Valence Isomer

In organic chemistry, two molecules are valence isomers when they are constitutional isomers that can interconvert through pericyclic reactions.

Benzene

There are many valence isomers one can draw for the C₆H₆ formula benzene. Some were originally proposed for benzene itself before the actual structure of benzene was known. Others were later synthesized in lab. Some have been observed to isomerize to benzene, whereas others tend to undergo other reactions instead, or isomerize by ways other than pericyclic reactions.

Image:Benzene-2D-flat.png, Benzene
Image:Historic Benzene Formulae Dewar(1867) V.1.svg, Dewar benzene
Image:Prisman2.svg, Prismane
Image:Benzvalene.png, Benzvalene
Image:Bicycloprop-2-enyl.svg, Bicyclopropenyl

Cyclooctatetraene

The valence isomers are not restricted to isomers of benzene. Valence isomers are also seen in the series (CH)₈. Due to the larger number of units, the number of possible valence isomers is also greater and at least 21:

Image:Cyclooctatetraen.svg, Cyclooctatetraene (COT)
Image:Barrelene structure.png, Barrelene
Image:Cuban.svg, Cubane
Image:Cuneane.png, Cuneane
Image:Semibullvalene.svg, Semibullvalene
Image:1,5-dihydropentalene.png, 1,5-dihydropentalene
Image:2a,2b,4a,4b-tetrahydrocyclopropa cd pentalene.png, 2a,2b,4a,4b-Tetrahydrocyclopropa dentalene
Image:Bicyclo420octa247triene.svg, Bicyclo .2.0cta-2,4,7-triene. Tautomer with COT by thermal 6e process or photochemical 4e process
Image:Tricyclo33006octa38diene.svg, Tricyclo ,3,0,0^2,6cta-3,8-diene. Isomerises to semibullvalene at room temperature, stable at −60 °C
Image:Cyclobutadiene dimer cis trans.svg, Tricyclo ,2,0,0^2,5cta-3,7-diene. The dimer of cyclobutadiene occurs as a cis isomer and a trans isomer. Both isomers convert to COT (symmetry forbidden hence stable) with a half-life of 20 minutes at 140 °C
Image:Tetracyclooctaene.svg, Tetracyclo ,3,0,0^2,4,0^3,6octa-7-ene is only known as its 4-carbomethoxy derivative.
Image:Tetracyclooct-7-ene.svg, Tetracyclo ,2,0,0^2,4,0^3,5octa-7-ene has been prepared from benzvalene and isomerises to COT
Image:Octabisvalene.svg, Pentacyclo .1.0.0^2,4. 0^3,5.0^6,8ctane (octabisvalene) is the third saturated valence isomer. The (Z)-3,7-phenylsulfonyl derivative is stable up to 200 °C.
Image:Octavalene.svg, Tricyclo .1.0.0^2,8cta-3,5-diene (octavalene) was reported synthesised from homobenzvalene and converts to COT at 50 °C ''Electronic structure of octavalene. Photoelectron spectroscopic investigations'' Rolf Gleiter, Peter Bischof, Manfred Christl J. Org. Chem., 1986, 51 (15), pp 2895–2898

Naphthalene and azulene

Perhaps no pair of valence isomers differ more strongly in appearance than colourless naphthalene and the intensely violet azulene.

Image:Naphthalene-2D-Skeletal.svg, Naphthalene
Image:Azulene-numbers.png, Azulene

Benzene oxide and oxepin

References

External links

*{{Commonscatinline

Isomerism