Olefin Strain Energy in trans-Cycloalkenes
With the geometric similarity established between the
twisted trans-2-butene model and the trans cycloalkenes, we
felt that the model could be used to approximate the contribution
to the strain energy caused exclusively by the trans
CC double bond in a series of trans cycloalkenes. That is,
we will use the 2-butene model to isolate the strain owing to
twisting alone. Then, by subtracting this from the total strain
energy in cycloalkenes, we can determine the proportion of
the strain that comes from twisting as opposed to other
sources.
Four nonideal geometric consequences have been calculated,
and in some cases measured experimentally, in the trans
cycloalkene isomers of medium-sized rings. These geometric
distortions are necessary to allow the two ends of the double
bond to be reconnected to one another through the available
(CH2)n tether when n < 8:
1. Nonoptimal bond lengths (both shortening of some
and lengthening of other bonds);
2. Expansion or contraction of sp3 carbon-centered bond
angles;
3. Twisting (torsion) about the CC double bond; and
4. Pyramidalization of the sp2 hybridized carbons.