: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.

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

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 {{cite journal|last1=Huisgen|first1=R.|last2=Mietzsch|first2=F.|title=The Valence Tautomerism of Cyclooctatetraene|journal=Angewandte Chemie International Edition in English|volume=3|pages=83|year=1964|doi=10.1002/anie.196400831|issue=2}}Bicyclo[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}}{{cite journal|last1=Huisgen|first1=Rolf.|last2=Konz|first2=Will E.|last3=Gream|first3=George E.|title=Evidence for different valence tautomers of bromocyclooctatetraene|journal=Journal of the American Chemical Society|volume=92|pages=4105|year=1970|doi=10.1021/ja00716a048|issue=13|bibcode=1970JAChS..92.4105H }}

Image:Tricyclo33006octa38diene.svg|Tricyclo[3,3,0,0^2,6]octa-3,8-diene. Isomerises to semibullvalene at room temperature, stable at −60 °C {{cite journal|last1=Meinwald|first1=Jerrold|last2=Tsuruta|first2=Haruki|title=Tricyclo[3.3.0.02,6]octa-3,7-diene|journal=Journal of the American Chemical Society|volume=91|pages=5877|year=1969|doi=10.1021/ja01049a034|issue=21|bibcode=1969JAChS..91R5877M }}{{cite journal|last1=Meinwald|first1=Jerrold|last2=Schmidt|first2=Douglass|title=Semibullvalene from tricyclo[3.3.0.02,6]octane|journal=Journal of the American Chemical Society|volume=91|pages=5877|year=1969|doi=10.1021/ja01049a033|issue=21|bibcode=1969JAChS..91Q5877M }}{{cite journal|last1=Zimmerman|first1=Howard Elliot|last2=Robbins|first2=Jeffrey D.|last3=Schantl|first3=Joachim|title=C8H8 interconversions. An unusual rearrangement providing a new route to semibullvalene|journal=Journal of the American Chemical Society|volume=91|pages=5878|year=1969|doi=10.1021/ja01049a035|issue=21|bibcode=1969JAChS..91.5878Z }}{{cite journal|last1=Meinwald|first1=Jerrold.|last2=Tsuruta|first2=Haruki.|title=(CH)8 hydrocarbons. Photochemistry of tricyclo[3.3.0.02,6]octa-3,7-diene|journal=Journal of the American Chemical Society|volume=92|pages=2579|year=1970|doi=10.1021/ja00711a078|issue=8|bibcode=1970JAChS..92.2579M }}

Image:Cyclobutadiene dimer cis trans.svg|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 Untersuchungen in der Cyclobutanreihe, XII. Zwei stereoisomere Dimere des Cyclobutadiens Margarete Avram, Ilie G. Dinulescu, Elise Marica, Georg Mateescu, Elvira Sliam, Costin D. Nenitzescu Chemische Berichte Volume 97, Issue 2, pages 382–389, February 1964 {{doi|10.1002/cber.19640970210}}

Image:Tetracyclooctaene.svg|Tetracyclo[3,3,0,0^2,4,0^3,6] octa-7-ene is only known as its 4-carbomethoxy derivative.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}}

Image:Tetracyclooct-7-ene.svg|Tetracyclo[4,2,0,0^2,4,0^3,5] octa-7-ene has been prepared from benzvalene and isomerises to COT Synthesis and reactions of tetracyclo[4.2.0.0^2,4.0^3,5]octanes Leverett R. Smith, George E. Gream, Jerrold Meinwald J. Org. Chem., 1977, 42 (6), pp 927–936 {{doi|10.1021/jo00426a001}}

Image:Octabisvalene.svg| 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.(Z)-3,7 Bis(phenylsulfonyl)pentacyclo[5.1.0.02,4.03,5.06,8]octane, an Octabisvalene Derivative (1985) Angewandte Chemie International Edition in English Volume 24, Issue 5, pages 411–412 {{doi|10.1002/anie.198504111}}

Image:Octavalene.svg| Tricyclo[5.1.0.0^2,8]octa-3,5-diene (octavalene) was reported synthesised from homobenzvalene and converts to COT at 50 °C 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}}Electronic structure of octavalene. Photoelectron spectroscopic investigations Rolf Gleiter, Peter Bischof, Manfred Christl J. Org. Chem., 1986, 51 (15), pp 2895–2898 {{doi|10.1021/jo00365a007}}

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

File:Oxepin-benzene oxide.png exists in dynamic equilibrium with its valence isomer oxepin.{{cite journal | doi = 10.1002/anie.196703851 | journal = Angewandte Chemie International Edition in English | title=Benzene Oxide-Oxepin Valence Tautomerism| volume=6 |issue=5 |pages =385–401 |author=E. Vogel, H. Günther | year = 1967}}]]

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Category:Isomerism