Reasoning By Analogy

For twelve years I’ve taught organic chemistry to a mixture of chemistry and biology students. I always begin Organic I by asking my students this same question: Why are you taking this class? Some students respond that the curriculum plan for their major or career requires the organic chemistry course sequence. For other students, organic chemistry is a hoop through which they must jump in order to prove to admissions boards that they have the ability to succeed in their chosen graduate or professional programs. Many students just shrug their shoulders—they, in fact, honestly don’t know the answer. In response, I tell my students that the universe is filled with countless molecules that can undergo innumerable reactions. I tell them that, even so, this overwhelming multitude can be organized by a rational set of structural and reactivity principles. And I tell them that this organizational framework has power, both descriptive and predictive. I tell them that, no matter the reason they are taking organic chemistry, they are here, in part, to learn to reason by analogy, to learn to recognize, to explain, and to extend patterns.

Most organic chemistry textbooks help students develop reasoning-by-analogy skills through the use of functional groups as an organizational framework. With such texts, students march through a chapter on haloalkanes, a chapter on alcohols, a chapter on ethers, and so on. While I’ve used such texts for most of my career, I’ve often been frustrated with them. Why? Because with these texts students tend to overlook the analogy that haloalkanes, alcohols, and ethers are unified by their ability to undergo nucleophilic substitution and elimination reactions. They think that the chapters on alcohols and ethers present something new from the chapter on haloalkanes, rather than something similar. Instead of seeing the connections between these functional group classes, they see more reactions. And I have found that this often discourages them.

That’s why I appreciate Karty’s use of mechanism class, rather than functional group, as an organizational framework. In Chapter 10, for example, students see how a wide range of functional groups can react by nucleophilic substitution and elimination pathways in synthetically useful ways. Students learn to look for molecules that are potential nucleophiles, as well as molecules that have a good leaving group (or a site that can be easily turned into one). Students are then readily able to predict the outcomes of possible nucleophilic substitution or elimination reactions using concepts developed in earlier chapters. This year, instead of seeing the reactions of haloalkanes and alcohols, even enolates, separated from each other as functional groups, students saw them presented together as a mechanism class. The result? I saw more “Aha!” moments, less discouragement, and a greater willingness to venture into the unknown using the skills that they had developed.

For example, every year, after addressing the key concepts of nucleophilic substitution and elimination as they pertain to haloalkanes, I like to ask my students to predict the product and mechanism of the reaction of an alcohol with a hydrohalic acid. This year, after reading Karty’s text, students were more willing (and better able) to extend the concepts they had just learned, and I saw more confident and knowing smiles from them when I drew the answer on the board. They had recognized that, after turning the hydroxyl group into a good leaving via a proton transfer from the hydrohalic acid, this was simply another example of a nucleophilic substitution reaction!

I tell my students on the first day of class that I don’t care if twenty years from now they no longer remember that a bimolecular nucleophilic substitution reaction proceeds with the approach of the nucleophile from the side opposite of the leaving group, but I do care deeply that they have the ability to recognize patterns and the courage to extend those patterns. (Well, at least those students who don’t intend on becoming practicing chemists!) In large part, my students intend on becoming health, physical, and biology scientists. They will be working on the frontiers where knowledge is created, and their success will, in part, be determined by their ability to recognize something familiar in the unknown and then to reason by analogy. In my mind, a foundational purpose of organic chemistry is to develop this skill, and, in my opinion, a mechanistically-organized text like Karty’s offers a better framework in which to do so.

— Brad Chamberlain, Luther College

Brad Chamberlain teaches at Luther College and is currently class-testing the Preliminary Edition of Joel Karty’s forthcoming textbook. Click here to learn more about Prof. Chamberlain.

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