Allene

For the organic compound with the common name allene, see propadiene.
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 D2d 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:

As a ligand and substrate in homogeneous catalysis

Allenes function as ligands, not unlike alkenes. A typical complex is Pt(η2-allene)(PPh3)2. Ni(0) reagents catalyze the cyclooligomerization of allene.[7] Using a suitable catalyst (e.g. Wilkinson's catalyst), it is possible to reduce just one of the double bonds of an allene.[8]

See also

References

  1. ↑ Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, ISBN 0-471-72091-7
  2. ↑ Rivera Fuentes, Pablo; Diederich, François (2012). "Allenes in Molecular Materials". Angew. Chem. Int. Ed. Engl. 51 (12): 2818–2828. doi:10.1002/anie.201108001. PMID 22308109.
  3. ↑ Crabbé, Pierre; Nassim, Bahman; Robert-Lopes, Maria-Teresa. "One-Step Homologation of Acetylenes to Allenes: 4-Hydroxynona-1,2-diene [1,2-Nonadien-4-ol]". Org. Synth. 63: 203. doi:10.15227/orgsyn.063.0203.; Coll. Vol., 7, p. 276
  4. ↑ Cripps, H. N.; Kiefer, E. F. "Allene". Org. Synth. 42: 12. doi:10.15227/orgsyn.042.0012.; Coll. Vol., 5, p. 22
  5. ↑ Lang, Robert W.; Hansen, Hans-Jürgen (1980). "Eine einfache Allencarbonsäureester-Synthese mittels der Wittig-Reaktion" [A simple synthesis of allene carboxylic acid esters by means of the Wittig reaction]. Helv. Chim. Acta. 63 (2): 438–455. doi:10.1002/hlca.19800630215.
  6. ↑ Lang, Robert W.; Hansen, Hans-Jürgen. "α-Allenic Esters from α-Phosphoranylidene Esters and Acid Chlorides: Ethyl 2,3-Pentadienoate [2,3-Pentadienoic acid, ethyl ester]". Org. Synth. 62: 202. doi:10.15227/orgsyn.062.0202.; Coll. Vol., 7, p. 232
  7. ↑ Otsuka, Sei; Nakamura, Akira "Acetylene and allene complexes: their implication in homogeneous catalysis" Advances in Organometallic Chemistry 1976, volume 14, pp. 245-83. doi:10.1016/S0065-3055(08)60654-1.
  8. ↑ Bhagwat, M. M.; Devaprabhakara, D. (1972). "Selective hydrogenation of allenes with chlorotris-(triphenylphosphine) rhodium catalyst". Tetrahedron Lett. 13 (15): 1391–1392. doi:10.1016/S0040-4039(01)84636-0.

Further reading

External links

This article is issued from Wikipedia - version of the 8/26/2016. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.