trans-Cyclooctene
trans-Cyclooctene is a cyclic hydrocarbon with the formula [–(CH2)6CH=CH–], where the two C–C single bonds adjacent to the double bond are on opposite sides of the latter's plane. It is a colorless liquid with a disagreeable odor.
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| Preferred IUPAC name
(E)-Cyclooctene | |
| Other names
trans-Cyclooctene | |
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| Properties | |
| C8H14 | |
| Molar mass | 110.200 g·mol−1 |
| Appearance | colorless liquid |
| Density | 0.848 g/mL |
| Melting point | −59 °C (−74 °F; 214 K) |
| Boiling point | 143 °C (1 atm); 68-72 °C (100 torr) |
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Cyclooctene is notable as the smallest cycloalkene that is readily isolated as its trans-isomer. The cis-isomer is much more stable; the ring-strain energies being 16.7 and 7.4 kcal/mol, respectively.
| cis-Cyclooctene in chair conformation | (Rp)-trans-Cyclooctene in crown conformation |
A planar arrangement of the ring carbons would be too strained, and therefore the stable conformations of the trans form have a bent (non-planar) ring. Computations indicate that the most stable "crown" conformation has the carbon atoms alternately above and below the plane of the ring. A "half-chair" conformation, with about 6 kcal/mol higher energy, has carbons 2,3,5,6, and 8 on the same side of the plane of carbons 1,4, and 7.
All conformations of trans-cyclooctene are chiral (specifically, what some call planar-chiral) and the enantiomers can be separated. In theory, conversion of between the enantiomers can be done, without breaking any bonds, by twisting the whole –CH=CH– group, rigidly, by 180 degrees. However, that entails passing one of its hydrogens through the crowded ring.