heterolys

Heterolysis is a type of chemical bond cleavage in which the two electrons of a covalent bond are localized on one of the two fragments, producing a cation and an anion. This contrasts with homolysis, where each fragment takes one electron and radicals are formed. Heterolytic bond cleavage is common in polar solvents and in reactions that proceed through ionic intermediates such as carbocations or carbanions.

In mechanism terms, heterolysis is often the rate-determining step in reactions that generate charged species. For example, in SN1 and E1 reactions, the bond between a carbon atom and a leaving group cleaves heterolytically to form a carbocation and a leaving-group anion. The stability of the resulting ions and the solvent’s ability to stabilize charges strongly influence the likelihood and rate of heterolytic cleavage.

Typical examples include the acid-catalyzed hydrolysis of alkyl halides, where the carbon–halogen bond undergoes heterolysis to give a carbocation (or a tightly solvated ion pair) and a halide ion, with subsequent nucleophilic attack or rearrangement leading to products. In organometallic and inorganic chemistry, heterolysis can describe bond cleavage that yields a cationic metal center paired with an anionic ligand, or vice versa, depending on the system and solvents.

Factors that favor heterolysis include high bond polarity, a good leaving group, and solvents that stabilize ions (especially polar protic solvents). Temperature, solvent polarity, and the presence of catalysts or strong acids can also modulate the extent and rate of heterolytic cleavage. Heterolysis is a fundamental concept for understanding ionic reaction mechanisms and the formation of charged intermediates.