98-06-6Relevant articles and documents
Accurate oxidation potentials of benzene and biphenyl derivatives via electron-transfer equilibria and transient kinetics
Merkel, Paul B.,Luo, Pu,Dinnocenzo, Joseph P.,Farid, Samir
experimental part, p. 5163 - 5173 (2009/12/06)
(Graph Presented) Nanosecond transient absorption methods were used to determine accurate oxidation potentials (Eox) in acetonitrile for benzene and a number of its alkyl-substituted derivatives. Eox values were obtained from a combination of equilibrium electron-transfer measurements and electron-transfer kinetics of radical cations produced from pairs of benzene and biphenyl derivatives, with one member of the pair acting as a reference. Using a redox-ladder approach, thermodynamic oxidation potentials were determined for 21 benzene and biphenyl derivatives. Of particular interest, Eox values of 2.48 ± 0.03 and 2.26 ± 0.02 V vs SCE were obtained for benzene and toluene, respectively. Because of a significant increase in solvent stabilization of the radical cations with decreasing alkyl substitution, the difference between ionization and oxidation potentials of benzene is ～0.5 eV larger than that of hexamethylbenzene. Oxidation potentials of the biphenyl derivatives show an excellent correlation with substituent σ+ values, which allows Eox predictions for other biphenyl derivatives. Significant dimer radical cation formation was observed in several cases and equilibrium constants for dimerization were determined. Methodologies are described for determining accurate electrontransfer equilibrium constants even when dimer radical cations are formed. Additional equilibrium measurements in trifluoroacetic acid, methylene chloride, and ethyl acetate demonstrated that solvation differences can substantially alter and even reverse relative Eox values.
Absorption Spectra and Photochemical Rearrangements of Alkyl- and Dialkylbenzene Cations in Solid Argon
Kelsall, Benuel J.,Andrews, Lester
, p. 5893 - 5898 (2007/10/02)
Matrix photoionization of alkyl- and dialkylbenzenes produced and trapped the parent radical cations.Irradiation in the visible parent cation absorption induced α-H transfer to the cation ring to give substituted methylenecyclohexadiene cations.The ease of 1,3-hydrogen transfer in these experiments suggests that this may be an important rearrangement in gaseous alkylbenzene cations.Subsequent ultraviolet photolysis of these samples produced substituted styrene cations.
Carbon-Hydrogen Bond Dissociation Energies in Alkylbenzenes. Proton Affinities of the Radicals and the Absolute Proton Affinity Scale
Meot-Ner (Mautner), Michael
, p. 5 - 10 (2007/10/02)
Rate constants (k) were measured for proton-transfer reactions from alkylbenzene ions RH+ to a series of reference bases B, i.e., RH+ + B -> BH+ + R*.For exothermic reactions (ΔH -1.For example, the reaction C6H5CH3+ + B -> BH+ + C6H5CH2* is fast (reaction efficiency = k/kcol >/= 0.5) when B = MeO-t-Bu or stronger bases, but k/kcol is significantly smaller when B is n-Pr2O or weaker bases.From the falloff curve of reaction efficiency vs.PA(B), we find PA(n-Pr2O) = PA(C6H5CH2*) + 0.8 kcal mol-1 = 200.0 kcal mol-1.Since PA(C6H5CH2*) is obtained from known thermochemical data, this relation defines the absolute PA of n-Pr2O.Through a ladder of known PA, we then obtain PA(i-C4H8) = 186.8 kcal mol-1; we also obtain the absolute PAs of other oxygen bases.Falloff curves of reaction efficiencies of 3-FC6H4CH3+, C6H5C2H5+, C6H5-n-C3H7+, and C6H5-i-C3H7+ with these reference bases give then the following PAs of R* and R-H bond dissociation energies (Do) (all in kcal mol-1) as R*, PA(R*), Do(R-H): 3-FC6H4CH2*, 197.2, 89.4; , 197.9, 86.2; , 199.1, 86.1; , 199.6, 86.1.In similar manner, rate constants for H+ transfer from C6H5NH2+ to reference pyridines and amines yield PA(C6H5NH*) = 221.5 and Do(C6H5NH-H) = 85.1 kcal mol-1 (1 kcal mol-1 = 4.18 kJ mol-1).
