Dewar benzene (also spelled dewarbenzene) or bicyclo2.2.0hexa-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.
Structure and properties
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-
Ring fusion 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.
Synthesis
The compound itself was first synthesized in 1962 as a
tert-butyl derivative
[
] 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.
- and Pappas
Hexamethyl Dewar benzene has been prepared by bicyclotrimerization of dimethylacetylene with aluminium chloride.
"Dewar benzene" and benzene
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
and believed that his experiments on benzene supported the (correct) structure that had been proposed by Friedrich August Kekulé von Stradonitz.
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.
