Chlorine abstraction reaction from chloropentafluorobenzene by the CF<sub>3</sub> radical

Nieto, Jorge D.; Herrera, O. S.; Lane, Silvia I.; Oexler, E. V.

doi:10.1002/bbpc.19981020605

Cited by 3 publications

(2 citation statements)

References 32 publications

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“…The fact that the observed substituent effects do not arise from variations in reaction enthalpy suggests that polar effects 1a,b,d,21a,24,25 play a critical role in controlling the reaction efficiency of the charged radicals, i.e., the neutral substituents lower the energy of the transition state by enhancing its polar character (Figure c). Due to the polarizability of halogen atoms, halogen atom transfer reactions have been predicted to be more sensitive to polar effects than, for example, hydrogen atom abstraction. 21b, Examination of iodine abstraction by the nucleophilic methyl radical from CF 3 I and CH 3 CH 2 I has demonstrated the importance of polar effects in these iodine transfer reactions: despite the same exothermicity, the reaction with CF 3 I is associated with a 3 kcal/mol lower barrier (4.3 kcal/mol) than the reaction with CH 3 CH 2 I 21b…”

Section: Resultsmentioning

confidence: 99%

Polar Effects on Iodine Atom Abstraction by Charged Phenyl Radicals

2000

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The ability of differently substituted charged phenyl radicals (a class of distonic radical cations) to abstract an iodine atom from allyl iodide was systematically examined in the gas phase by using Fourier transform ion cyclotron resonance mass spectrometry. The reaction products and second-order reaction rate constants were determined for several radicals that differ by the type and/or number of substituents located in the ortho- and/or meta-position with respect to the radical site. All the radicals also carry a para-pyridinium group needed for mass spectrometric manipulation. These electron-deficient phenyl radicals react with allyl iodide by predominant iodine atom abstraction. The reaction is facilitated by the presence of neutral electron-withdrawing substituents, such as F, CF3, Cl, or CN. The extent of rate increase depends on the type and number of the substituents, as well as their location relative to the radical site. Based on molecular orbital calculations (PM3 and Becke3LYP/6-31G(d)+ZPVE), the indicated variations in the transition state energy are not related to enthalpic factors. Instead, the results are rationalized by polar effects arising from a variable contribution of a stabilizing charge transfer resonance structure to the transition state. A semiquantitative measure for the barrier-lowering effect of each substituent is provided by its influence on the electron affinity of the radical (the electron affinities were calculated by Becke3LYP/ 6-31+G(d) and AM1, which were found to produce similar values). Methyl substitution does not significantly affect the electron affinity, and accordingly, does not have a detectable effect on reactivity. Methyl groups located at ortho-positions are an exception, however. o-Methyl-substituted phenyl radicals undergo exothermic rearrangement to a benzyl radical in competition with iodine abstraction from allyl iodide.

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Section: Resultsmentioning

confidence: 99%

Polar Effects on Iodine Atom Abstraction by Charged Phenyl Radicals

2000

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“…Previously, we have shown 42,57,58,67,69 that polar effects − play a crucial role in controlling the reaction efficiencies of charged σ-radicals and σ,σ-biradicals. Specifically, the energy of the transition state for a radical reaction is significantly influenced by low-lying charge-transfer resonance structures, which contribute to the electronic structure of the transition state.…”

Section: Resultsmentioning

confidence: 98%

Gas-Phase Reactivity of Charged π-Type Biradicals

et al. 2004

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Four pi,pi-biradicals, 2,6-dimethylenepyridinium and the novel isomers N-(3-methylenephenyl)-3-methylenepyridinium, N-phenyl-3,5-dimethylenepyridinium, and N-(3,5-dimethylenephenyl)pyridinium ions, were generated and structurally characterized in a Fourier transform ion cyclotron resonance mass spectrometer. Their gas-phase reactivity toward various reagents was compared to that of the corresponding monoradicals, 2-methylenepyridinium, N-phenyl-3-methylenepyridinium, and N-(3-methylenephenyl)pyridinium ions. The biradicals reactivity was found to reflect their predicted multiplicity. The 2,6-dimethylenepyridinium ion, the only biradical in this study predicted to have a closed-shell singlet ground state, reacts significantly faster than the other biradicals, which are predicted to have triplet ground states. In fact, this biradical reacts at a higher rate than the analogous monoradical, which suggests that to avoid the costly uncoupling of its unpaired electrons, the biradical favors ionic mechanisms over barriered radical pathways. In contrast, the second-order reaction rate constants of the isomeric biradicals with triplet ground states are well approximated by those of the analogous monoradicals, although the final reaction products are sometimes different. This difference arises from rapid radical-radical recombination of the initial monoradical reaction products. The overall reactivity toward the hydrogen-atom donors benzeneselenol and tributylgermanium hydride is significantly greater for the radicals with the charged site in the same ring system as the radical site. This finding indicates that polar effects play an important role in controlling the reactivity of pi,pi-biradicals, just as has been demonstrated for sigma,sigma-biradicals.

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The gas-phase reaction of the CF₃radical with thiophene

Herrera¹,

Nieto²,

Lane³

et al. 2003

Can. J. Chem.

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The reaction of CF3 radicals, generated by photolysis of CF3I or hexafluoroacetone with thiophene, was studied in the gas phase at 25 °C. At conversion of thiophene less than 20%, monosubstituted CF3-thiophenes were found as the main reaction products, in addition to CF3H, C2F6, and monosubstituted dihydro-CF3-thiophene, the latter in very low proportion. An isomeric mixture of 2- and 3-CF3-thiophene was obtained in a ratio of about 16, independent of the radical source used (CF3I or hexafluoroacetone) to produce the CF3 radicals. A plausible mechanism that accounts for the observed products is proposed, and the reactivity of thiophene toward the CF3 radical at 25 °C was determined as kadd/kc1/2 = 106 ± 4 cm3/2 mol–1/2 s–1/2.Key words: thiophene, trifluoromethyl radical, reaction mechanism, reactivity.

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Chlorine abstraction reaction from chloropentafluorobenzene by the CF₃ radical

Cited by 3 publications

References 32 publications

Polar Effects on Iodine Atom Abstraction by Charged Phenyl Radicals

Polar Effects on Iodine Atom Abstraction by Charged Phenyl Radicals

Gas-Phase Reactivity of Charged π-Type Biradicals

The gas-phase reaction of the CF₃radical with thiophene

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Chlorine abstraction reaction from chloropentafluorobenzene by the CF3 radical

Cited by 3 publications

References 32 publications

Polar Effects on Iodine Atom Abstraction by Charged Phenyl Radicals

Polar Effects on Iodine Atom Abstraction by Charged Phenyl Radicals

Gas-Phase Reactivity of Charged π-Type Biradicals

The gas-phase reaction of the CF3radical with thiophene

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Chlorine abstraction reaction from chloropentafluorobenzene by the CF₃ radical

The gas-phase reaction of the CF₃radical with thiophene