Allyl radicals electron affinity

Another early acidity investigation of propene by the thermodynamic method involved the determination of the electron affinity of allyl radical by photodetachment from allyl anion34. Extrapolation of the data to a photodetachment threshold gave an electron affinity (EA) of allyl radical of 0.55 eV which, combined with a bond dissociation energy of allyl-H of 89 kcalmol-1, gave A//ac d = 390 kcalmol-1. 

The formation of organometallic intermediates, however, can be initiated by an allylic radical anion on the metal surface (Figure 4.3). Noteworthy is the high value of the electron affinity of allylic bromide and, to a lesser extent, chloride (Moyano et al, 1990). As a matter of fact, in tin-mediated reactions, Wurtz coupling products were sometimes detected (Kim et al, 1993), but, unlike the zinc-mediated allylation, no dimerization of the carbonyl compounds was observed in the absence of the halide (Einhom and Luche, 1987). 

The data show that cyclopropenyl cation is remarkably well stabilized, that the anion is somewhat destabilized, and that the radical is essentially unstabilized. This may also be seen in comparing the ionization energies and electron affinities of allyl radical and cyclopropenyl radical in Table 6. Here, the cyclopropenyl cation is 46 kcal/mol more easily formed than allyl cation, but the cyclopropenyl anion is 18 kcal/mol less easily formed than allyl anion. The two allyl ion have, of course, some stabilization energies, and correction for this would increase the stabilization for the cation and decrease the destabilization for the anion. Thus, all of the data are in good accord. 

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