Dewar benzene

Dewar benzene

Skeletal formula The conjoined cyclobutene rings of Dewar benzene form an obtuse angle.
Ball-and-stick model
Names
IUPAC name Bicyclo[2.2.0]hexa-2,5-diene
Identifiers
CAS Number	5649-95-6 N
ChemSpider	89242 Y
Jmol 3D model	Interactive image
PubChem	98808
InChI InChI=1S/C6H6/c1-2-6-4-3-5(1)6/h1-6H Y Key: CTLSARLLLBZBRV-UHFFFAOYSA-N Y InChI=1/C6H6/c1-2-6-4-3-5(1)6/h1-6H Key: CTLSARLLLBZBRV-UHFFFAOYAO
SMILES C\1=C\C2/C=C\C/12
Properties
Chemical formula	C6H6
Molar mass	78.1 g·mol−1
Vapor pressure	{{{value}}}
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

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.

Synthesis and properties of bicyclo[2.2.0]hexa-2,5-diene

The compound itself was first synthesized in 1962 as a tert-butyl derivative and then as the unsubstituted compound by E.E. van Tamelen in 1963 by photolysis of cis-1,2-dihydro derivative of phthalic anhydride followed by oxidation with lead tetraacetate.

Dewar benzene synthesis E. E. Van Tamelen, S. P. Pappas

Unlike benzene, Dewar benzene is not flat; the two cyclobutene rings make an angle. 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.

"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 possible isomers, and believed that his experiments on benzene supported the (correct) structure which had been proposed by Kekulé.

After the development in 1928 of the valence bond theory, the three possible Dewar structures were considered as minor resonance contributors in the overall description of benzene. The major resonance contributors are of course the two possible Kekulé structures.

Other classic structures which have been considered as possible benzene isomers are prismane, benzvalene and Claus' benzene. Prismane and benzvalene were synthesized in the 1970s; Claus' benzene is impossible to synthesize.

Hexamethyl Dewar benzene

Hexamethyl Dewar benzene has been prepared by bicyclotrimerization of dimethylacetylene with aluminium chloride. It undergoes an unusual rearrangement reaction with hydrohalic acids to form a pentamethylcyclopentadiene derivative, and consequently can be used as a starting material for synthesising some pentamethylcyclopentadienyl organometallic compounds.

Synthesis of the rhodium(III) dimer [Cp*RhCl2]2 from hexamethyl Dewar benzene