Using porphyrins to develop comprehensive visual-evidence-based teaching of core organic chemistry

We are undertaking a long-term project to provide comprehensive visual, photographic and videographic supporting evidence for the concepts, reaction outcomes and reaction mechanisms covered in core Organic Chemistry courses. The brilliant colors and chelating ability of porphyrins make them valuable teaching tools for these courses. Four demonstrations that illustrate their pedagogical value are presented. In all cases, the demonstration is ”layered,” meaning that it can be repeated and extended to reinforce later material within the course. In addition, each experiment is designed to meet one or more of the following criteria: A. It demonstrates a process of historical importance. B. It illustrates a process or concept that has real-world applications. C. It should have pedagogical value in other disciplines. Experiment 1 provides visual evidence for the existence of antibonding orbitals by using chlorophyll fluorescence. It can later be used to introduce the concept of photosynthesis. Experiment 2 demonstrates porphyrin chelation as an example of a one-pot Brønsted acid-base and Lewis acid-base reaction. The visual impact of the experiment was maximized by using a divided U-tube. Experiment 3 extends Experiment 2 into a competitive metallation study that illustrates the distinction between kinetic and thermodynamic control. Experiment 4 demonstrates the existence of π-cation radicals. All four experiments can also be carried out as real-time demonstrations. These experiments clearly illustrate the pedagogical value of porphyrins as visual tools in the core Organic Chemistry curriculum.