- Evaluation of dissociation energies of S-H bonds in thiophenols and thioalcohols on the basis of kinetic measurements
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Kinetic data on the reactions of alkyl and benzyl radicals with thiophenol C6H5SH were analyzed within the framework of the parabolic model of transition state. The values of the parameter that establishes a correlation between the activation energy of a reaction and its enthalpy were calculated for reactions of alkyl and benzyl radicals with the C6H5SH. The equations of the parabolic model were used to calculate the bond dissociation energies for 11 thiophenols and 4 thioalcohols. The activation energies for reactions of 12 thiophenoxy radicals with cumene and of C6H5S? radical with several alkyl-aromatic hydrocarbons were obtained.
- Denisov
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p. 238 - 241
(2007/10/03)
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- Hydrogen-Abstraction Reactivity of Excited-State Para-Substituted Benzyl Radicals in Solution at Room Temperature
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In contrast to almost no H-abstraction reactivity of the ground-state para-substituted benzyl radicals (p-X-benzyls: X=CN, Cl, F, and OCH3) toward 1,4-cyclohexadiene (CHD), a significant fluorescence quenching by CHD in hexane at room temperature was conf
- Tokumura, Kunihiro,Ozaki, Tomomi,Itoh, Michiya
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p. 5999 - 6004
(2007/10/02)
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- Kinetic Study for Reactions of Nitrate Radical (NO3.) with Substituted Toluenes in Acetonitrile Solution
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The absolute rate constants for the reactions of the nitrate radical (NO3.) with substituted toluenes in acetonitrile have been determined by the flash photolysis method.From the plots of the rate constants against the ionization energies, it was revealed that the reaction path for toluene derivatives with low ionization energies is different from that for toluene derivatives with high ionization energies.For toluene, a deuterium isotope effect was observed to be ca. 1.6, suggesting the direct hydrogen atom abstraction reaction; in this group, xylenes and p-chlorotoluene belong.For toluene derivatives with electron-withdrawing substit uents, NO3. may add to the phenyl rings followed by successive reactions.For both groups, linear correlations against ionization energies with negative slopes show that NO3. is highly electrophilic and that strong polar effects exist in the transition states of both reactions.For toluenes with methoxy groups, the electron-transfer reaction from methoxytoluene to NO3. is a main initial path, since the transient absorption band due to the cation radical of methoxytoluene was detected.
- Ito, Osamu,Akhido, Seiji,Iino, Masashi
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p. 2436 - 2440
(2007/10/02)
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- Electron Spin Resonance Studies. Part 71. Side-chain Oxidation Pathways in the Reactions of .OH and SO4-. with Some Phenyl-substituted Carboxylic Acids, their Anions, and Some related Compounds
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A series of arene radical-cations has been generated in situ by the reactions of methylbenzene, phenylethanoic acid, and some derivatives and cyclic analogues with both SO4-. and .OH (the latter in acid solution).The results are interpreted in terms of a variety of subsequent rapid reactions including hydration, deprotonation (to give benzylic radicals), and fragmentation (decarboxylation: for a series of radical-zwitterions +.Ar(CH2)nCO2- (n=1-3) decarboxylation (k >/= 1E9 dm3 mol-1 s-1) appears to proceed via direct intramolecular elecron-transfer, though in some cases formation of a discrete ?-bonded intermediate cannot be ruled out.
- Gilbert, Bruce C.,Scarratt, Cathryn J.,Thomas, C. Barry,Young, John
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p. 371 - 380
(2007/10/02)
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- Self-Termination and Electronic Spectra of Substituted Benzyl Radicals in Solution
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Several substituted benzyl radicals have been generated in liquid cyclohexane by the photolysis of symmetrically substituted dibenzyl ketones.The rate constants for self-termination to substituted bibenzyls, and absorption spectra of the transient radicals, have been measured by optical modulation spectroscopy.The termination rates are generally well described by the von Smoluchowski equation with a spin factor of 1/4.Deviations are discussed in terms of steric retardation and the unreliability of the estimated reaction diameters of radicals with bulky substituent groups.Absorption spectra of the three monomethyl-, the three monochloro-, two dichloro-, and the three monomethoxybenzyl radicals demonstrate the influence of the substituents on the energy levels of the benzyl system.
- Claridge, Rodney F. C.,Fischer, Hanns
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p. 1960 - 1967
(2007/10/02)
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- Substituent Effects on Benzyl Radical ESR Hyperfine Coupling Constants. The ?α*Scale Based upon Spin Delocalization
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The electron spin resonance spectra of 21 para- and meta-substituted benzyl radicals have been analyzed.A substituent constant, ?α*, has been defined from the benzylic α-hydrogen hyperfine coupling constant.The ?α* constant reflects the component of energy that may be attributed to spin delocalization in a substituted benzyl radical in comparison to the unsubstituted radical.For the derivatives studied, para substitution is stabilizing, except when fluorine is the substituent; meta substitution destabilizes the benzyl radical.A series of radical reactions is reexamined with an extended Hammett relation.The relative importance of spin delocalization vs. polar effects is assessed by the ratio of ρ* to ρ.The importance of choosing substituents for which ?a* and ? differ widely for such an assessment is emphasized.
- Dust, Julian M.,Arnold, Donald R.
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p. 1221 - 1227
(2007/10/02)
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