[[Image:Allene.png|right|thumb|1,2-propadiene, the simplest allene, is also known as allene.]]

An '''allene''' is a [[hydrocarbon]] in which one atom of [[carbon]] is connected by [[covalent bond|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 [[alkene]]s.
For example, their reactivity with gaseous [[chlorine]] is more like the reactivity of [[alkyne]]s. Allenes can be chiral.

==Geometry==
[[Image:Allene3D.png|thumb|right|3D view of allene.]]
The central carbon of allene forms two [[sigma bond]]s and two [[pi bond]]s. The central carbon is [[Orbital hybridisation|sp-hybridised]], and the two terminal carbons are sp2-hybridised. The bond angle formed by the three carbons is 180 degrees, indicating linear geometry for the carbons of allene.

==Symmetry==
[[Image:Allene symmetry.png|none|left|300px]]
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 D_2d [[point group]].

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[[Image:Allene chirality.png|none|left|300px]]
An allene with only two substituents, '''A''' and '''B''' will be [[chirality (chemistry)|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.
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Allene has no dipole moments

==Synthesis==
Laboratory methods for the formation of allenes are:
* from geminal dihalocyclopropanes and organolithium compounds in the [[Skattebøl rearrangement]].
* from reaction of certain terminal [[alkyne]]s with [[formaldehyde]] [[copper(II) bromide]] and added base<ref>[[Organic Syntheses]], Coll. Vol. 7, p.276 (1990); Vol. 63, p.203 (1985). [http://www.orgsynth.org/orgsyn/pdfs/CV7P0276.pdf Link]</ref>
* from [[dehydrohalogenation]] of certain [[halide|dihalides]].<ref>[[Organic Syntheses]], Coll. Vol. 5, p.22 (1973); Vol. 42, p.12 (1962) [http://www.orgsynth.org/orgsyn/pdfs/CV5P0022.pdf Link]</ref>

==See also==
* Compounds with three or more adjacent carbon-carbon double bonds are called [[cumulene]]s.
* The allene motif is frequently encountered in [[carbomer]]s.

==External links==
*[[IUPAC]]s [[Gold Book]] definition 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]

==References==
<div class="references-small"><references/></div>

[[Category:Dienes]]
[[Category:Functional groups]]

[[ar:ألين]]
[[de:Propadien]]
[[es:Aleno]]
[[fr:Allène]]
[[it:Allene]]
[[ja:アレン (化学)]]
[[la:Propa-1,2-dienum]]
[[lv:Allēns]]
[[nl:Alleen (scheikunde)]]
[[pl:Alleny]]
[[sv:Allen]]