20.1: Radicals: Electronic Structure and Geometry
This lesson delves into the geometry of a radical, which is influenced by the electronic structure of the molecule. The principle is similar to that of a lone pair, where the unpaired electron influences the geometry at the radical center.
Accordingly, the structure of a trivalent radical lies between the geometries of carbocations and carbanions. An sp2-hybridized carbocation is trigonal planar, while an sp3-hybridized carbanion is trigonal pyramidal. Here, the difference in geometry is attributed to the difference in the number of nonbonding electrons. While the former has nil, the latter has two nonbonding electrons. Hence, a carbon radical with one nonbonding electron present in the p orbital falls between these two cases.
It is therefore reasonable to expect the radical geometry of a trivalent carbon species to lie between trigonal planar and trigonal pyramidal. As observed experimentally, the trivalent carbon-centered radicals typically possess superficially pyramidal geometry but are nearly planar. For instance, this is observed in oxygenated radicals like •CH2OH and •CMe2OH. Certain carbon-centered radicals, such as •CF3, are closer in geometry to the sp3-hybridized carbanions, which are trigonal pyramidal. In contrast, the methyl radical is fully trigonal planar like an sp2-hybridized carbocation.
The chirality of radicals is also a good indicator of the geometry. While the carbanion could be chiral given its resistance to pyramidal inversion, radicals can be achiral, since carbon-centered radicals with alkyl substituents that are superficially pyramidal readily undergo pyramidal inversion to become nearly planar.
