Hexamethyl Dewar benzene
.PNG) The conjoined cyclobutene rings of Dewar benzene form an obtuse angle.
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IUPAC name
Bicyclo[2.2.0]hexa-2,5-diene
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3D model (JSmol)
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| C6H6 | |
| Molar mass | 78.1 g·mol−1 |
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Dewar benzene 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 1867. 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.
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 nevertheless considerable strain energy and reverts 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.
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.
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 possible isomers and believed that his experiments on benzene supported the (correct) structure that had been proposed by Kekulé.
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