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Propadiene, the simplest allene, is also known as allene.

An allene is a compound in which one carbon atom has double bonds with each of its two adjacent carbon centres. Allenes are classified as polyenes with cumulated dienes. The parent compound of allene is propadiene. Compounds with an allene-type structure but with more than three carbon atoms are called cumulenes. Allenes are much more reactive than most other alkenes. For example, their reactivity with gaseous chlorine is more like the reactivity of alkynes than that of alkenes.

Structure and bonding

Geometry

3D view of propadiene (allene).

The central carbon of allene forms two sigma bonds and two pi bonds. The central carbon is sp-hybridized, and the two terminal carbons are sp²-hybridized. The bond angle formed by the three carbons is 180°, indicating linear geometry for the carbons of allene. It can also be viewed as an "extended tetrahedral" with a similar shape to methane.

Symmetry

The symmetry and isomerism of allenes has long fascinated organic chemists.^[1] For allenes with four identical substituents, there exist two twofold axes of rotation through the center carbon, inclined at 45° to the CH₂ planes at either end of the molecule. The molecule can thus be thought of as a two-bladed propeller. A third 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 belong to the D_2d point group. Because of the symmetry, an unsubstituted allene has no net dipole moment.

R and S configurations are determined by precedences of the groups attached to the axial section of the molecule when viewed along that axis. The front plane is given higher priority over the other and the final assignment is given from priority 2 to 3 (i.e. the relationship between the two planes).

An allene with two different substituents on each of the two carbons 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 of the axial chirality can be determined by considering the substituents on the front atom followed by the back atom when viewed along the allene axis. For the bottom, only the group of higher priority need be considered. Chiral allenes have been recently used as building blocks in the construction of organic materials with exceptional chiroptical properties.^[2]

Synthesis

Although allenes often require specialized syntheses, the parent, propadiene is produced on a large scale as an equilibrium mixture with methylacetylene:

H₂C=C=CH₂ CH₃C≡CH

This mixture, known as MAPP gas, is commercially available.

Laboratory methods for the formation of allenes include:

from geminal dihalocyclopropanes and organolithium compounds in the Skattebøl rearrangement.
from reaction of certain terminal alkynes with formaldehyde, copper(I) bromide and added base^[3]
from dehydrohalogenation of certain dihalides.^[4]
from reaction of a triphenylphosphinyl ester with an acid halide, a Wittig reaction accompanied by dehydrohalogenation^[5]

References

^ Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, ISBN 0-471-72091-7
^ Angew Chem Int Ed Engl. 2012 Mar 19;51(12):2818-28. doi: 10.1002/anie.201108001
^ Organic Syntheses, Coll. Vol. 7, p.276 (1990); Vol. 63, p.203 (1985). Link
^ Organic Syntheses, Coll. Vol. 5, p.22 (1973); Vol. 42, p.12 (1962) Link
^ Helv. Chim. Acta, 63, 438 (1980); Org. Synth. Coll., 7, 232 (1990)
^ Bhagwat; Devaprabhakara (1972). "Selective hydrogenation of allenes with chlorotris-(triphenylphosphine) rhodium catalyst". Tetrahedron Lett. 13 (15): 1391. doi:10.1016/S0040-4039(01)84636-0.

External links

Stereochemistry study guide
Synthesis of allenes

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Chemo-, regio- and stereocontrolled palladium-catalyzed preparations of enantiopure morpholines, oxocines, and dioxonines have been developed starting from 2-azetidinone-tethered γ,δ-, δ,ε-, and ε,ζ-allendiols. The palladium-catalyzed cyclizative coupling reaction of γ,δ-allendiols 2 with al
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Demethylation of an Allene Bearing Two Dimethoxythioxanthene Groups by Oxidation via a Vinyl Cation Intermediate

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Enantioselective formal total synthesis of aplysin utilizing a palladium-catalyzed addition of an arylboronic acid to an allenic alcohol-Eschenmoser/Claisen rearrangement