Conclusion
A CC double bond is not rigid and inflexible. Significant
distortions of a double bond are possible without “fatal”
reduction in the strength of the π bond. This is due to
the pyramidalization of sp2 hybridized carbon atoms that accompanies
twisting. Far from being an esoteric point suitable
only for advanced students, this idea can easily be
explained to undergraduates. When introducing the topic of
alkene structure, one could reinforce the idea of a flexible
double bond by using a model set that demonstrates some
degree of twisting about the CC double bond is possible.
Then, simple reference to a diagram like Figure 4 will clearly
show students the increase in p-orbital overlap that accompanies
(and explains) pyramidalization in twisted alkenes.
Furthermore, measurements of OSE allow us to see explicitly
that the π bond is an ideal location for expressing strain.
Indeed, trans cycloalkenes smaller than cyclononene probably
could not exist if not for the geometric suppleness of
the CC double bond.