Allene

The title of this article is ambiguous. Further meanings are listed under Allene (disambiguation) .

General formula of the allenes (above). The radicals R ¹ to R ⁴ are organyl radicals ( alkyl radicals, aryl radicals, alkylaryl radicals, etc.) or other radicals (for example halogens) which can be identical or different. Propadiene (below) is the parent compound of the allenes. The allen group is marked in blue .

As Allene (stress on the second syllable: All e ne ) refers to a group of substances in organic chemistry , whose members by the sequence of their carbon distinguishes carbon-link: they have adjacent right two, so cumulative double bonds . Thus they contain the structural element C = C = C.

The simplest representative of the allenes is the Propadien .

Whether the allenes should be added to the group of substances of the cumulene is controversially discussed in the specialist literature. The Römpp affirms this, the Gold Book, on the other hand, defines the cumulens as compounds with a sequence of at least three cumulative double bonds and thus excludes the allenes from the cumulens.

Relative stability

A comparison of the relative stability shows that conjugated dienes are more stable than isolated dienes and those are more stable than allenes.

Symmetry and stereochemistry

Newman projection : determination of the configuration of axially chiral allenes with two different residues A and B on the terminal sp ² -hybridized carbon atoms

The C = C = C group has a linear structure, the middle carbon atom is sp hybridized . The marginal carbon atoms are sp ² hybridized. The substituents of allenes or cumulenes with an even number of cumulative double bonds do not lie in one plane, but are rotated by 90 ° to one another. It follows that the molecules can have an axial chirality under the conditions R ¹ ≠ R ² and R ³ ≠ R ⁴ . Individual isomers can be specifically synthesized using stereoselective synthesis strategies or isolated from the mixture using suitable methods. The absolute configuration of an isomer is determined according to the Cahn-Ingold-Prelog system . Either the stereochemical descriptors R _a and S _a or the plus (P) / minus (M) notation are used for identification.

Occurrence

Substances with an allene structure occur as natural substances , e.g. B. in peridinin , a carotenoid found in dinoflagellates . The pheromone of the male bean beetle (Acanthoscelides obtectus) also has an Allen unit. Cyclic allenes are also known from nature.

synthesis

Allen can be represented in different ways in the laboratory.

• One possibility for this is the conjugate 1,6-addition to enynes .
• The reduction of propargylic halides or acetals also leads to allenes.
• Terminal alkynes can be converted to allenes with formaldehyde and copper (I) bromide with the addition of bases.
• From geminal Dihalocyclopropanen a can by means of organolithium carbene be obtained after a skattebøl rearrangement leads to allenes.

Reaction scheme for the Allen synthesis using the Skattebøl rearrangement

• The dehalogenation of 2,3-dihalopropenes with zinc also gives allenes.
• The dehydrohalogenation of trisubstituted vinyl halides leads to allenes.

literature

• Norbert Krause , A. Stephen Hashmi: Modern allene chemistry. Wiley-VCH Weinheim 2004, ISBN 3-527-30671-4 ( excerpt in the Google book search)
• Alan R. Katritzky, Otto Meth-Cohn, Charles Wayne Rees: Comprehensive organic functional group transformations. Elsevier Science 1995, ISBN 0-08-040604-1 , pp. 953-996 ( limited preview in Google book search)
• Kay M. Brummond (Ed.) Allene chemistry Thematic series in the Open Access Beilstein Journal of Organic Chemistry

Individual evidence

1. ↑ entry to Allenes . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.A00238 Version: 2.1.5.
2. ↑ Entry on cumulative. In: Römpp Online . Georg Thieme Verlag, accessed on June 21, 2014.
3. ↑ Entry on Cumulenes . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.C01440 Version: 2.1.5.
4. ↑ Paula Yurkanis Bruice: Organic Chemistry , Pearson Education Inc., 2004, 4th edition, p 302, ISBN 0-13-121730-5 .
5. ^ Ivan Ernest: Binding, Structure and Reaction Mechanisms in Organic Chemistry , Springer-Verlag, 1972, p. 28, ISBN 3-211-81060-9 .
6. ↑ Entry on Axial Chirality . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.A00547 Version: 2.1.5.
7. ↑ Horler, DF: (-) - Methyl n-tetradeca-trans-2,4,5-trienoate, an allenic ester produced by the male dried bean beetle, Acanthoscelides obtectus (Say). J. Chem. Soc. 1970, pp. 859-862 doi: 10.1039 / J39700000859 .
8. ↑ Ferdinand Bohlmann , Jasmin Jakupovic, Rajinder K. Gupta, Robert M. King, Harold Robinson: Allenic germacranolides, bourbonene derived lactones and other constituents from Vernonia species. In: Phytochemistry , Volume 20, Issue 3, 13 March 1981, pp. 473-480 doi: 10.1016 / S0031-9422 (00) 84169-2 .
9. ↑ N. Krause, A. Gerold: Regio- and stereoselective syntheses with organocopper reagents. In: Angewandte Chemie 1997, Vol. 109 (3), pp. 194-213. doi: 10.1002 / anie.19971090304 .
10. ↑ L. Brandsma, HD Verkruijsse: Synthesis of acetylenes, allenes and cumulenes: methods and techniques. Elsevier Oxford 2004, ISBN 0-12-125751-7 ( limited preview in Google Book Search).
11. ↑ Pierre Crabbé, Bahman Nassim, Maria-Teresa Robert-Lopes: One-Step Homologation of Acatylenes to Allenes: 4-Hydroxynona-1,2-diene In: Organic Syntheses . 63, 1985, p. 203, doi : 10.15227 / orgsyn.063.0203 ; Coll. Vol. 7, 1990, p. 276 ( PDF ).
12. ^ Lars Skattebøl: Chemistry of gem-Dihalocyclopropanes. V.1 Formation of Tricyclo [4.1.0.0 ^4.6 ] heptane and Derivatives. in: J. Org. Chem. 1966, 31 (9), pp. 2789-2794 doi: 10.1021 / jo01347a014 .
13. ^ Siegfried Hauptmann : Organic Chemistry , 2nd Edition, VEB Deutscher Verlag für Grundstoffindindustrie, Leipzig, 1985, p. 245, ISBN 3-342-00280-8 .