Dewar benzene: Difference between revisions
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==Structure and properties== |
==Structure and properties== |
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Unlike benzene, Dewar benzene is not flat because the carbons where the rings join are bonded to four atoms rather than three. These carbons tend toward [[tetrahedral geometry]], and the two cyclobutene rings make an angle |
Unlike benzene, Dewar benzene is not flat because the carbons where the rings join are bonded to four atoms rather than three. These carbons tend toward [[tetrahedral geometry]], and the two cyclobutene rings make an angle where they are ''[[Cis–trans isomerism|cis]]''-[[Ring fusion|fused]] to each each other. The compound has nevertheless considerable [[strain energy]] and reverts to benzene with a [[chemical half-life]] of two days. This thermal conversion is relatively slow because it is [[symmetry forbidden]] based on orbital symmetry arguments.<ref>{{cite journal |
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| author=James O. Jensen |
| author=James O. Jensen |
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| title = Vibrational Frequencies and Structural Determination of Dewar Benzene |
| title = Vibrational Frequencies and Structural Determination of Dewar Benzene |
Revision as of 02:34, 22 February 2017
The conjoined cyclobutene rings of Dewar benzene form an obtuse angle.
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Names | |
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IUPAC name
Bicyclo[2.2.0]hexa-2,5-diene
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Identifiers | |
3D model (JSmol)
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ChemSpider | |
PubChem CID
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CompTox Dashboard (EPA)
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Properties | |
C6H6 | |
Molar mass | 78.1 g·mol−1 |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Dewar benzene or bicyclo[2.2.0]hexa-2,5-diene is a bicyclic isomer of benzene with the molecular formula C6H6. The compound is named after James Dewar who included this structure in a list of possible C6H6 structures in 1867.[1] However, he did not propose it as the structure of benzene, and in fact he supported the correct structure previously proposed by August Kekulé in 1865.[2]
Structure and properties
Unlike benzene, Dewar benzene is not flat because the carbons where the rings join are bonded to four atoms rather than three. These carbons tend toward tetrahedral geometry, and the two cyclobutene rings make an angle where they are cis-fused to each each other. The compound has nevertheless considerable strain energy and reverts to benzene with a chemical half-life of two days. This thermal conversion is relatively slow because it is symmetry forbidden based on orbital symmetry arguments.[3]
Synthesis
The compound itself was first synthesized in 1962 as a tert-butyl derivative[4] and then as the unsubstituted compound by E.E. van Tamelen in 1963 by photolysis of cis-1,2-dihydro derivative of phthalic anhydride followed by oxidation with lead tetraacetate.[5][6]
"Dewar benzene" and benzene
It is sometimes incorrectly claimed that Dewar proposed his structure as the true structure of benzene. In fact, Dewar merely wrote the structure as one of seven possible isomers and believed that his experiments on benzene supported the (correct) structure that had been proposed by Kekulé.[2]
After the development in 1928 of the valence bond theory, the three possible Dewar structures were considered as minor resonance contributors in the overall description of benzene. The major resonance contributors are of course the two possible Kekulé structures.
Other classic structures that have been considered as possible benzene isomers are prismane, benzvalene and Claus' benzene. Prismane and benzvalene were synthesized in the 1970s; Claus' benzene is impossible to synthesize.[7]
Hexamethyl Dewar benzene
Hexamethyl Dewar benzene has been prepared by bicyclotrimerization of dimethylacetylene with aluminium chloride.[8] It undergoes an unusual rearrangement reaction with hydrohalic acids to form a pentamethylcyclopentadiene derivative,[9][10] and consequently can be used as a starting material for synthesising some pentamethylcyclopentadienyl organometallic compounds.[11][12]
One of the alkenes can be epoxidized using mCPBA,[13] peroxybenzoic acid,[14] or dimethyldioxirane (DMDO).[15] Using a peracid (mCPBA or peroxybenzoic acid), the epoxy product quickly rearranges, catalyzed by the acid byproduct of the epoxidation.[13] Using DMDO gives the epoxide as a stable product—the byproduct of the epoxidation is neutral acetone. By varying the amount of DMDO, either the mono- or diepoxide can be formed, with the oxygen atoms syn on the bicyclic carbon framework.[15]
References
- ^ J. Dewar (1867). "On the Oxidation af Phenyl Alcohol, and a Mechanical Arrangement adapted to illustrate Structure in the Non-saturated Hydrocarbons". Proc. Royal Soc. Edinburgh. 6: 82–86.
