A covalent bond is formed when two atoms share a pair of electrons. When this bond breaks, the process is called bond cleavage. The key point is not simply that the bond is broken but how the shared electrons are divided between the two atoms. Based on this, bond cleavage is classified into two types: homolytic cleavage and heterolytic cleavage. ^[1–4]

Homolytic Cleavage

Homolytic cleavage, or homolysis, is the breaking of a covalent bond in such a way that each bonded atom takes one electron from the shared pair. As a result, free radicals are formed. These radicals are generally neutral species, but because they contain an unpaired electron, they are usually highly reactive. ^[1–3]

Homolytic cleavage is commonly associated with conditions such as heat, light, or radical initiators, and it is often discussed in the case of relatively nonpolar or weak bonds. However, it should not be treated as an absolute rule in every situation. A common misconception is that neutral species are not very reactive. In reality, free radicals are often highly reactive because of their unpaired electron.

Examples

1. When chlorine gas is exposed to UV light, the Cl–Cl bond absorbs energy and breaks evenly. Each chlorine atom takes one electron from the shared pair, forming two chlorine radicals.
2. The Br–Br bond can also break equally when sufficient energy is supplied by heat or light. One electron goes to each bromine atom, producing two bromine radicals.

Heterolytic Cleavage

Heterolytic cleavage, or heterolysis, is the breaking of a covalent bond in such a way that one atom takes both electrons from the shared pair. This unequal splitting produces ions, usually a positively charged ion and a negatively charged ion. The atom that retains both bonding electrons becomes negatively charged, while the other becomes positively charged. ^[1–3]

Heterolytic cleavage is more common in polar bonds, especially when one atom attracts electrons more strongly than the other. It is also favored when the resulting ions can be stabilized, such as in polar solvents or when a good leaving group is present.

Examples

1. Carbon-halogen bonds commonly undergo heterolytic cleavage. In tert-butyl bromide, the C–Br bond breaks, and both bonding electrons go to bromine. As a result, a bromide ion (Br^–) and a tert-butyl carbocation ((CH₃)₃C⁺) are formed.
2. When H–Cl breaks in water, both bonding electrons move toward chlorine because chlorine is more electronegative. In a simplified representation, this reaction gives Cl^– and H⁺. However, in reality, the proton does not exist freely. It is transferred to a water molecule to form the hydronium ion (H₃O⁺).

Bond cleavage is fundamental to chemistry because it determines how reactions begin and what reactive species are formed. Whether the bond breaks homolytically or heterolytically influences the reaction pathway and overall reactivity.