Direct observation of the equilibrium between cyclohexenyl radicals, O2, and cyclohexenylperoxy radicals

Zils, R.; Inomata, Satoshi; Yukio, Okunuki; Washida, Nobuaki

doi:10.1016/s0301-0104(98)00023-8

Cited by 6 publications

(6 citation statements)

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“…Of the radicals reported in these investigations, benzyl (stabilised similarly to allyl) was adopted as the closest model for 14 and 15. As judged from allyl systems, [56] the reactions of benzyl, 14 and 15 with oxygen should be irreversible under our conditions. The value determined for benzyl was 2.36 ϫ 10 9  Ϫ1 s Ϫ1 in cyclohexane [54] (2.77 ϫ 10 9  Ϫ1 s Ϫ1 in water [55] ).…”

Section: Dimerisation Of 9 In the Presence Of Atmospheric Oxygenmentioning

confidence: 85%

The Thermal [2+2] Cyclodimerisation of (E,Z)-Cycloocta-1,3-diene Revisited − Chemical Trapping and Properties of the Intermediate 1,4-Diradicals

et al. 2002

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Keywords: Cycloaddition / Radicals / Reaction mechanisms / ThermochemistryThe thermal [2+2] cyclodimerisation of (E,Z)-cycloocta-1,3diene (9), which is known to afford the cyclobutane dimers 11, 12, and 13, has been investigated in the presence of the nitroxyls 17 and 18 and of atmospheric dioxygen, all of which are known to be efficient trapping agents for carbon-centred free radicals. The nitroxyls have been found to divert the reaction from formation of the dimers to formation of 2:2 adducts of two molecules of 9 and two molecules of nitroxyl. The rate constant for the formation of the overall sum of the dimers plus the 2:2 adducts in the presence of nitroxyl has been found to equal the rate constant for the formation of dimers in the absence of nitroxyl. This and the molecular structures of the 2:2 adducts prove that two molecules of 9 combine irreversibly to produce the two epimeric bis(allylic) 1,4-diradicals 14 and 15 (meso and rac, respectively) which undergo two competing reactions: ring-closure to dimers 11, 12, and 13, and trapping by nitroxyl to form the 2:2 adducts. Dioxygen, too, was found to trap 14 and 15 efficiently. From the kinetics of the latter trapping reaction, studied at six temperatures between 5 and 55°C, the heights of the activation barriers separating 14 from 11 and 15 from 12 + 13 were estimated at 11.1 ± 1.5 and 10.2 ± 1.5 kcal·mol −1 , respectively, corresponding to diradical lifetimes of ca. 0.5 µs. These unexpectedly high barriers have been verified by MM3 force-field calculations and by an investigation of the kinetics of the gas-phase thermolysis of 12 (to give 13 and 16 which is an epimer of 12 and 13) and of 13 (to give 12 and 16). When the [a]

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Section: Dimerisation Of 9 In the Presence Of Atmospheric Oxygenmentioning

confidence: 85%

The Thermal [2+2] Cyclodimerisation of (E,Z)-Cycloocta-1,3-diene Revisited − Chemical Trapping and Properties of the Intermediate 1,4-Diradicals

et al. 2002

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“…Over the temperature range from 268 to 308 K the decay profile of the pentadienyl radical is governed by reactions 8 and -8 and the wall-loss processes of pentadienyl radicals (k w1 ) and pentadienylperoxy radicals (k w2 ). The analysis of rate equations of this system has been described in the previous paper 6 and also by Benson. 22 Briefly, the concentration of the pentadienyl radical [R] and the pentadienylperoxy radical [RO 2 ] will be given by the coupled equations…”

Section: Resultsmentioning

confidence: 99%

“…The experimental setup used in this work is similar to that described previously. 6 The radical precursors and O 2 were introduced into a tubular Pyrex reactor (13 mm i.d.) with N 2 as the carrier gas.…”

