Aromatic stabilization energy radical

Loss of the first H atom from PH2 gives a free radical PH which is strongly stabilized due to the formation of one fully aromatic unit. The effective 4 —H bond dissociation energy, D(PH—H) assumes a very low value of ca. 47 Kcal/mole (or below) just because of this aromatic stabilization This value of D(PH-H) should be compared with the usual C—H bond dissociation energy, e.g., D((CH3)3C—H) =... 

The authors also considered the relative influence of para substitution in the phenylethynyl compared to simply phenyl (i.e., compared to the analogous styrenes). They found that over the four substituents noted above, the stabilization energy from Eq. (6.14) varied by 5.2 kJ mol for phenylethynyl and 7.0 kJ mol for phenyl. Thus, insertion of the acetylene unit between the radical center and the aromatic ring is predicted to decrease the influence of the aryl substituent by only about 25 percent. 

Brocks JJ, Beckhaus H-D, Beckwith ALJ, Ruchardt C (1998) Estimation of bond dissociation energies and radical stabilization energies by ESR spectroscopy. J Org Chem 63 1935-1943 Buxton GV, Langan JR, Lindsay Smith JR (1986) Aromatic hydroxylation. 8. A radiation chemical study of the oxidation of hydroxycyclohexadienyl radicals. J Phys Chem 90 6309-6313 Buxton GV, GreenstockCL, Helman WP, Ross AB (1988) Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals ( OH/O -) in aqueous solution. J Phys Chem Ref Data 17 513-886... 

Of this group only benzyl chloride is not an aryl halide its halogen is not attached to the aromatic ring but to an. v/r -hybridized carbon. Benzyl chloride has the weakest carbon-halogen bond, its measured carbon-chlorine bond dissociation energy being only 293 kJ/mol (70 kcal/mol). Homolytic cleavage of this bond produces a resonance-stabilized benzyl radical. 

The bond dissociation energy (BDE) of the S-H bond of aryl thiols is approximately 75 kcal/mole (14), which would result in the formation of a resonance-stabilized phenylthio radical. The BDE of the aromatic C-S bond is approximately 85 kcal/mole (14), and cleavage of thiophenol at the C-S bond would result in the formation of an unstable and reactive phenyl radical. Very little decomposition of neat thiophenol occurred at a reaction temperature of 375°C for 30 minutes, confirming a report in the... 

K lO". The second correction factor is due to the reorganization energy A of the solvate shells of the redox ions A is in the order of 0.5-1 eV. This factor then becomes to 10". Thus fchet drops in reality to 10-0.1. Most of the measured rate constants for aromatic systems (molecule/radical ion) are of this order of magnitude [55]. The molecular interpretation of such high rates is a stabilization of the radical ion in an (extended) aromatic system. 

We have also discussed the use of the electrostatic potential for the analysis of substituent effects in aromatic systems. Substituent effects on gas phase and solution acidities of benzoic acids and phenols are dominantly determined by the relative stabilization of the negative charge in the ionized forms of these systems. The oxygen Vmin is an excellent tool for the analysis of this stabilization effect. On the other hand, we have found that the homol5dic O-H bond dissociation energy in phenols depends both on the substituent s ability to stabilize the parent molecule (the phenol) and the radical. The relative stabilization energies of the parent molecule and the radical can be estimated from their computed Vmin and surface maxima in the spin density, respectively. 

Ferry, Atkinson, and Fitts (264, 265) also derived heats of formation of the OH-aromatlc adducts from the temperatures at which nonexponential behavior was observed. This led to an estimate of the resonance stabilization energy of these substituted cyclohexadlenyl radicals of 16.5 + 5 kcal mole for the aromatic hydrocarbons (264), and 19+5 kcal mole for methyoxybenzene and -cresol (265). 

The involvement of charged species in aromatic-aromatic interactions increases the stabilization energy. For the benzene pair radical cation. experimental... 

In aprotic nonaqueous media, the organic electrochemistry of anodic and cathodic reactions is concerned predominantly with radical-ion chemistry in many cases involving aromatic substances, the radicals are of sufficient stability for them to be characterized spectroscopically by conventional absorption spectrophotometry and by esr spectroscopy. Linear relations are found between the cathodic and anodic half-wave potentials and the ionization potentials or electron affinities determined in the gas phase. The oxidation and reduction potentials can also be related to the theoretically calculated energies of the highest occupied (anodic process) or lowest vacant (cathodic process) molecular orbitals. 

Radical-solvent complexes are more difficult to detect spectroscopically however, they do provide a plausible explanation for many of the solvent effects observed in free-radical homopolymerization—particularly those involving unstable radical intermediates (such as vinyl acetate) where complexation can lead to stabilization. For instance, Kamachi (50) observed that the homopropagation rate of vinyl acetate in a variety of aromatic solvents was correlated with the calculated delocalization stabilization energy for complexes between the radical and solvent. If such solvent effects are detected in the homopolymerization of one or both of the comonomers, then they are likely to be present in the copolymerization systems as well. Indeed, radical-complex models have been invoked to explain solvent effects in the copolymerization of vinyl acetate with acrylic acid (51). Radical-solvent complexes are probably not restricted merely to systems with highly unstable propagating radicals. In fact, radical-solvent complexes have even been proposed to explain the effects of some solvents (such as benzyl alcohol, A7 / 7 -dimethyl for-mamide, and acetonitrile) on the homo- and/or copolymerizations of styrene and methyl methacrylate (52-54). Certainly, radical-solvent complexes should be considered in systems where there is a demonstrable solvent effect in the copolymerizations and/or in the respective homopolymerizations. 

In systems containing numerous stabilizing groups, the situation may be intermediate between those observed in aromatic and aliphatic radical anions. Here the radical anion may or may not dimerize. Since dimerization removes from the conjugated system the atoms through which combination takes place, the ease of the reaction should be greater, the lower are the localization energies at those positions. 

Calculations using the complete basis set ab initio method for the cyclopropenyl radical give an ionization energy of 6.17 eV, in good agreement with an experimental energy of 6.60 eV, and an electron affinity of 0.45 eV. The very low value of the former is indicative of the large aromatic stabilization of the cation, and the low value of the latter indicates the instability of the cyclopropenyl anion. The radical is intermediate between the two, but these results do not permit an estimate of any antiaromatic destabilization of the radical. 

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