Valence isomer

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.

• Some known valence isomers of benzene
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  Benzene
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  Dewar benzene
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  Prismane
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  Benzvalene
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  Bicyclopropenyl

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:

• Valence isomers of cyclooctatetraene
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  Cyclooctatetraene (COT)
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  Barrelene
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  Cubane
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  Cuneane
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  Semibullvalene
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  1,5-dihydropentalene
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  2a,2b,4a,4b-Tetrahydrocyclopropa[cd]pentalene
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  Bicyclo[4.2.0]octa-2,4,7-triene. Tautomer with COT by thermal 6e process or photochemical 4e process ^[3][4][5]
•  
  Tricyclo[3,3,0,0^2,6]octa-3,8-diene. Isomerises to semibullvalene at room temperature, stable at −60 °C ^[6][7][8][9]
•  
  Tricyclo[4,2,0,0^2,5]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 ^[10]
•  
  Tetracyclo[3,3,0,0^2,4,0^3,6] octa-7-ene is only known as its 4-carbomethoxy derivative.^[11]
•  
  Tetracyclo[4,2,0,0^2,4,0^3,5] octa-7-ene has been prepared from benzvalene and isomerises to COT ^[12]
•  
  Pentacyclo[5.1.0.0^2,4. 0^3,5.0^6,8]octane (octabisvalene) is the third saturated valence isomer. The (Z)-3,7-phenylsulfonyl derivative is stable up to 200 °C.^[13]
•  
  Tricyclo[5.1.0.0^2,8]octa-3,5-diene (octavalene) was reported synthesised from homobenzvalene and converts to COT at 50 °C ^[14][15]

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

• The valence isomers of naphthalene
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  Naphthalene
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  Azulene

•   Media related to Valence isomer at Wikimedia Commons