Radicals are stabilized by conjugating, electron-withdrawing, and electron-donating groups

Let’s consider first what happens when a radical centre finds itself next to an electron-with drawing group. Groups like C=O and C≡N are electron withdrawing because they have a low-lying empty π* orbital. By overlapping with the (usually-p) orbital containing the radical (the SOMO), two new molecular orbitals are generated. One electron (the one in the old SOMO) is available to fill the two new orbitals. It enters the new SOMO, which is of lower energy than the old one, and the radical experiences stabilization because this electron drops in energy

We can analyse what happens with electron-rich groups, such as RO groups, in a similar way. Ether oxygen atoms have relatively high-energy filled n orbitals, their lone pairs. Interacting this with the SOMO again gives two new molecular orbitals. Three electrons are available to fill them. The SOMO is now higher in energy than it was to start with, but the lone pair is lower. Because two electrons have dropped in energy and only one has risen, there is an overall stabilization of the system, even though the new SOMO is of higher energy than the old one. We shall see later what effect the energy of the SOMO, rather than the overall energy of the radical, has on its reactivity.

they stabilize radicals in the same way, which is why tertiary radicals are more stable than primary ones. Conjugation, too, is effective at stabilizing radicals. We know from their ESR spectra that radicals next to double bonds are delocalized; that they are more stable is evident from the bond dissociation energies of allylic and benzylic C–H bonds.

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مواضيع ذات صلة

The Hammett relationship

Systematic structural variation

Crossover experiments

Labelling experiments reveal the fate of individual atoms

Be sure of the structure of the product

Variation in the structure of the aldehyde

formation of an ester intermediate

formation of an intermediate dimeric adduct

Be sure of the structure of the product

Variation in the structure of the aldehyde

Proposed mechanism C: formation of an ester intermediate

Proposed mechanism B: formation of an intermediate dimeric adduct