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The study of Conjugated Systems is often combined with some other chapter in textbooks, but is treated
separately, here. This allows for its placement at any point needed.
The topics addressed are types of unsaturated systems, preparations
of conjugated systems, as well as reactions, the impact of conditions,
and the molecular orbital picture.
After a discussion of the types of dienes that can exist, the unit
focuses on the characteristics of conjugated systems and notes that
reactions in general are 1,2 and 1,4 addition reactions. The conditions
which favor 1, 2 vs. 1,4 additions are addressed in light of activation
energies and energy diagrams.
Pictorial representations of the bonding orbital concept for conjugated
systems are used to facilitate understanding of constructive and
destructive overlap, in-phase and out-of-phase overlap, and the
formation and identification of nodal planes. The designations of
LOMO (lowest occupied molecular orbital), HOMO (highest occupied
molecular orbital), LUMO(lowest unoccupied molecular orbital), and
HUMO (highest unoccupied molecular orbital) are explained and ranked
in energy. The multicolored diagrams as well as the explaining narration
help the organic student to understand the model used to represent
these occupied and unoccupied molecular orbitals and their relative
energies. No longer will the organic student see conjugated dienes
as static bonding systems, rather they will see them reflect the
dynamic, interactive character these conjugated systems demonstrate
in the laboratory.
Students frequently have difficulty understanding why certain multi-cyclic
structures containing conjugated dienes cannot undergo Diels-Alder
reactions, yet others can. The concept of s-trans and s-cis structures
and their positional orientations are presented. Following that,
the concept of symmetry allowed and symmetry disallowed reactions
is used to introduce the need for light rather than heat for certain
Diels-Alder reactions to be successful. Students sometimes have
difficulty understanding the need to specify heat or light in these
reactions, but when they see the results that occur when a conjugated
diene system is treated with light relative to these discussions,
it becomes clear. All of this is addressed using the molecular orbital
picture and extensive narration. The idea that pericyclic reactions
that may be thermally disallowed but photochemically allowed is
introduced and explained. Throughout this discussion, examples are
used to enhance and explain the concepts.
Although this unit is treated separate and apart from other units
in the program, it is recommended that the student have a good understanding
of alkene structure at the very minimum before embarking on this
study. Having covered alkynes will only enhance the understanding.
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