Enzymes which catalyse bond cleavage are known as lyases, unless they operate by hydrolysis or oxidoreduction, in which case they are known as hydrolases and oxidoreductases respectively. In biochemistry, the process of breaking down large molecules by splitting their internal bonds is catabolism. For example, an epoxide ring can be opened by heterolytic cleavage of one of the polar carbon–oxygen bonds to give a single acyclic structure. The bond breaks, but the two fragments remain attached by other parts of the structure. In a ring-opening, the cleaved molecule remains as a single unit. Heterolysis occurs naturally in reactions that involve electron donor ligands and transition metals which have empty orbitals. 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. 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. This process is also known as ionic fission. Thus, a fragment gains an electron, having both bonding electrons, while the other fragment loses an electron. 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. Main article: Heterolysis (chemistry) Heterolytic cleavage Due to the high bond-dissociation energy of C−H bonds, around 100 kcal/mol (420 kJ/mol), a large amount of energy is required to cleave the hydrogen atom from the carbon and bond a different atom to the carbon. In some cases, bond cleavage requires catalysts. A metal−metal sigma bond is an exception because the bond's excitation energy is extremely high, thus cannot be used for observation purposes. 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. In general, there are two classifications for bond cleavage: homolytic and heterolytic, depending on the nature of the process. This can be generally referred to as dissociation when a molecule is cleaved into two or more fragments. In chemistry, bond cleavage, or bond fission, is the splitting of chemical bonds. JSTOR ( February 2018) ( Learn how and when to remove this template message).Unsourced material may be challenged and removed. Please help improve this article by adding citations to reliable sources. This article needs additional citations for verification.
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