Cycloheptene

Cycloheptene is a 7-membered cycloalkene. It is a raw material in organic chemistry and a monomer in polymer synthesis.

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. The double bond in the trans isomer is very strained. 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°. Because the barrier for rotation of the double bond in ethylene is approximetly 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 at very low temperatures 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. 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.