Correlations of various indices of the stability and reactivity of carbon-centered radicals with ESR hyperfine splitting constants have been examined. For a large number of mono-and disubstituted radicals there is a moderately good linear correlation of a-proton hyperfine splitting constants (a(Ha)) with radical stabilization enthalpies (RSE) and with BDE(C-H), the C-H bond dissociation energies for the corresponding parent compounds determined from thermodynamic and kinetic studies of C-C homolysis reactions. There is a similarly satisfactory linear correlation of a(H a) with BDE-(C-H) determined by Bordwell's electrochemical and acidity function method. In all cases the correlations fail for nonplanar radicals. As expected, j3-proton hyperfine splitting constants (a(H,Me)) for radicals with a freely rotating methyl substituent are less sensitive to deviations from planarity and give better linear correlations with RSE and BDE(C-H). The correlations cover a range of more than 20 kcal/mol and are reliable predictors of RSE and BDE(C-H) for a variety of radicals including captodative species. However, the correlations fail for significantly nonplanar radicals and for radicals with cyclic delocalized systems, e.g., cyclopentadienyl. The ratio a(HfiMe)/ a(Ha) for suitably substituted radicals provides an index of pyramidalization and allows one to decide for which compounds values of RSE and BDE(C-H) can be confidently estimated.

Uncatalyzed transfer hydrogenations are H2‐transfer reactions in which donors that contain weakly bound hydrogen atoms undergo an H‐atom transfer (retrodisproportionation) onto an acceptor containing unsaturated bonds such as CC, CO, CN, NN, NO. Transfer hydrogenolyses are reactions in which σ bonds are cleaved upon additon of H2. These hydrogenations are terminated by H, transfer; they do not follow a radical‐chain mechanism. The initial steps of both types of reactions, H‐atom transfer, complement the bimolecular formation of 1,4‐diradicals from alkenes or heteroalkenes within the scope of bimolecular radical formation (Molecule‐Induced Radical Formation, MIRF). The title reactions play an important role in coal liquefaction, aromatization reactions with nitroarenes or quinones, and possibly biochemical dehydrogenations. This review focuses on mechanistic studies, structure–reactivity relationships, and current applications of these reactions.

An efficient and simple method for the synthesis of two uniform buckminsterfullerene hydrogenation products, C60H18 and C60H36, is offered by the title reaction. The picture on the right shows a probable structure of C60H36 obtained from MM2 calculations. These results are an important step on the way to preparative organic chemistry with C60.

The enthalpy of formation ( = 57.51 ± 0.70 kcal/mol) of triquinacene (1), newly determined by measuring its energy of combustion in a microcalorimeter, is about 4 kcal/mol higher than that previously reported and corresponds to ab initio and density functional theory computational results. As a consequence, the previously derived homoaromatic stabilization energy (claimed to be 4.5 kcal/mol) from enthalpy of hydrogenation measurements is not present in 1. The lack of homoaromaticity in 1 is supported by evaluation of geometric, energetic, and magnetic criteria. In contrast, the isomerization transition state from diademane (5) to 1 is highly aromatic on the basis of the same criteria. The enthalpy of isomerization of 5 to 1 was experimentally determined by differential scanning calorimetry (DSC) to be −29.4 ± 0.3 kcal/mol (measured at 368.2 K). The enthalpy of activation for this rearrangement as determined from the DSC measurements (28.4 ± 0.2 kcal/mol) is 2.5 kcal/mol higher than the value computed at B3LYP/6-311+G**+ZPE.

An analysis of the relations between structure and reactivity in free-radical chemistry has shown that the usual interpretation of reactivities by means of the stability of the radicals involved is greatly simplified and often incorrect. The C-X bond energies of the alkanes and simple alkyl derivatives can be explained qualitatively by strain efects in the ground state on the basis of the VSEPR theory and nonbonding interactions. To be able to explain reactivities in free-radical chemistry, it is necessary to deduce injormation about the geometry of the transition states during free-radical formation from experimental measurements. The relations between structure and reactivity in freeradical chemistry are interpreted in terms of bond dissociation energies, as well as polar and sieric effects.

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