Homolysis (chemistry) in the context of Bond-dissociation energy

The bond dissociation energy (BDE, D₀, or DH°) is one measure of the strength of a chemical bond A−B. It can be defined as the standard enthalpy change when A−B is cleaved by homolysis to give fragments A and B, which are usually radical species. The enthalpy change is temperature-dependent, and the bond dissociation energy is often defined to be the enthalpy change of the homolysis at 0 K (absolute zero), although the enthalpy change at 298 K (standard conditions) is also a frequently encountered parameter.

As a typical example, the bond dissociation energy for one of the C−H bonds in ethane (C₂H₆) is defined as the standard enthalpy change of the process

In chemistry, bond cleavage, or bond fission, is the splitting of chemical bonds. This can be generally referred to as dissociation when a molecule is cleaved into two or more fragments.

In general, there are two classifications for bond cleavage: homolytic and heterolytic, depending on the nature of the process. The triplet and singlet excitation energies of a sigma bond can be used to determine if a bond will follow the homolytic or heterolytic pathway. A metal−metal sigma bond is an exception because the bond's excitation energy is extremely high, thus cannot be used for observation purposes.