Section: Methodsmentioning

confidence: 99%

“…Subsequent studies observed the equilibria for several other hydrocarbon radicals (CH 3 , C 2 H 5 , i-C 3 H 7 , t-C 4 H 9 , and benzyl) with O 2 using similar methods. [2][3][4][5] Recently, we reported 6 the observation of the equilibrium between cyclohexenyl radicals, O 2 , and cyclohexenylperoxy radicals at 361 K. The cyclohexenyl radical can be thought of as a cyclic allyl-type radical. The C-O bond energy in the cyclohexenylperoxy radical, 80 kJ mol -1 , is similar to the C-O bond energy in the allylperoxy radical, 76 kJ mol -1 .…”

Section: Introductionmentioning

confidence: 99%

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Determination of the Equilibrium Constant and Thermodynamic Parameters for the Reaction of Pentadienyl Radicals with O₂

et al. 2001

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The reaction of pentadienyl radicals (C 5 H 7 ) with O 2 has been studied by a combination of pulsed laser photolysis and photoionization mass spectrometry. These radicals were generated either by the photolysis of 1,3-pentadiene or by a two step process, photolyzing carbon tetrachloride to form Cl atoms, which then abstracted a hydrogen atom from 1,4-pentadiene. The equilibrium between pentadienyl radicals, O 2 and pentadienylperoxy radicals could be observed over the temperature range 268-308 K. An analysis of the time-dependent signal of pentadienyl radicals was used to evaluate the equilibrium constant. From the temperature dependence of the equilibrium constant, the enthalpy change for the reaction C 5 H 7 + O 2 f C 5 H 7 O 2 was found to be 56 ( 5 kJ mol -1 . The C-O bond energy in the C 5 H 7 -O 2 adduct is weaker than those of allyl-type peroxy radicals. Possible values of the heats of formation of C 5 H 7 and C 5 H 7 -O 2 radicals and the resonance stabilization energy of the C 5 H 7 radical are discussed.

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“…Rate constants for irreversible trapping of carbon-centred monoradicals in organic solvents by oxygen at 27°C have been determined, [54] and were very similar to those determined in water. As judged from allyl systems, [56] the reactions of benzyl, 14 and 15 with oxygen should be irreversible under our conditions. Of the radicals reported in these investigations, benzyl (stabilised similarly to allyl) was adopted as the closest model for 14 and 15.…”

Section: Dimerisation Of 9 In the Presence Of Atmospheric Oxygenmentioning

confidence: 85%

The Thermal [2+2] Cyclodimerisation of (E,Z)-Cycloocta-1,3-diene Revisited − Chemical Trapping and Properties of the Intermediate 1,4-Diradicals

et al. 2002

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Direct observation of the equilibrium between cyclohexenyl radicals, O2, and cyclohexenylperoxy radicals

Cited by 6 publications

References 23 publications

The Thermal [2+2] Cyclodimerisation of (E,Z)-Cycloocta-1,3-diene Revisited − Chemical Trapping and Properties of the Intermediate 1,4-Diradicals

The Thermal [2+2] Cyclodimerisation of (E,Z)-Cycloocta-1,3-diene Revisited − Chemical Trapping and Properties of the Intermediate 1,4-Diradicals

Determination of the Equilibrium Constant and Thermodynamic Parameters for the Reaction of Pentadienyl Radicals with O₂

The Thermal [2+2] Cyclodimerisation of (E,Z)-Cycloocta-1,3-diene Revisited − Chemical Trapping and Properties of the Intermediate 1,4-Diradicals

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Direct observation of the equilibrium between cyclohexenyl radicals, O2, and cyclohexenylperoxy radicals

Cited by 6 publications

References 23 publications

The Thermal [2+2] Cyclodimerisation of (E,Z)-Cycloocta-1,3-diene Revisited − Chemical Trapping and Properties of the Intermediate 1,4-Diradicals

The Thermal [2+2] Cyclodimerisation of (E,Z)-Cycloocta-1,3-diene Revisited − Chemical Trapping and Properties of the Intermediate 1,4-Diradicals

Determination of the Equilibrium Constant and Thermodynamic Parameters for the Reaction of Pentadienyl Radicals with O2

The Thermal [2+2] Cyclodimerisation of (E,Z)-Cycloocta-1,3-diene Revisited − Chemical Trapping and Properties of the Intermediate 1,4-Diradicals

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Determination of the Equilibrium Constant and Thermodynamic Parameters for the Reaction of Pentadienyl Radicals with O₂