Heterolytically

Heterolytically is an adverb describing a mode of bond cleavage in chemistry in which the two electrons of a bond are not shared equally. Instead, both electrons are transferred to one of the fragments, generating a charged species such as a cation and an anion. This is contrasted with homolytic cleavage, in which each fragment retains one electron and neutral radicals are formed.

In organic reaction mechanisms, heterolytic bond breaking is common when a leaving group departs in a polar solvent, producing a carbocation or other electrophilic center along with a counterion. Classic examples include SN1 and E1 pathways, where cleavage of a carbon–leaving-group bond yields a carbocation and a leaving-group anion. Proton transfers and acid-base steps often proceed via heterolysis as well, such as the formation of H+ (often solvated as H3O+) and the conjugate base.

Factors that favor heterolysis include highly polar solvents, stabilization of ions by solvation or resonance, and the presence of a strong leaving group. The process is often accompanied by significant ionic character in the transition state. In separate contexts, heterolytic cleavage can be part of catalytic cycles and ion-pair formation.

Note on use: the term heterolytically is used to describe the manner of bond cleavage rather than a separate reaction type. The opposite process is homolytically, where a bond breaks to form two radicals.