrogenation of Organic Solids-Basic Problaems
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The hydrogenation of organic solid substances like phenols and phenoxides under mild conditions has been studied.It is shown that those reactions take place in the solid state.Because of the lack of fluid phase the classical catalysis processes cannot explain the high conversion ratios obtained; the spillover effect and the existence of mobile hydrogenating species are discussed.
- Lamartine, Roger,Perrin, Robert,Thozet, Alain,Perrin, Monique
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- THE REACTIONS OF UNACTIVATED ARYL HALIDES WITH SODIUM METHOXIDE IN HMPA; SYNTHESIS OF PHENOLS, ANISOLES, AND METHOXYPHENOLS
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Sodium methoxide reacts with dichlorobenzenes in HMPA to give the chloroanisoles as a result of a SNAr process.Excess MeONa then effects the demethylation of the ethers to give the chlorophenols via an SN2 reaction.With tri- and tetrachlorobenzenes the initially formed chloroanisoles can be dealkylated to chlorophenols or can suffer further substitution to give the chlorodimethoxybenzenes; these react with excess MeONa to give the chloromethoxyphenols.The results obtained with the various isomers of the di-, tri-, and tetrachlorobenzenes are presented and discussed on the basis of the electronic effects of the substituents.
- Testaferri, L.,Tiecco, M.,Tingoli, M.,Chianelli, D.,Montanucci, M.
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p. 193 - 198
(2007/10/02)
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- Participation of Oligochlorobenzenes in the Base-Catalyzed Halogen Dance
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The three trichlorobenzenes fail to participate in the base-catalyzed halogen dance even on treatment with the favorable base/solvent combination potassium tert-butoxide in hexamethylphosphoric triamide.However, 1,2,3,5- and 1,2,4,5-tetrachlorobenzenes undergo disproportionation to penta- and trichlorobenzenes as well as interconversion into each other.Pentachlorobenzene disproportionates to hexa- and tetrachlorobenzenes, but further reactions of C6Cl6 form pentachlorophenol.Substitution reactions to form aryl tert-butyl ethers are observed as side reactions and are believed to occur by the SNAr mechanism.The phenols produced in several reactions apparently result from E2 cleavage of these ethers.These observations are possibly relevant to the mechanism of formation of 2,3,7,8-tetrachlorodibenzo-p-dioxin from 1,2,4,5-tetrachlorobenzene.
- Mach, Martin H.,Bunnett, Joseph F.
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p. 4660 - 4666
(2007/10/02)
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