Sequence of Reduction and Cycloaddition Laboratories for Understanding Stereochemistry and Thermodynamics in Organic Chemistry II

We have developed, tested, and introduced a sequence of two laboratory experiments intended to reinforce concepts of endo and exo products and approaches; reversible and irreversible reactions; and thermodynamic and kinetic products, cementing their importance in various topics of Organic Chemistry II. Two bicyclic ketone reductions introduce the possibility of generating epimers that 1H-NMR and GC-MS can quantify. Students deduce that the difference in stereoselectivity must come from the transition states of each reaction. GC-MS percentages can be used to assign 1H-NMR peaks and percentages, fortifying the predicted major products of the reactions. Next, a Diels-Alder cycloaddition performed at varying temperatures emphasizes the distinction between thermodynamic and kinetic conditions and products, while reinforcing the endo and exo topics of the previous week's experiment. This second experiment highlights the characteristics of a reversible reaction where 1H-NMR quantifies the thermodynamic and kinetic products for each reaction temperature. The introduction of the Karplus equation fortifies the relationship between dihedral angles and coupling constants, while allowing students to assign the correct signals for endo and exo Hs. Through these two experiments, students gain experience in several laboratory practices, computer modeling, computation, and product analysis.