Dewar benzene

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Dewar benzene

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Template:Longitem	C6H6
Molar mass	78.1 g·mol−1

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 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.

Dewar benzene synthesis reported by E. E. van Tamelen and Pappas[5]

Hexamethyl Dewar benzene has been prepared by bicyclotrimerization of dimethylacetylene with aluminium chloride.

"Dewar benzene" and benzene

File:Historic Benzene Formula Dewar 1869 proposals (original).png

Seven possible isomers proposed by Dewar, with "Dewar benzene" in second row, right

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.