Radical Substitution Reactions at the Saturated C Atom

Abstract

In a substitution reaction, a part X of a molecule R–X is replaced by a group Y. This chapter focuses on substitution reactions in which a part X that is bound to a sp3-hybridized C atom is replaced by a group Y via radical intermediates. At the so-called radical center, an organic radical R has an electron septet, which is an electron deficiency in comparison to the electron octet of valence-saturated compounds. Carbon atoms are the most frequently found radical centers. Nitrogen-centered radicals or oxygen-centered radicals are less stable than C-centered radicals. Nitrogen- or oxygen centered radicals of the cited substitution pattern consequently have only a limited chance to exist. The preferred geometries of carbenium ions and carbanions are correctly predicted by the valence shell electron pair repulsion (VSEPR) theory. The VSEPR theory analyzes the stereo structure of these compounds in the environment of the central atom. This stereo structure depends on the number of atoms or atom groups linked to the central atom and the number of nonbonding valence electron pairs localized at the central atom. Stability in chemistry is not an absolute, but a relative concept. Evaluation of the relative rates of formation of these radicals is done by the Bell–Evans–Polanyi principle or the Hammond postulate. All radical substitution reactions are chain reactions. There is a great diversity of starting reaction(s) and propagation steps for radical substitution reactions that include bond homolyses, fragmentations, atom abstraction reactions, and addition reactions to C=C double bonds.