65 Kinds of dienes
Dienes have two or more carbon-carbon double bonds which may be eitherisolated(R’-
C=C-R-C=C-R’),cumulated(R-C=C=C-R), orconjugated(R-C=C-C=C-R). Each state
represents a different stability due to electron delocalization, and the conjugated form is
generally favored in organic molecules.
65.0.4 Isolated Dienes
Isolated dienes typically show no ”special” stability. Their twoπ-bonds interact independent
ofoneanotherandtheycompeteasintermolecularreactionsitesdependingonsteric, kinetic,
and thermodynamic factors. Often, these types of dienes can be treated merely as larger,
more complex alkenes, because the two bonds cannot be said to have ”communication”
(proximal electron delocalization due to location).
Isolated dienes are not considered to be especially useful in common synthesis reactions,
although intermediates may often take the form of an isolated diene.bb
65.0.5 Cumulated Dienes
Cumulated dienes are typically less stable than other alkenes. The main reason for the
instability is the fact that this sort of diene is a probable transition state for an alkyne’s
triple bond to move down the carbon chain towards the most stable position. As you may
recall, rotation does not occur around anyπ-bond, which means that cumulated double
bonds can lead to a less stable, higher energy compound being formed.
Typically, cumulated dienes are discussed only in advanced courses in organic chemistry,
and so they will not be discussed in detail here. Beginning organic chemistry students
should merely remember that cumulated dienes are 1) high energy and 2) most likely found
as transition states.
65.0.6 Conjugated Dienes
Conjugated dienes are dienes which have at least two double bonds separated by a single
carbon-carbon bond, and for this reason conjugated dienes are observed to have a special
stabilityduetotheoverlapofelectronorbitals. Theareasofconcentrationofnegativecharge
(electron density) overlap across the three bonds (two double bonds and one single bond)
forming what behaves essentially as a single, continuousπ-bond across three carbon atoms.
This delocalization of electron density stabilizes the molecule, resulting in the arrangement
of lowest energy.