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  • 1
    Electronic Resource
    Electronic Resource
    Reversibility in the Reaction of Cyclohexadienyl Radicals with Oxygen in Aqueous Solution (1995)
    Weinheim : Wiley-Blackwell
    Chemistry - A European Journal 1 (1995), S. 423-429 
    Details
    ISSN: 0947-6539
    Keywords: kinetics ; peroxyl radicals ; pulse radiolysis ; radicals ; superoxide radicals ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Hydroxyl radicals were generated radiolytically and reacted with a number of benzene derivatives (PhH). In the presence of oxygen, the hydroxycyclohexadienyl radicals thus formed were converted into their corresponding peroxyl radicals. Pulse radiolysis has shown the oxygen addition (forward reaction, f) to be reversible (reverse reaction, r) [Eq. (1)]. The peroxyl radicals can eliminate HO2· to yield phenols, alongside some ring-fragmentation products (product-forming reaction, p). The rate constants for the forward and reverse reactions (kf and kr) and the corresponding stability constants K (= kf/kr) were determined for the hydroxy-cyclohexadienyl radicals derived from anisole, toluene, fluorobenzene, benzene, chlorobenzene, benzyl chloride, benzoate ion, phenylalanine, and terephthalate ion. The constants kf lie between 8 × 108 (anisole) and 1.6 × 107 dm3 mol-1 s-1 (terephthalate ion), and kr between 7.5 × 104 (toluene) and 3.4 × 103 s-1 (terephthalate ion). The stability constants lie between 2.6 × 104 (benzene) and 3.3 × 103 dm3 mol-1 (phenylalanine). The rate constants for the product-forming reactions kp are between 5.5 × 103 (anisole) and 3.4 × 102 s-1 (benzoate). For the peroxyl radical derived from phenylalanine, a bond dissociation energy of 5.5 kcal mol-1 has been derived. A number of hydroxy-cyclohexadienyl radicals (e.g., those derived from benzoic acid, ethylbenzoate, benzonitrile, and nitrobenzene) react too slowly to allow the equilibrium constant to be determined by means of pulse radiolysis. These reactions have rate constants kf in the order of 5 × 106 dm3 mol-1 s-1, except for nitrobenzene where the reaction is too slow for measurement. The rate constants kr are below 500 s-1, and the product-forming reaction is too slow to be detected by pulse radiolysis. γ-Radiolysis of N2O/O2(4:1)-saturated aqueous solutions of benzonitrile gave dimeric compounds (e.g., dicyanobiphenyls) in low yield, alongside the three isomeric phenols; this again proves the low reactivity of its hydroxycyclohexadienyl radical toward oxygen.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/chem.19950010706
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  • 2
    Electronic Resource
    Electronic Resource
    Synthesis of homologues of deoxyribouridylic acid in preparative amounts using the triester method (1986)
    New York : Wiley-Blackwell
    Die Makromolekulare Chemie 187 (1986), S. 809-827 
    Details
    ISSN: 0025-116X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The homologous oligodeoxyribouridylic acids dU2, dU4, dU8 and dU16 were chemically synthesized in amounts of 70 to 800 mg by block condensation using the phosphotriester method. The partially protected hydroxyl and phosphate components were condensed without any protection for the nucleobase uracil, using a mixture of 2,4,6-triisopropylbenzenesulfonyl chloride and tetrazole in anhydrous pyridine as condensing agent. Demonstrable side reactions of the uracil residue occur under this condensation conditions. However, this undesired derivatisation of uracil residues is totally eliminated by a simple deamination of the deblocked oligodeoxyribouridylic acids. After synthesis, cleavage of the protecting groups and column chromatographical fractionation using QAE-Sephadex, the oligodeoxyribouridylic acids were deaminated and obtained as pure products, as was positively evidenced by “fingerprints”.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/macp.1986.021870412
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  • 3
    Electronic Resource
    Electronic Resource
    OH-radical-induced crosslinking and strand breakage of poly(vinyl alcohol) in aqueous solution in the absence and presence of oxygen. A pulse radiolysis and product study (1994)
    Weinheim : Wiley-Blackwell
    Macromolecular Chemistry and Physics 195 (1994), S. 1443-1461 
    Details
    ISSN: 1022-1352
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Hydroxyl radicals were generated radiolytically in N2O- or N2O/O2(4:1)-saturated aqueous solutions of poly(vinyl alcohol) (PVAL) or its low-molecular-weight model compound pentane-2,4-diol (PD). Using the pulse radiolysis technique, the rate constant of OH with PD and PVAL has been determined to be 2,3 · 109 dm3 · mol-1 · s-1 and 1,5 · 108 dm3 · mol-1 · s-1, respectively. Upon OH attack two kinds of radicals are generated: (i) tertiary α-hydroxyalkyl radicals and (ii) alkyl radicals. The former rapidly reduce tetranitromethane yielding the stable nitroform anion, and from its yield it is calculated that 70-75% of the radicals are α-hydroxyalkyl radicals in both systems. In the presence of oxygen, the carbon-centered radicals are converted into the corresponding peroxyl radicals. The α-hydroxyalkylperoxyl radicals eliminate HO2·/O2· in spontaneous and base-catalyzed reactions (rate constants ≈700 s-1 and ≈2,3 · 109 dm3 · mol-1 · s-1, respectively). There are marked differences in the rates of the bimolecular radical decay reactions. The carbon-centered radicals of PD decay with a rate constant of 5 · 108 dm3 · mol-1 · s-1, whereas those of PVAL decay, under pulse-radiolytic conditions, mainly intramolecularly (loop formation), with rate constants which reach, at high radical numbers per macromolecule, values as high as 1010 dm3 · mol-1 · s-1. With PD it was shown by product studies that the radicals mainly (90%) disproportionate and dimerize only to 10%, thus limiting the yield of crosslinks in PVAL to the latter value. The bimolecular decay rates of the organic peroxyl radicals in O2· containing solutions are more difficult to investigate because of the effective unimolecular HO2·-elimination reaction. However, it is estimated that the peroxyl radicals of both compounds decay (under otherwise equal conditions) more than one order of magnitude slower than the carbon-centered radicals. The measured yields of strand breaks in oxygenated solutions as well as the yields of intermolecular crosslinks under anoxia, determined with PVAL by low-angle laser light-scattering at different dose-rates and pH values, support these conclusions.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/macp.1994.021950427
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