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Alpha-cleavage (α-cleavage) in
In molecules containing carbonyl groups, alpha-cleavage often competes with
organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J.; ...
refers to the act of breaking the carbon-carbon bond adjacent to the carbon bearing a specified functional group
In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the rest ...
.
Mass spectrometry
Generally this topic is discussed when coveringtandem mass spectrometry
Tandem mass spectrometry, also known as MS/MS or MS2, is a technique in instrumental analysis where two or more mass analyzers are coupled together using an additional reaction step to increase their abilities to analyse chemical samples. A comm ...
fragmentation and occurs generally by the same mechanisms.
For example, of a mechanism of alpha-cleavage, an electron is knocked off an atom (usually by electron collision) to form a radical cation. Electron removal generally happens in the following order: 1) lone pair
In chemistry, a lone pair refers to a pair of valence electrons that are not shared with another atom in a covalent bondIUPAC ''Gold Book'' definition''lone (electron) pair''/ref> and is sometimes called an unshared pair or non-bonding pair. Lone ...
electrons, 2) pi bond
In chemistry, pi bonds (π bonds) are covalent chemical bonds, in each of which two lobes of an orbital on one atom overlap with two lobes of an orbital on another atom, and in which this overlap occurs laterally. Each of these atomic orbitals ...
electrons, 3) sigma bond
In chemistry, sigma bonds (σ bonds) are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals. Sigma bonding is most simply defined for diatomic molecules using the language and tools of s ...
electrons.
One of the lone pair electrons moves down to form a pi bond with an electron from an adjacent (alpha) bond. The other electron from the bond moves to an adjacent atom (not one adjacent to the lone pair atom) creating a radical. This creates a double bond adjacent to the lone pair atom (oxygen is a good example) and breaks/cleaves the bond from which the two electrons were removed.
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In molecules containing carbonyl groups, alpha-cleavage often competes with McLafferty rearrangement The McLafferty rearrangement is a reaction observed in mass spectrometry during the fragmentation or dissociation of organic molecules. It is sometimes found that a molecule containing a keto-group undergoes β-cleavage, with the gain of the γ-hyd ...
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Photochemistry
Inphotochemistry
Photochemistry is the branch of chemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption of ultraviolet (wavelength from 100 to 400 nm), visible light (400–7 ...
, it is the homolytic cleavage of a bond adjacent to a specified group.
See also
*Inductive cleavage Inductive cleavage, in organic chemistry, is the charge-initiated counterpoint to radical initiated alpha-cleavage. Since inductive cleavage does not require unpairing and re-pairing electrons it can occur at both radical cationic and cationic site ...
References
{{Reflist Organic reactions Tandem mass spectrometry