2348-48-3Relevant articles and documents
Mechanism of Formation of o-Methylbenzyl Radical by Photodissociation of o-Xylene in Solution
Fujiwara, Masao,Tanimoto, Yoshifumi
, p. 5695 - 5700 (1994)
Photodissociation of o-xylene in room temperature n-heptane solution has been studied by means of two-pulse laser-induced fluorescence and transient absorption spectroscopy.Excitation of o-xylene at 266 nm into the S1 state causes the molecule to undergo carbon-hydrogen bond homolysis in its methyl group, resulting in formation of the o-methylbenzyl radical.The fluorescence of the o-methylbenzyl radical has been observed around 500 nm with a lifetime of 4.1 +/- 1.0 ns, when it has been excited with a 308-nm pulse after the photolysis pulse.The absorption of the o-methylbenzyl radical has been obtained with maxima at 309 and 320 nm.The formation raet constant of the o-methylbenzyl radical, (3.1 +/- 0.4)E7 s-1, agrees with the decay rate constant of the fluorescence of o-xylene, (2.7 +/- 0.3)E7 s-1.It is concluded that excitation with one photon at 266 nm followed by vibrational relaxation populates the thermal equilibrium.S1 state of o-xylene, from which predissociation occurs.
Kinetic Study of the Phthalimide N-Oxyl Radical in Acetic Acid. Hydrogen Abstraction from Substituted Toluenes, Benzaldehydes, and Benzyl Alcohols
Koshino, Nobuyoshi,Saha, Basudeb,Espenson, James H.
, p. 9364 - 9370 (2007/10/03)
The phthalimide N-oxyl (PINO) radical was generated by the oxidation of N-hydroxyphthalimide (NHPI) with Pb(OAc)4 in acetic acid. The molar absorptivity of PINO. is 1.36 × 103 L mol -1 cm-1 at λmax 382 nm. The PINO radical decomposes slowly with a second-order rate constant of 0.6 ± 0.1 L mol-1 s-1 at 25°C. The reactions of PINO . with substituted toluenes, benzaldehydes, and benzyl alcohols were investigated under an argon atmosphere. The second-order rate constants were correlated by means of a Hammett analysis. The reactions with toluenes and benzyl alcohols have better correlations with σ+ (ρ = -1.3 and -0.41), and the reaction with benzaldehydes correlates better with σ (ρ = -0.91). The kinetic isotope effect was also studied and significantly large values of kH/kD were obtained: 25.0 (p-xylene), 27. 1 (toluene), 27.5 (benzaldehyde), and 16.9 (benzyl alcohol) at 25°C. From the Arrhenius plot for the reactions with p-xylene and p-xylene-d10, the difference of the activation energies, EaD - E aH, was 12.6 ± 0.8 kJ mol-1 and the ratio of preexponential factors, AH/AD, was 0.17 ± 0.05. These findings indicate that quantum mechanical tunneling plays an important role in these reactions.
Kinetics of the reaction of the TEMPO radical with alkylarenes
Opeida,Matvienko,Bakurova,Voloshkin
, p. 900 - 904 (2007/10/03)
The kinetics of the reaction of the stable radical 2,2,6,6- tetramethylpiperidine-N-oxyl (TEMPO) with a series of alkylarenes containing primary and secondary benzyl C-H bonds was studied by ESR, and the reaction rate constants were determined. The scheme
Photo- and radiation-chemical production of radical cations of methylbenzenes and benzyl alcohols and their reactivity in aqueous solution
Russo-Caia, Claudia,Steenken, Steen
, p. 1478 - 1485 (2007/10/03)
Radical cations of methylated benzenes and benzyl alcohols were generated by photoionization and by reaction SO4·- in aqueous solution. The photoionization requires two 248 nm photons. The lifetimes with the oxidant SO4 and absorption spectra of the radical cations produced were determined by time-resolved conductance and optical detection, and the reaction products were measured by GC. As expected, the radical cation lifetimes increase strongly with increasing number of additional methyl groups, and so does the ratio of deprotonation from a methyl or hydroxymethyl group vs. addition (of water) to a ring position. In the case of toluene the radical cation appears to have a chemical lifetime τ of 10-100 ps ≤ τ ≤ 20 ns, i.e., longer than it takes for an ion pair to separate into the free (solvated) ions, and it reacts predominantly by addition of water to the ring rather than by deprotonation from the methyl group. A further observation is that, as compared to methoxylated analogues, the methylated benzyl alcohol radical cations are much more reactive, such that OH-induction of side-chain fragmentation, as often required with methoxylated benzyl alcohol-type radical cations, is not necessary.
