We have written a handful of blog posts that call attention to the benefits of Chapter 7, which introduces students to the 10 common elementary steps involving closed-shell species. Because of Chapter 7, students are exposed to the complete set of the elementary steps that make up the mechanisms for all reactions encountered through Chapter 23. In Chapter 7, students see how these elementary steps function and they become familiar with each step’s curved arrow notation before being held accountable for more complex mechanisms, predicting products of reactions, or incorporating those reactions in synthesis. This strategy works so well that I decided to use the same strategy in Chapter 25, which deals with radicals and radical reactions.
Toward the beginning of Chapter 25, the common elementary steps that involve radicals are introduced, along with their curved arrow notation, before students work with radical reactions to greater depth. Those four elementary steps are: (1) homolysis, (2) radical coupling, (3) bimolecular homolytic substitution (SH2), and (3) radical addition, examples of which are shown below.
Just as in Chapter 7, the emphasis of this presentation in Chapter 25 is on becoming familiar with both the curved arrow notation tied to each elementary step, and with the species involved. Moreover, when I teach these four elementary steps in class, I find it particularly helpful to show students how each elementary step involving free radicals compares to its closed-shell analog from Chapter 7, as shown below.
Homolysis involves the breaking of a sigma bond, analogous to heterolysis. In radical coupling, a sigma bond is formed, similar to what we see in coordination. One radical displaces another in an SH2 step, and similarly a nucleophile displaces a leaving group in SN2. Finally, radical addition and electrophilic addition both have electrons from a pi bond used to form a new bond to an electron-poor species.
By highlighting these comparisons between radical and closed-shell elementary steps, my aim is to give students a feeling of familiarity before delving further into mechanisms and reactions involving the new elementary steps. Perhaps more importantly, I want students to see that, even though we are dealing with new material, there are patterns to be found when maintaining focus on mechanisms. And just as we see in other parts of the textbook, mechanistic patterns simplify the chemistry.