In homolytic cleavage, or homolysis, the two electrons in a cleaved covalent bond are divided equally between the products. This process is also known as homolytic fission or radical fission. The bond-dissociation energy of a bond is the amount of energy required to cleave the bond homolytically. This enthalpy change is one measure of bond strength.

The triplet excitation energy of a sigma bond is the energy required for homolytic dissociation, but the actual excitation energy may be higher than the bond dissociation energy due to the repulsion between electrons in the triplet state.



•In heterolytic cleavage, or heterolysis, the bond breaks in such a fashion that the originally-shared pair of electrons remain with one of the fragments. Thus, a fragment gains an electron, having both bonding electrons, while the other fragment loses an electron.
 This process is also known as ionic fission.

The singlet excitation energy of a sigma bond is the energy required for heterolytic dissociation, but the actual singlet excitation energy may be lower than the bond dissociation energy of heterolysis as a result of the Coulombic attraction between the two ion fragments.

The singlet excitation energy of a silicon–silicon sigma bond is lower than the carbon–carbon sigma bond, even though their bond strengths are 80kJ/mol and 70kJ/mol respectively, because silicon has higher electron affinity and lower ionization potential than carbon.

Heterolytic cleavage and homolytic cleavage are two different types of bond breaking processes in chemical reactions. Let's understand each of them in detail.

Heterolytic Cleavage:
Heterolytic cleavage, also known as heterolysis, refers to the breaking of a covalent bond in a molecule or ion where the shared pair of electrons is unequally distributed between the two atoms involved. In this process, one atom retains both electrons from the bond, while the other atom becomes positively or negatively charged, resulting in the formation of ions. Heterolytic cleavage most commonly occurs in polar covalent bonds.

Homolytic Cleavage:
Homolytic cleavage, also known as homolysis, refers to the breaking of a covalent bond in a molecule or ion where the shared pair of electrons is equally distributed between the two atoms involved. In this process, each atom retains one electron from the bond, resulting in the formation of highly reactive species known as radicals. Homolytic cleavage primarily occurs in nonpolar covalent bonds.

Difference between Heterolytic and Homolytic Cleavage:
- The key difference between heterolytic and homolytic cleavage lies in the distribution of electrons during bond breaking. In heterolytic cleavage, the electrons are unequally distributed, leading to the formation of ions, whereas in homolytic cleavage, the electrons are equally distributed, resulting in the formation of radicals.
- Heterolytic cleavage is more common in polar covalent bonds, whereas homolytic cleavage is more common in nonpolar covalent bonds.
- Heterolytic cleavage results in the formation of charged species (ions) that are more stable and less reactive compared to the radicals formed in homolytic cleavage, which are highly reactive due to the unpaired electron.
- The reaction mechanisms and products differ for these cleavage processes. Heterolytic cleavage often involves ion-pair reactions, while homolytic cleavage leads to radical reactions.

Applications:
- Heterolytic cleavage is commonly observed in reactions involving acids and bases, where the acid donates a proton (H+) and the base accepts it, resulting in the formation of ions.
- Homolytic cleavage plays a significant role in radical reactions, such as the initiation step in free radical polymerization and some organic reactions involving radicals as intermediates.

In summary, heterolytic cleavage involves the uneven distribution of electrons, resulting in the formation of ions, while homolytic cleavage involves the equal distribution of electrons, leading to the formation of highly reactive radicals. These processes have different implications in various chemical reactions and play a crucial role in understanding reaction mechanisms.