Understanding Rotation about a C=C Double Bond
Many organic textbooks (1) indicate that 90 twisting
of a CC double bond results in breakage of the π bond
and formation of a diradical product. In other words, “for
rotation to occur around a double bond, the π bond must
break and re-form” (1a). Perhaps it is due to pictures like
Figure 1 (1a), which appear with minor variations in many
organic textbooks, or perhaps it is a result of working with
rigid plastic or wooden model sets that our students (and
maybe some organic instructors, too) believe the CC π
bond to be rigid and incapable of accommodating any flexing
and twisting. However, the twisted and pyramidalized
nature of the double bond in trans-cyclooctene has been
known since 1975 (2). This article focuses on the process and
energetic cost of twisting around a CC double bond. As a
result of this work, and an increased familiarity with the literature
published on this topic, we have made personal adjustments
in our mental picture of the flexibility of CC
double bonds.