Dewar benzene (also spelled ''dewarbenzene'') or bicyclo .2.0exa-2,5-diene is a

bicyclic A bicyclic molecule () is a molecule that features two joined rings. Bicyclic structures occur widely, for example in many biologically important molecules like α-thujene and camphor. A bicyclic compound can be carbocyclic (all of the ring ...

isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formula – that is, the same number of atoms of each element (chemistry), element – but distinct arrangements of atoms in space. ''Isomerism'' refers to the exi ...

benzene Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...

with the molecular formula C₆H₆. The compound is named after

James Dewar Sir James Dewar ( ; 20 September 1842 – 27 March 1923) was a Scottish chemist and physicist. He is best known for his invention of the vacuum flask, which he used in conjunction with research into the liquefaction of gases. He also studie ...

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 Friedrich August Kekulé, later Friedrich August Kekule von Stradonitz ( , ; 7 September 1829 – 13 July 1896), was a German organic chemist. From the 1850s until his death, Kekulé was one of the most prominent chemists in Europe, especially ...

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 In physics, the elastic potential energy gained by a wire during elongation with a tensile (stretching) or compressive (contractile) force is called strain energy. For linearly elastic materials, strain energy is: : U = \frac 1 2 V \sigma \v ...

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 In mathematics, the derivative is a fundamental tool that quantifies the sensitivity to change of a function's output with respect to its input. The derivative of a function of a single variable at a chosen input value, when it exists, is t ...

and then as the unsubstituted compound by Eugene van Tamelen in 1963 by

photolysis Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by absorption of light or photons. It is defined as the interaction of one or more photons wi ...

of the ''cis''-1,2-dihydro derivative of

phthalic anhydride Phthalic anhydride is the organic compound with the formula C6H4(CO)2O. It is the anhydride of phthalic acid. Phthalic anhydride is a principal commercial form of phthalic acid. It was the first anhydride of a dicarboxylic acid to be used commer ...

followed by

oxidation Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

with lead tetraacetate. : Dewar benzene synthesis

Hexamethyl Dewar benzene Hexamethyl Dewar benzene is a derivative of Dewar benzene with application in organometallic chemistry. It consists of the Dewar benzene core, with a methyl group substituent on each of its six carbon positions. Synthesis Hexamethyl Dewar benze ...

has been prepared by bicyclotrimerization of dimethylacetylene with

aluminium chloride Aluminium chloride, also known as aluminium trichloride, is an inorganic compound with the formula . It forms a hexahydrate with the formula , containing six water molecules of hydration. Both the anhydrous form and the hexahydrate are col ...

"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 Friedrich may refer to: Names *Friedrich (given name), people with the given name ''Friedrich'' *Friedrich (surname), people with the surname ''Friedrich'' Other *Friedrich (board game), a board game about Frederick the Great and the Seven Years' ...

. After the development of

valence bond theory In chemistry, valence bond (VB) theory is one of the two basic theories, along with molecular orbital (MO) theory, that were developed to use the methods of quantum mechanics to explain chemical bonding. It focuses on how the atomic orbitals of ...

in 1928, benzene was described primarily using its two major

resonance Resonance is a phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency (or resonance frequency) of the system, defined as a frequency that generates a maximu ...

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.

References

{{Reflist Cyclobutenes Substances discovered in the 1960s Bicyclic compounds