- Study of alkyl radicals fragmentation from 2-alkyl-2-propoxyl radicals
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The reaction of a series of 2-alkyl-2-propoxyl radicals 2 in cumene has been studied. Alkoxyl radicals 2 were generated from the thermolysis of the corresponding tert-alkyl peroxypivalates, and underwent several modes of unimolecular reactions, that is, β-scission to give methyl radical, β-scission to give alkyl radicals and 1,5-H shift, which competed with hydrogen abstraction from cumene. The ratios of the rate constant for alkyl radicals elimination to that for methyl radical elimination, kβ(R)/kβ(Me), were determined by using the radical trapping method employing TEMIO as the scavenger. The logarithm of the relative rate was satisfactorily correlated with the heat of formation of the leaving alkyl radicals containing the steric parameters of neopentyl radical 2d. Then the plot of ln [kβ(R)/kβ(Me)] vs. the SOMO energies of the corresponding alkyl radicals showed a linear relationship with a slope of -7.6 eV-1, which is comparable to that of formolysis of alkyl bromides. The mechanism involving a fairly polar transition state of the alkyl radicals fragmentation from tert-alkoxyl radicals 2 is discussed in terms of the MO diagrams and the solvent effects.
- Nakamura, Tomoyuki,Watanabe, Yasumasa,Suyama, Shuji,Tezuka, Hiroshi
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- Reaction of tert-alkoxyl and alkyl radicals with styrene studied by the nitroxide radical-trapping technique
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The reactions of tert-alkyl peroxypivalates 1 (R = methyl, ethyl, and n- propyl) with styrene in the presence of the free-radical scavenger (1,1,3,3- tetramethyl-2,3-dihydro-1H-isoindol-2-yl)oxyl (2) have been studied at 60 °C. tert-Butyl and tert-alkoxyl radicals (tert-butoxyl, tert-pentyloxyl, and tert-hexyloxyl radicals) were generated from the thermolysis of 1, and the derivative alkyl radicals (methyl, ethyl, n-propyl, and 4-hydroxy-4- methylpentyl radicals) were formed by subsequent unimolecular reactions (β- scission and 1,5-H shift) of the corresponding tert-alkoxyl radicals. The extent of the unimolecular reactions of the tert-alkoxyl radicals (versus addition to styrene) and the relative reactivity of alkyl radicals toward addition to styrene were obtained from the competitive addition/trapping reactions. The absolute rate constants for the addition of tert-butyl, ethyl, methyl, and n-propyl radicals to styrene at 60 °C were estimated to be (7.4, 4.7, 5, and 5.4) x 105 M-1 s-1, respectively.
- Nakamura, Tomoyuki,Busfield, W. Ken,Jenkins, Ian D.,Rizzardo, Ezio,Thang, San H.,Suyama, Shuji
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- Thermal decomposition mechanisms of tert-alkyl peroxypivalates studied by the nitroxide radical trapping technique
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The thermolysis of a series of tert-alkyl peroxypivalates 1 in cumene has been investigated by using the nitroxide radical-trapping technique. tert-Alkoxyl radicals generated from the thermolysis underwent the unimolecular reactions, β-scission, and 1,5-H shift, competing with hydrogen abstraction from cumene. The absolute rate constants for β-scission of tert- alkoxyl radicals, which vary over 4 orders of magnitude, indicate the vastly different behavior of alkoxyl radicals. However, the radical generation efficiencies of 1 varied only slightly, from 53 (R = Me) to 63% (R = Bu(t)), supporting a mechanism involving concerted two-bond scission within the solvent cage to generate the tert-butyl radical, CO2, and an alkoxyl radical. The thermolysis rate constants of tert-alkyl peroxypivalates 1 were influenced by both inductive and steric effects [Taft-Ingold equation, log(rel k(d)) = (0.97 ± 0.14)Σσ* - (0.31 ± 0.04)ΣE(s)(c), was obtained].
- Nakamura,Busfield,Jenkins,Rizzardo,Thang,Suyama
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- Reactions of tert-Butoxyl Radicals with Acyclic Ethers Studied by the Radical Trapping Technique
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1,1,3,3-Tetramethyl-2,3-dihydro-1H-isoindol-2-yloxyl has been used as a radical trap to investigate the pattern of hydrogen abstraction reactions occurring in a range of acyclic ethers by tert-butoxyl radicals.The results confirm the high degree of selectivity for abstraction at C atoms adjacent to ethereal O.The presence of two α ethereal O atoms is less effective in enhancing abstraction than one.This is posssibly due to a stereoelectronic effect whereby the 1,3-oxygen orbital interactions reduce the ability of either oxygen to interact effectively with the developing radical centre.Whereas the presence of an α-oxygen enhances the rate of hydrogen abstraction, the presence of a β-oxygen retards the rate of hydrogen abstraction.Abstraction at a methine C atom adjacent to ethereal O occurs at about the same rate or a little faster than at a methylene C atom, whereas abstraction at methyl groups is much slower.A temperature study of the abstraction from methyl and methylene sites in dimethoxymethane shows that the difference in abstraction rates at these sites is predominantly an entropy effect.A lower entropy of activation due to the loss of the internal rotational mode of the methyl group in the formation of the transition state is the probable reason.Ethers appear to be less reactive than alcohols in hydrogen abstraction reactions.
- Busfield, W. Ken,Grice, I. Darren,Jenkins, Jan D.,Monteiro, Michael J.
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p. 1071 - 1078
(2007/10/02)
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