Dewar benzene

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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.<ref>On the Oxidation af Phenyl Alcohol, and a Mechanical Arrangement adapted to illustrate Structure in the Non-saturated Hydrocarbons Dewar. J., Proc. Royal Soc. Edinburgh 6,62, 96 1867.</ref>

The compound itself was first synthesized in 1962 as a tert-butyl derivative<ref>Chemistry of Dewar Benzene. 1,2,5-Tri-t-Butylbicyclo[2.2.0]Hexa-2,5-Diene E. E. Van Tamelen and S. P. Pappas J. Am. Chem. Soc.; 1962; 84(19) pp 3789 - 3791; DOI:10.1021/ja00878a054</ref> and then as the unsubstituted compound in 1963 by photolysis of phthalic anhydride followed by oxidation with lead tetraacetate.<ref>Bicyclo [2.2.0]hexa-2,5-diene Eugene E. van Tamelen and Socrates P. Pappas J. Am. Chem. Soc.; 1963; 85(20) pp 3297 - 3298; DOI:10.1021/ja00903a056</ref> Unlike benzene, Dewar benzene is not flat; the two cyclobutane 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 Dewars original claim, one study did not find any evidence for this in the original article<ref>Para-Bond or "Dewar" Benzene? Wilson Baker and Dennis H. Rouvray Journal of Chemical Eduction Volume 55, Number 10, 1978 1 645</ref> 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.