| cis-Cycloheptene[1] | |
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(Z)-Cycloheptene |
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Other names
cis-Cycloheptene |
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| Identifiers | |
| CAS number | 628-92-2 |
| PubChem | 12363 |
| ChemSpider | 11857 |
| Jmol-3D images | Image 1 |
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| Properties | |
| Molecular formula | C7H12 |
| Molar mass | 96.17 g/mol |
| Density | 0.824 g/cm3 |
| Boiling point |
112-114.7 °C |
| (verify) (what is: /?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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| Infobox references | |
Cycloheptene is a 7-membered cycloalkene with a flash point of -6 C°. It is a raw material in organic chemistry and a monomer in polymer synthesis. Cycloheptene can exist as either the cis- or the trans-isomer.
| cis-Cycloheptene | trans-Cycloheptene |
With cycloheptene, the cis-isomer is always assumed but the trans-isomer does also exist. One procedure for the organic synthesis of trans-cycloheptene is by singlet photosensitization of cis-cycloheptene with methyl benzoate and ultraviolet light at -35°C.[2] The double bond in the trans isomer is very strained.[3] Part of the steric strain is relieved by pyramidalization of the alkene carbons, the pyramidalization angle is estimated at 37° (compared to a zero angle in an unstrained alkene) and the p-orbital misalignment is 30.1°.[2] Because the barrier for rotation of the double bond in ethylene is approximately 65 kcal/mol (270 kJ/mol) and can only be lowered by the estimated strain energy of 30 kcal/mol (125 kJ/mol) present in the trans-isomer, trans-cycloheptene should be a stable molecule just as its homologue trans-cyclooctene. In fact, it is not; and unless the temperature is kept very low, quick isomerization to the cis-isomer takes place. In a 2005 publication, it is argued that trans-cycloheptene isomerization occurs by an alternative lower energy pathway.[2] Based on the experimentally observed second order reaction kinetics for isomerization, two trans-cycloheptene molecules in the proposed pathway first form a diradical dimer. The two heptane radical rings then untwist to an unstrained conformation and in the final step the dimer is cleaved again into two cis-cycloheptene molecules. Note that the photoisomerization of maleic acid to fumaric acid with bromine is also bimolecular.
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