For years I told my students they shouldn’t merely memorize a list of reactions. But what were my actions really telling them?
In the last textbook I used, the alkene chapter began with nomenclature, then covered Markovnikov addition of H-X and water, halogenation and halohydrin formation, and ended with hydroboration/oxidation. The next chapter that covered alkynes follows a similar path: nomenclature, Markovnikov addition of H-X and water, hydroboration/oxidation, but finished with formation and reaction of the acetylide anion.
I always told my students (and still do) to look for patterns. Organic chemistry is more a pattern-recognition course than memorization or algorithm-based. But what patterns were my students supposed to see in these chapters? The common theme was “reactions of ____ functional group.” Alkenes do a bunch of reactions, but each new mechanism is different from the last. It looks like alkynes do some similar reactions, but their products don’t always follow the alkene pattern. Plus, the alkyne can be made into an SN2 nucleophile. Epoxides wouldn’t be formed until a later chapter.
It’s hard not to feel sympathy for the student who feels that memorizing is the only chance at survival.
Consider the same text from another perspective. The SN2 reaction is a reaction of a strong nucleophile with an electrophile containing a competent leaving group. As such, this text had an entire chapter dedicated to nucleophilic substitution. But, two chapters later, introduced four non-consecutive sections revising the Sn2 reaction. Another two chapters later, alkynes became SN2 nucleophiles after deprotonation. Twelve chapters later, enolates finally became SN2 nucleophiles. There is certainly a pattern to other textbooks, but not one conducive to predicting new reactions of alkenes.
For this reason, I very much appreciated Chapter 7 in Karty’s textbook: “An Overview of the Most Common Elementary Steps.” I’ll admit, I wasn’t sure about this chapter when my department switched texts, but it turned out to be the chapter—really more the payoff this chapter provided in later chapters—that convinced me to buy into Karty’s philosophy. Instead of looking at functional groups as a collection of atoms, functional groups have a personality. Narrowing nearly all of organic chemistry down to ten elementary steps unified my course.
Now, when we look at a new functional group, we characterize it by the small handful of elementary steps it can undergo. Comparing that to the handful of elementary steps the other reagent can undergo shows where the two reactivity patterns overlap, and provides insight into how the mechanism might work—even if the students have never seen this particular reaction before.
An alcohol no longer reacts one way with this reagent, and randomly some other way with some other reagent. An alkoxide can undergo acid/base as a base, SN2, E2, coordination, and nucleophilic addition. A tertiary alkyl halide can undergo heterolysis, or E2. The overlap shows the E2 to be a likely mechanism for this reaction. When an alkoxide reacts with an ester, it’s not so much a randomly different reaction as it is a different overlap of potential elementary steps.
Students no longer believe they have to memorize a list of reactions and examples from class to predict mechanisms. They can use their knowledge of the potential elementary steps to logically work through any novel scenario. They realize the course has a logic that they can understand if they integrate the elementary steps into their thought process.
-Adam Azman, Butler University
