Dewar benzene or bicyclo[2.2.0]hexa-2,5-diene is the bicyclic isomer of benzene with the molecular formula C6H6. The compound is named after James Dewar who, it is said, (incorrectly) proposed this structure for benzene in 1867.[1]
The compound itself was first synthesized in 1962 as a tert-butyl derivative[2] and then as the unsubstituted compound in 1963 by photolysis of cis-1,2-dihydro derivative of phthalic anhydride followed by oxidation with lead tetraacetate.[3] Unlike benzene, Dewar benzene is not flat; the two cyclobutene rings make an angle. The compound has nevertheless considerable strain energy and reverts back 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. With regards to Dewar's original claim, one study did not find any evidence for this in the original article[4] and only goes as far as to state that Dewar in it identified the structure as one of 7 possible isomers. Other classic benzene isomers are prismane, benzvalene and Claus benzene.
External links
- Hexamethyl Dewar benzene Organic Syntheses, Coll. Vol. 7, p.256 (1990); Vol. 61, p.62 (1983) Link
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 6,62: 96.
- ^ 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". Journal of the American Chemical Society 84 (19): 3789 - 3791. doi:10.1021/ja00878a054.
- ^ E. E. Van Tamelen, S. P. Pappas (1963). "Bicyclo [2.2.0]hexa-2,5-diene". Journal of the American Chemical Society 85 (20): 3297 - 3298. doi:10.1021/ja00903a056.
- ^ W. Baker, D. H. Rouvray (1978). "Para-Bond or "Dewar" Benzene?". Journal of Chemical Education 55 (10): 645. doi:10.1021/ja00903a056.
- James O. Jensen (2004). "Vibrational frequencies and structural determination of dewar benzene". Journal of Molecular Structure: THEOCHEM 680: 227-236. doi:10.1016/j.theochem.2004.03.042.
- 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)". Journal of the American Chemical Society 93 (23): 6092-6101. doi:10.1021/ja00752a021.
