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Dewar benzene

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.

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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.

Category: Hydrocarbons