Conjugated Systems

Overview:

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.