Valence isomer

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.svg|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[cd]pentalene Image:Bicyclo420octa247triene.svg|Bicyclo[4.2.0]octa-2,4,7-triene. Tautomer with COT by thermal 6e process or photochemical 4e process Image:Tricyclo33006octa38diene.svg|Tricyclo[3,3,0,02,6]octa-3,8-diene. Isomerises to semibullvalene at room temperature, stable at −60 °C Image:Cyclobutadiene dimer cis trans.svg|Tricyclo[4,2,0,02,5]octa-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[3,3,0,02,4,03,6] octa-7-ene is only known as its 4-carbomethoxy derivative. Image:Tetracyclooct-7-ene.svg|Tetracyclo[4,2,0,02,4,03,5] octa-7-ene has been prepared from benzvalene and isomerises to COT Image:Octabisvalene.svg| Pentacyclo[5.1.0.02,4. 03,5.06,8]octane (octabisvalene) is the third saturated valence isomer. The (Z)-3,7-phenylsulfonyl derivative is stable up to 200 °C. Image:Octavalene.svg| Tricyclo[5.1.0.02,8]octa-3,5-diene (octavalene) was reported synthesised from homobenzvalene and converts to COT at 50 °C

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:Azulen num.svg| Azulene

Benzene oxide and oxepin

References

References

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4. B''icyclo[4,2,0]octa-2,4,7-triene'' Emanuel Vogel, H. Kiefer, W. R. Roth Volume 3, Issue 6, pages 442–443, June '''1964''' {{doi. 10.1002/anie.196404422
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11. Methyl tetracyclo[3.3.0.0^2,4O^3,6]cot-7-ene-4-carboxylate Gerhard W. Klumpp, W. G. J. Rietman, J. J. Vrielink [[J. Am. Chem. Soc.]], '''1970''', 92 (17), pp 5266–5267 {{doi. 10.1021/ja00720a071
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14. ''The synthesis of octavalene (tricyclo[5.1.0.02,8]octa-3,5-diene) and several substituted octavalenes'' Tetrahedron Volume 42, Issue 6, '''1986''', Pages 1585-1596 Manfred Christl, Reinhard Lang and Clemens Herzog {{doi. 10.1016/S0040-4020(01)87575-X
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