All Downhill After the Aldol

Ah yes, it is that time of year again; a seven week coast to the Organic Chemistry II finish line. All the elementary steps have been introduced and discussed multiple times and it feels like review from here. Most texts seem to end with reactions like Aldol condensation and Robinson Annulation. A functional group approach to organic chemistry forces these key carbon-carbon bond forming reactions to be at the end of the second semester because they do not create a simple functional group like an alkene, aldehyde, or an amine. This leaves little time for them to be reviewed via retrosynthetic analysis problems in future chapters. How are chemists going to be appropriately trained when all C-C bond forming reactions are forced to be covered last? How are we to make anything interesting without access to them?  Joel’s text doesn’t create this problem.

With a traditional approach, carbon-carbon bond forming reactions are often the most difficult for students to master. This is not because of intrinsic difficulty, it is because the way they are taught does not follow the pattern of other material in a text. In addition, they are found at the end of a text and I’ve noticed that students tend to think that what is covered last must be the most difficult. When Karty chose to place Aldol reactions in the midst of less clunky nucleophilic addition reactions, he made students less fearful of drawing products that may take two or three linear inches of notebook space. Just because the final product takes twice as much real estate as one of the starting materials does not mean it should be feared. Instead it should be celebrated; organic chemists are architects on a molecular level. Students are now closer to being able to make molecules of practical significance. By the end of March, my students can completely understand the following relevant multistep synthetic transformation. Below are the synthesis of avobenzone, a chief component in sunscreen, and benzoyl peroxide, the active ingredient in acne medication:

Organic chemistry is now more real to the students and has a concrete implied purpose, besides being a steeplechase barrier between them and medical or graduate school. This is where organic chemistry gets exciting. I have nearly two months to build on this excitement and to hopefully leave a lasting impression of the importance of synthetic chemistry. Since all the heavy lifting is done, the mechanisms move to the background as a review of elementary steps.

After Aldol, it’s all downhill. The most profound reaction in Chapter 21 is a Claisen condensation, and if a student can do an Aldol they can certainly do a Claisen. Instead of a nucleophilic addition reaction, they just need to do an addition elimination reaction. Chapters 22 and 23 provide a nice break by teaching electrophilic aromatic substitution. We breeze through these chapters as EAS is merely electrophilic addition followed by electrophilic elimination. Both of these elementary steps have already been seen many times in previous chapters. EAS gives students a breather as we hit new Diels-Alder material in Chapter 24. Chapter 25 is radicals. I ask students if they are tired of moving electrons. They jokingly say yes and I tell them they are in luck because now we only have to move one electron at a time. It is not less work but it is refreshing to learn something a little different at the end of the semester.

Using a mechanistic approach makes me look forward to the last few weeks of Organic II compared to when I used other approaches. Instead of fighting to the end, I now feel like I glide to the finish line.

-James Wollack, St. Catherine University
Click here to learn more about Prof. Wollack

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