Unimolecular net heterolysis of symmetric and homopolar σ-bonds.

The unimolecular heterolysis of covalent σ-bonds is integral to many chemical transformations, including S1-, E1- and 1,2-migration reactions. To a first approximation, the unequal redistribution of electron density during bond heterolysis is governed by the difference in polarity of the two departing bonding partners. This means that if a σ-bond consists of two identical groups (that is, symmetric σ-bonds), its unimolecular fission from the S, S, or T states only occurs homolytically after thermal or photochemical activation.