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.

The compound itself was first synthesized in 1962 as a tert-butyl derivative 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. 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 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.