Dewar benzene
Encyclopedia
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.
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.
Isomer
In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...
of benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
with the molecular formula C6H6. The compound is named after James Dewar
James Dewar
Sir James Dewar FRS was a Scottish chemist and physicist. He is probably best-known today for his invention of the Dewar flask, which he used in conjunction with extensive research into the liquefaction of gases...
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 derivativeDerivative (chemistry)
In chemistry, a derivative is a compound that is derived from a similar compound by some chemical or physical process. In the past it was also used to mean a compound that can be imagined to arise from another compound, if one atom is replaced with another atom or group of atoms, but modern...
and then as the unsubstituted compound by E.E. van Tamelen in 1963 by photolysis of cis-1,2-dihydro derivative of phthalic anhydride
Phthalic anhydride
Phthalic anhydride is the organic compound with the formula C6H42O. It is the anhydride of phthalic acid. This colourless solid is an important industrial chemical, especially for the large-scale production of plasticizers for plastics. In 2002, approximately 4.6 billion kilograms were...
followed by oxidation with lead tetraacetate.
Unlike benzene, Dewar benzene is not flat; the two cyclobutene rings make an angle. The compound has nevertheless considerable strain energyStrain energyIn a molecule, strain energy is released when the constituent atoms are allowed to rearrange themselves in a chemical reaction or a change of chemical conformation in a way that:* angle strain,* torsional strain,* ring strain and/or steric strain,...
and reverts back 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 KekuléFriedrich August Kekulé von StradonitzFriedrich August Kekule von Stradonitz was a German organic chemist. From the 1850s until his death, Kekule was one of the most prominent chemists in Europe, especially in theoretical chemistry...
.
After the development in 1928 of the valence bond theory, the three possible Dewar structures were considered as minor resonanceResonance (chemistry)In chemistry, resonance or mesomerism is a way of describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by one single Lewis formula...
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 prismanePrismanePrismane is a polycyclic hydrocarbon with the formula C6H6. It is an isomer of benzene, more specific: a valence isomer. Prismane is far less stable than benzene. The carbon atoms of the prismane molecule are arranged in the shape of a six-atom triangular prism. Albert Ladenburg proposed this...
, benzvaleneBenzvaleneBenzvalene is an organic compound and one of several isomers of benzene.. It was synthesised in 1971 by Thomas J. Katz et al.The 1971 synthesis consisted of treating cyclopentadiene with methyllithium in dimethylether and then with dichloromethane and methyllithium in at -45°C. The hydrocarbon in...
and Claus' benzeneClaus' benzeneClaus' benzene is a hypothetical hydrocarbon and an isomer of benzene . It was proposed by Adolf Karl Ludwig Claus in 1867 as a possible structure for benzene at a time when the structure of benzene was debated...
. In fact prismane and benzvalene were synthesized in the 1970s, and Claus' benzene has never been synthesized.
Hexamethyl Dewar benzene
Hexamethyl Dewar benzene has been prepared by bicyclotrimerization of dimethylacetyleneDimethylacetyleneDimethylacetylene is an alkyne with chemical formula CH3C≡CCH3. Produced artificially, it is a colorless, volatile, pungent liquid at standard temperature and pressure....
with aluminium chlorideAluminium chlorideAluminium chloride is the main compound of aluminium and chlorine. It is white, but samples are often contaminated with iron trichloride, giving it a yellow colour. The solid has a low melting and boiling point. It is mainly produced and consumed in the production of aluminium metal, but large...
. It undergoes an unusual rearrangement reaction with hydrohalic acids to form a pentamethylcyclopentadienePentamethylcyclopentadiene1,2,3,4,5-Pentamethylcyclopentadiene is a cyclic diolefin with the formula C5Me5H . 1,2,3,4,5-Pentamethylcyclopentadiene is the precursor to the ligand 1,2,3,4,5-pentamethylcyclopentadienyl, which is often denoted as Cp*...
derivative, and consequently can be used as a starting material for synthesising some pentamethylcyclopentadienyl organometallic compounds.