Bond cleavage
Bond cleavage, or scission, is the splitting of chemical bond
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...


If the two electrons in a cleaved covalent bond
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....

 are divided between the products, the process is known as homolytic fission and free redicals are generated by homolytic cleavage the process is known as homolytic fission or homolysis
In general it means breakdown to equal pieces There are separate meanings for the word in chemistry and biology.-Homolysis in chemistry:...

. 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 homolytic fission and (heterolysis
In chemistry, heterolysis or heterolytic fission is chemical bond cleavage of a neutral molecule generating a cation and an anion. In this process the two electrons that make up the bond are assigned to the same fragment...

). The term "bond dissociation energy
Bond dissociation energy
In chemistry, bond-dissociation energy or D0, is one measure of the bond strength in a chemical bond. It is defined as the standard enthalpy change when a bond is cleaved by homolysis, with reactants and products of the homolysis reaction at 0 K...

" 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
Catabolism is the set of metabolic pathways that break down molecules into smaller units and release energy. In catabolism, large molecules such as polysaccharides, lipids, nucleic acids and proteins are broken down into smaller units such as monosaccharides, fatty acids, nucleotides, and amino...

. Enzymes which catalyse bond cleavage are known as lyase
In biochemistry, a lyase is an enzyme that catalyzes the breaking of various chemical bonds by means other than hydrolysis and oxidation, often forming a new double bond or a new ring structure...

s, unless they operate by hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...

 or oxidation/reduction, in which case they are known as hydrolase
In biochemistry, a hydrolase is an enzyme that catalyzes the hydrolysis of a chemical bond. For example, an enzyme that catalyzed the following reaction is a hydrolase:-Nomenclature:...

s and oxidoreductase
In biochemistry, an oxidoreductase is an enzyme that catalyzes the transfer of electrons from one molecule to another...

s respectively.

Heterolytic cleavage

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.
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