Trans geometry cycloalkenes

Compare geometries of the cis and trans cycloalkenes. Are the double bonds incorporated into the trans compounds significantly more distorted than those incorporated into the analogous cis cycloalkenes Consider carbon-carbon bond lengths and the twisting and/ or puckering of the double bond. Are any distortions greater in trans-cycloheptene than in trans-cyclooctene ... 

Trans Cycloalkenes Another difference between cyclic and acyclic alkenes is the relationship between cis and trans isomers. In acyclic alkenes, the trans isomers are usually more stable but the trans isomers of small cycloalkenes are rare, and those with fewer than eight carbon atoms are unstable at room temperature. The problem with making a trans cycloalkene lies in the geometry of the trans double bond. The two alkyl groups on a trans double bond are so far apart that several carbon atoms are needed to complete the ring. 

Cycloalkenes having fewer than eight carbon atoms have a cis geometry. A trans cycloalkene must have a carbon chain long enough to connect the ends of the double bond without introducing too much strain. /rani-Cyclooctene is the smallest, isolable trans cycloalkene, but it... 

In 1972, Mock considered double-bond reactivity and its relationship to torsional strain, by which he understood the strain imposed on a double bond in medium-ring fra 5-cycloalkenes or by steric compression of large cis substituents [28]. He argued that the loss of 7t overlap due to a torsion about the double bond can be partially compensated by rehybridization in these two situations, leading, respectively, to syn and anti pyramidalization of the double bond consequently, such bonds will favor different modes of addition (cis and trans). The proposition was supported by examples of X-ray structures of strained olefins, STO-3G energy calculations for the twisted and pyramidalized ethylene geometries, and by analysis of the out-of-plane vibrational frequencies of ethylene. Mock concluded that small ground-state distortions may produce sizable effects in the transition states. 

The geometry of a carbon-carbon double bond can also be distorted in monocyclic compounds. Cycloalkenes with a trans double bond show significant deviations from normal bond angles when the rings are small. Nevertheless, frans-cyclooctene (33) has been isolated, and trflws-cyclohep-tene and fra s-cyclohexene have been implicated as reaction intermedi-ates. ° ° Even cis double bonds may be highly strained in very small rings. Billups and Haley obtained evidence for the existence of spirocyclopenta-diene (34) as a transient species whose existence was confirmed by trapping with cyclopentadiene. °... 

The required anti and coplanar transition state geometry can also be used to predict the regiochemistry of E2 elimination in halocyclohexanes such as chlorocyclohex-anes. In these molecules, anti and coplanar correspond to trans and diaxial. Consider the E2 reaction of the cis isomer of l-chloro-2-isopropylcyclohexane.The major product is 1-isopropylcyclohexene, the more substituted cycloalkene. 

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