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In chemistry, alpha elimination refers to particular types of
Both α- and β-eliminations proceed via agostic intermediates.
elimination reaction
An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one- or two-step mechanism. The one-step mechanism is known as the E2 reaction, and the two-step mechanism is known as the E1 r ...
s. The definition of alpha elimination differs for organometallic and organic chemistry.
Organic chemistry
In organic chemistry, alpha-elimination refers to reactions of this type: :R2CHX → R2C: + HX The reaction is employed to generate carbenes and nitrenes. The formation ofdichlorocarbene
Dichlorocarbene is the reactive intermediate with chemical formula CCl2. Although this chemical species has not been isolated, it is a common intermediate in organic chemistry, being generated from chloroform. This bent diamagnetic molecule rapidly ...
from chloroform
Chloroform, or trichloromethane, is an organic compound with formula C H Cl3 and a common organic solvent. It is a colorless, strong-smelling, dense liquid produced on a large scale as a precursor to PTFE. It is also a precursor to various re ...
is an example.
Alpha eliminations contrasts with beta eliminations, which are commonly used to generate alkene
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
s:
:R2CHCXR'2 → R2C=CR'2 + HX
Both alpha- and beta-eliminations typically require strong base.
Organometallic chemistry
In organometallic chemistry, alpha elimination refers to reactions of this type (otherspectator ligand In coordination chemistry, a spectator ligand is a ligand that does not participate in chemical reactions of the complex. Instead, spectator ligands (vs "actor ligands") occupy coordination sites. Spectator ligands tend to be of polydentate, such th ...
s omitted):
:X-M-CH2R → M=CHR + HX
Well studied case are found in organotantalum chemistrys leading to an alkylidene derivatives. Specifically, tetraalkyl-monochloro-tantalum complex undergoes α-hydrogen elimination, followed by alkylation of the remaining chloride to give a derivative with a Ta=C bond.
Alpha elimination contrasts with β-hydride elimination, whereby an alkyl
In organic chemistry, an alkyl group is an alkane missing one hydrogen.
The term ''alkyl'' is intentionally unspecific to include many possible substitutions.
An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloal ...
group bonded to a metal centre is converted into the corresponding metal-bonded hydride
In chemistry, a hydride is formally the anion of hydrogen( H−). The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride of ...
and an alkene
In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond.
Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...
.
Both α- and β-eliminations proceed via agostic intermediates.
See also
*Elimination reaction
An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one- or two-step mechanism. The one-step mechanism is known as the E2 reaction, and the two-step mechanism is known as the E1 r ...
, mainly focused on organic substrates.
References