Dewar benzene
.svg) The conjoined cyclobutene rings of Dewar benzene form an obtuse angle.
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| Preferred IUPAC name
Bicyclo[2.2.0]hexa-2,5-diene | |
| Identifiers | |
3D model (JSmol)
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PubChem CID
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| UNII | |
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 (also spelled dewarbenzene) 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 1869.[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
[edit]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 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 orbital symmetries forbid it.[3]
Synthesis
[edit]The compound itself was first synthesized in 1962 as a tert-butyl derivative[4] and then as the unsubstituted compound by Eugene van Tamelen in 1963 by photolysis of the cis-1,2-dihydro derivative of phthalic anhydride followed by oxidation with lead tetraacetate.[5][6]
![Dewar benzene synthesis reported by E. E. van Tamelen and Pappas[5]](/wiki/File:Dewar_benzene_synthesis.svg)
Hexamethyl Dewar benzene has been prepared by bicyclotrimerization of dimethylacetylene with aluminium chloride.[7]
"Dewar benzene" and benzene
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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 example isomers[1] and believed that his experiments on benzene supported the (correct) structure that had been proposed by Friedrich August Kekulé von Stradonitz.[2]
After the development of valence bond theory in 1928, benzene was described primarily using its two major resonance contributors, the two Kekulé structures. The three possible Dewar structures were considered as minor resonance contributors in the overall description of benzene, alongside other classic structures such as the isomers prismane, benzvalene and Claus' benzene. Prismane and benzvalene were synthesized in the 1970s; Claus' benzene is impossible.[8]
References
[edit]- ^ a b Dewar, James (1869). "On the Oxidation af Phenyl Alcohol, and a Mechanical Arrangement adapted to illustrate Structure in the Non-saturated Hydrocarbons". Proc. R. Soc. Edinb. 6: 82–86. doi:10.1017/S0370164600045387.
- ^ a b Baker, Wilson; Rouvray, Dennis H. (1978). "Para-Bond or "Dewar" Benzene?". J. Chem. Educ. 55 (10): 645. Bibcode:1978JChEd..55..645B. doi:10.1021/ed055p645.
- ^ Jensen, James O. (2004). "Vibrational Frequencies and Structural Determination of Dewar Benzene". Journal of Molecular Structure: THEOCHEM. 680 (1–3): 227–236. doi:10.1016/j.theochem.2004.03.042.
- ^ van Tamelen, Eugene E.; Pappas, S. P. (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.
- ^ a b van Tamelen, Eugene E.; Pappas, S. P. (1963). "Bicyclo [2.2.0]hexa-2,5-diene". J. Am. Chem. Soc. 85 (20): 3297–3298. doi:10.1021/ja00903a056.
- ^ van Tamelen, Eugene E.; Pappas, S. P.; Kirk, K. L. (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.
- ^ Shama, Sami A.; Wamser, Carl C. (1990). "Hexamethyl Dewar Benzene". Organic Syntheses. 61: 62. doi:10.15227/orgsyn.061.0062; Collected Volumes, vol. 7, p. 256.
- ^ Hoffmann, Roald; Hopf, Henning (2008). "Learning from Molecules in Distress". Angew. Chem. Int. Ed. 47 (24): 4474–4481. doi:10.1002/anie.200705775. PMID 18418829.