Dewar benzene
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| Surname | Dewar benzene | |||||||||
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| Molecular formula | C 6 H 6 | |||||||||
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| Molar mass | 78.11 g mol −1 | |||||||||
| Physical state |
liquid |
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| As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions . | ||||||||||
Dewar benzene (also Dewar benzene ) is the common name for the compound bicyclo [2.2.0] hexa-2,5-diene, a valence isomer of benzene. The name describes the formula proposed by James Dewar for C 6 H 6 , which in his opinion was not benzene . Dewar himself considered the formula proposed by August Kekulé to be correct.
In 1962 a derivative of this basic structure could be produced for the first time , the following year the synthesis of pure Dewar benzene was successful . Because of the four-atom rings and the double bonds in them, it has a high ring tension and is therefore much more unstable than benzene. In addition, unlike benzene, it is not aromatic and shows two instead of one signal in the NMR spectrum .
Under normal conditions it isomerizes to benzene with a half-life of 37 hours. The transformation is kinetically inhibited: for the concerted ring opening, the Woodward-Hoffmann rules (preservation of the orbital symmetry ) require a conrotatory course (same direction of rotation of the CH bonds on the opening, central CC bond). It is irrelevant whether 4 or 6 electrons are included in the consideration:
However, a conrotatory ring opening would lead to a benzene structure with an endocyclic proton, which would correspond to a trans double bond in the six-membered ring. It is prevented by the ring tension that builds up, which explains the comparatively long half-life of Dewar benzene.
presentation
It is obtained by irradiating cis -1,2-dihydrophthalic anhydride and subsequent oxidation with lead (IV) acetate .
Further valence isomers of benzene
Individual evidence
- ↑ This substance has either not yet been classified with regard to its hazardousness or a reliable and citable source has not yet been found.
- ↑ Entry on Dewar benzene. In: Römpp Online . Georg Thieme Verlag, accessed on December 25, 2014.
- ↑ EE van Tamelen, SP Pappas, KL Kirk: Valence bond isomers of aromatic systems. Bicyclo [2.2.0] hexa-2,5-dienes (Dewar benzenes) . In: J. Am. Chem. Soc . tape 93 , no. 23 , 1971, p. 6092-6101 , doi : 10.1021 / ja00752a021 .