- ^ a b W. Baker; D. H. Rouvray (1978). "Para-Bond or "Dewar" Benzene?". J. Chem. Educ. 55 (10): 645. doi:10.1021/ed055p645.
- ^ James O. Jensen (2004). "Vibrational Frequencies and Structural Determination of Dewar Benzene". J. Mol. Struct.:THEOCHEM. 680: 227–236. doi:10.1016/j.theochem.2004.03.042.
- ^ E. E. van Tamelen, S. P. Pappas (1962). "Chemistry of Dewar Benzene. 1,2,5-Tri-t-Butylbicyclo[2.2.0]Hexa-2,5-Diene". J. Am. Chem. Soc. 84 (19): 3789–3791. doi:10.1021/ja00878a054.
- ^ E. E. van Tamelen, S. P. Pappas (1963). "Bicyclo [2.2.0]hexa-2,5-diene". J. Am. Chem. Soc. 85 (20): 3297–3298. doi:10.1021/ja00903a056.
- ^ E. E. van Tamelen, S. P. Pappas, K. L. Kirk (1971). "Valence Bond Isomers of Aromatic Systems. Bicyclo[2.2.0]hexa-2,5-dienes (Dewar benzenes)". J. Am. Chem. Soc. 93 (23): 6092–6101. doi:10.1021/ja00752a021.
{{cite journal}}
: CS1 maint: multiple names: authors list (link) - ^ Hoffmann, R.; Hopf, H. (2008). "Learning from molecules in distress". Angewandte Chemie International Edition in English. 47 (24): 4474–4481. doi:10.1002/anie.200705775. PMID 18418829.
- ^ Sami A. Shama and Carl C. Wamser (1990). "Hexamethyl Dewar Benzene". Organic Syntheses; Collected Volumes, vol. 7, p. 256.
- ^ Paquette, L. A.; Krow, G. R. (1968). "Electrophilic Additions to Hexamethyldewarbenzene". Tetrahedron Lett. 9 (17): 2139–2142. doi:10.1016/S0040-4039(00)89761-0.
- ^ Criegee, R.; Gruner, H. (1968). "Acid-catalyzed Rearrangements of Hexamethyl-prismane and Hexamethyl-Dewar-benzene". Angew. Chem. Int. Ed. 7 (6): 467–468. doi:10.1002/anie.196804672.
- ^ Kang, J. W.; Mosley, K.; Maitlis, P. M. (1968). "Mechanisms of Reactions of Dewar Hexamethylbenzene with Rhodium and Iridium Chlorides". Chem. Commun. (21): 1304–1305. doi:10.1039/C19680001304.
- ^ Kang, J. W.; Maitlis, P. M. (1968). "Conversion of Dewar Hexamethylbenzene to Pentamethylcyclopentadienylrhodium(III) Chloride". J. Am. Chem. Soc. 90 (12): 3259–3261. doi:10.1021/ja01014a063.
- ^ a b King, R. B.; Douglas, W. M.; Efraty, A. (1977). "5-Acetyl-1,2,3,4,5-pentamethylcyclopentadiene". Organic Syntheses. 56: 1.
- ^ Junker, Hans-Nikolaus; Schäfer, Wolfgang; Niedenbrück, Hans (1967). "Oxydationsreaktionen mit Hexamethyl-bicyclo[2.2.0]-hexadien-(2.5) (= Hexamethyl-Dewar-Benzol)" [Oxidation reactions with hexamethylbicyclo[2.2.0]-hexa-2,5-diene]. Chemische Berichte. 100 (8): 2508–2514. doi:10.1002/cber.19671000807.
- ^ a b Asouti, Amalia; Hadjiarapoglou, Lazaros P. (2000). "Regioselective and diastereoselective dimethyldioxirane epoxidation of substituted norbornenes and hexamethyl Dewar benzene". Tetrahedron Letters. 41 (4): 539–542. doi:10.1016/S0040-4039(99)02113-9.