Cycloheptene is a 7-membered cycloalkene. 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.
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Image:Cis-cycloheptene3D.png|cis-Cycloheptene
Image:Trans-cycloheptene3D.png|trans-Cycloheptene
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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..[1] The double bond in the trans isomer is very strained.[2] 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°..[1] 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.[1] Based on the experimentally observed second order reaction kinetics for isomerization, two trans-cycloheptene molecules in the proposed pathway first form a diradicaldimer. 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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