Bond cleavage

Bond cleavage, or scission, is the splitting of chemical bonds.

If the two electrons in a cleaved covalent bond are divided between the products, the process is known as homolytic fission or homolysis and free redicals are generated by homolytic cleavage. Alternatively, the case where both electrons are retained by one product and charged species that is nucleophile and electrophile are generated by the process is known as heterolytic fission and (heterolysis). The term "bond dissociation energy" refers to the energy required to cleave a bond (typically homolytically).

In biochemistry the process of breaking down large molecules (by splitting their internal bonds) is catabolism. Enzymes which catalyse bond cleavage are known as lyases, unless they operate by hydrolysis or oxidation/reduction, in which case they are known as hydrolases and oxidoreductases respectively.

Heterolytic cleavage

Main article: Heterolysis

In heterolytic cleavage, or heterolysis, the bond breaks in such a fashion that the shared pair of electrons remain with one of the fragments. After heterolysis, one atom has a sextext electronic structure and a positive charge and other, a valence octet with at least one lone pair and a negative charge.