Bond cleavage

Source: Wikipedia, the free encyclopedia.

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.[1]

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.[2] A metal−metal sigma bond is an exception because the bond's excitation energy is extremely high, thus cannot be used for observation purposes.[2]

In some cases, bond cleavage requires

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.[3]

Homolytic cleavage

Main article: Homolysis (chemistry)
Homolytic cleavage

In homolytic cleavage, or

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.[2]

Heterolytic cleavage

Main article: Heterolysis (chemistry)
Heterolytic cleavage

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.[4] 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

ionization potential than carbon.[2]

Heterolysis occurs naturally in reactions that involve

ligands and transition metals which have empty orbitals.[4]

Ring-opening

Epoxide opening

In a ring-opening, the cleaved molecule remains as a single unit.[5] The bond breaks, but the two fragments remain attached by other parts of the structure. 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.[5]

Applications

In

oxidoreduction, in which case they are known as hydrolases and oxidoreductases
respectively.

In proteomics, cleaving agents are used in proteome analysis, where proteins are cleaved into smaller peptide fragments.[6] Examples of cleaving agents used are cyanogen bromide, pepsin, and trypsin.[6]

References

  1. S2CID 195819485
    .
  2. ^
    doi:10.1021/ar00173a001
    .
  3. PMID 29168816
    .
  4. ^
    S2CID 95234750. Archived from the original
    (PDF) on August 11, 2017 – via Jack Simons 's Home Page - The University of Utah.
  5. ^
    doi:10.1021/cr50028a006
    .
  6. ^
    ISBN 978-0-08-045381-1
    . Retrieved 23 February 2018.
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