An allene is a
hydrocarbonin which one atom of carbonis connected by double bonds with two other atoms of carbon. Allene also is the common name for the parent compound of this series, 1,2-propadiene.
Such pair of bonds make allenes much more reactive than other
alkenes.For example, their reactivity with gaseous chlorineis more like the reactivity of alkynes. Allenes can be chiral.
The central carbon of allene forms two
sigma bonds and two pi bonds. The central carbon is sp-hybridised, and the two terminal carbons are sp2-hybridised. The bond angle formed by the three carbons is 180°, indicating linear geometry for the carbons of allene.
For allenes with four identical substituents, the compound will have a twofold axis of rotation through the center carbon. The axis is inclined at 45° to the CH2 planes on either side of the molecule. The molecule can thus be thought of as a two-bladed
propeller. Another twofold axis of rotation passes through the C=C=C bonds, and there is a mirror plane passing through both CH2 planes. Thus this class of molecules will have a D2d point group.
An allene with only two substituents, A and B will be chiral because there will no longer be any mirror planes. Where A has a greater priority than B according to the
Cahn-Ingold-Prelog priority rule, the configuration can be determined by considering the top, then the bottom. For the bottom, only the group of higher priority need be considered.
Because of the symmetry, an unsubstituted allene has no
Laboratory methods for the formation of allenes are:
* from geminal dihalocyclopropanes and organolithium compounds in the
* from reaction of certain terminal
alkynes with formaldehyde copper(II) bromideand added base [ Organic Syntheses, Coll. Vol. 7, p.276 (1990); Vol. 63, p.203 (1985). [http://www.orgsynth.org/orgsyn/pdfs/CV7P0276.pdf Link] ]
dehydrohalogenationof certain dihalides. [ Organic Syntheses, Coll. Vol. 5, p.22 (1973); Vol. 42, p.12 (1962) [http://www.orgsynth.org/orgsyn/pdfs/CV5P0022.pdf Link] ]
* Compounds with three or more adjacent carbon-carbon double bonds are called
* The allene motif is frequently encountered in
* Using a suitable catalyst (e.g.
Wilkinson's catalyst), it is possible to reduce just one of the double bonds of an allene. [Bhagwat; Devaprabhakara, Tetrahedron Lett. p. 1391 (1972) [http://dx.doi.org/10.1016/S0040-4039(01)84636-0 Link] ]
IUPACs Gold Bookdefinition of allenes [http://www.iupac.org/goldbook/A00238.pdf ]
* [http://www.sparknotes.com/chemistry/organic3/stereochemistry/section2.rhtml Stereochemistry study guide]
* [http://www.organic-chemistry.org/synthesis/C1C/chains/allenes.shtm Synthesis of allenes]
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