Dewar benzene
chemical compound
Dewar benzene (also spelled dewarbenzene) or bicyclo[2.2.0]hexa-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. However, he did not propose it as the structure of benzene, and in fact he supported the correct structure previously proposed by August Kekulé in 1865.
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-fused to each other. The compound has 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 and derivatives
The first synthesis of a compound containing a Dewar benzene core was a tri-tert-butyl derivative, by photoisomerization of the analogous benzene structure in 1962. The large groups ortho to each other have a destabilizing effect in the planar benzene-ring isomer more-so than in the non-planar Dewar benzene structure.
The next year, the unsubstituted Dewar benzene itself was synthesized by photoisomerization of the cis-1,2-dihydro derivative of phthalic anhydride followed by oxidation with lead tetraacetate.
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