Reaction pathways involved in the quenching of the photoactivated aromatic ketones xanthone and 1-azaxanthone by polyalkylbenzenes
Coenjarts,Scaiano
, p. 3635 - 3641 (2007/10/03)
The reactions of the photoexcited aromatic ketones, xanthone and 1-azaxanthone, with polyalkylbenzene donors yields the corresponding ketyl radicals as detected by nanosecond laser flash photolysis. On the basis of formation of these photoreduced products, the quenching of the photoexcited species is expected to occur either by a one-step hydrogen abstraction from the donor, electron transfer followed by proton transfer from the donor, or by formation of a charge-transfer type encounter complex prior to hydrogen atom transfer. The reactions of triplet xanthone and triplet 1-azaxanthone with polyalkylbenzene donors in acetonitrile were investigated to probe the effect of the nature of the triplet state and the redox properties on the relative importance of each quenching pathway. Determination of bimolecular rate constants, as well as analysis of kinetic isotope effects and ketyl radical yields, suggests that for both xanthone and 1-azaxanthone the quenching process is dominated by formation of charge-transfer encounter complexes between excited-state aromatic ketone acceptor and ground-state polyalkylbenzene donor. The reactivites of the xanthone π,π* triplet and 1-azaxanthone n,π* triplet toward these donors is shown to be governed by their reduction potentials, with their electronic configuration being unimportant to the kinetics of encounter complex formation. The only exception to this is found when sterically encumbered polyalkylbenzene donors are employed. Results with these compounds suggest that π,π* and n,π* states form encounter complexes of different structure which affects their ability to react with hindered donors. Additionally, product yields with all of the donors are controlled by both the extent of charge transfer within encounter complexes and the encounter complex structure.
One-Electron Oxidation of Alkylbenzenes in Acetonitrile by Photochemically Produced NO3.: Evidence for an Inner-Sphere Mechanism
Giacco, Tiziana Del,Baciocchi, Enrico,Steenken, Steen
, p. 5451 - 5456 (2007/10/02)
The reaction between NO3. and polyalkylbenzenes was studied using 308-nm laser flash photolysis of cerium(IV) ammonium nitrate in the presence of the alkylbenzenes in acetonitrile solution.For all benzenes, with the exception of monoalkylbenzenes and o- and m-xylene, the reaction with NO3. was found to yield the corresponding radical cations and to proceed in an apparently straightforward bimolecular manner.For monoalkylbenzenes and o- and m-xylene, radicals were seen which are derived from the parents by formal loss of H. from the side chain of the aromatic.This reaction proceeds via a complex between the aromatic and NO3. with the decomposition of the complex being rate determining at higher concentrations of aromatic (rate constants for decomposition between 6 * 105 and 4 * 107 s-1).In the complex, electron transfer from the aromatic to NO3. is suggested to be concerted with deprotonation of the incipient radical cation.Formation of a complex between NO3. and aromatics is likely even in those cases where radical cations are observed, with the assumption that in these cases the complex decomposition rate is greater than 6 * 107 s-1.
C-C and C-H Bond Splits of Laser-Excitated Aromatic Molecules. 1. Specific and Thermally Averaged Rate Constants
Brand, U.,Hippler, H.,Lindemann, L.,Troe, J.
, p. 6305 - 6316 (2007/10/02)
Toluene, m-,o-,and p-xylene, mesitylene, ethyl-, isopropyl-, and tert-butylbenzene were irradiated by nanosecond laser flashes at 193 nm.After fast internal conversion to the electronic ground state, the molecules dissociate by C-C or C-H bond splits.The
Primary Processes in the Type I Photocleavage of Dibenzyl Ketones. A Pulsed Laser and Photochemically Induced Dynamic Nuclear Polarization Study
Gould, Ian R.,Baretz, Bruce H.,Turro, Nicholas J.
, p. 925 - 929 (2007/10/02)
The primary homolytic α-cleavage, and subsequent decarbonylation of the intermediate phenacyl radicals, have been studied for the type I photoreaction of a series of dibenzyl ketones, using the techiques of pulsed laser photolysis and photochemically induced dynamic nuclear polarization (photo-CIDNP).Evidence for selective primary cleavage to produce the most stable radical pairs is obtained for unsymmetrical ketones.The absolute rate constants and activation parameters for the decarboxylations are obtained, and the relationships between these and the stabilities of the product radicals is discussed.
Pyrolysis of Alkyl Benzenes. Relative Stabilities of Methyl-Substituted Benzyl Radicals
Barton, Barrie D.,Stein, Stephen E.
, p. 2141 - 2145 (2007/10/02)
Decomposition rates for β C-C bond scission in several methyl-substituted ethylbenzenes were measured in a very low pressure pyroly (VLPP) system from 1050 to 1200 K, and relative stabilities of the resulting substituted benzyl radicals were derived.Relative to ethylbenzene, rates were highest for o-methyl-substituted ethylbenzenes, and little affected by meta and para substitution.Activation energy differences were approximately separable into ortho and nonortho contributions, amounting to a lowering of the activation energy, relative to ethylbenzene, of 1.3-1.7 kJ/mol per m- or p-CH3, and 5.0-6.3 kJ/mol per o-CH3.Rate differences were explained in terms of a partial relaxation of steric interaction in the activated complex.Methyl inductive effects were found to be very small, amounting to a decrease of 1.7 kJ/mol per CH3 in the Gibbs energy of formation of the radicals produced.
