Chapter 7: Elementary Steps but Giant Conceptual Leaps

If writing mechanisms is like giving good directions, then each elementary step is similar to saying “turn left at the stop sign.”  You might have to turn right many times during one trip just as you might need multiple acid-base steps during one mechanistic pathway. Joel’s “Most Common Elementary Steps” chapter lays out each possible direction you might give when directing electron flow in the transformation of the starting material into the product. At this stage, most students won’t have the ability (or confidence) to write complex multistep mechanisms. However, the chapter provides a valuable framework and comprehensive summary of the key steps for writing mechanisms for polar reactions.

While Chapter 7 may at first seem daunting to students, by continually emphasizing the names of the elementary steps, the instructor can simplify what might be a very complex process into a series of simple steps. And by reminding students that they don’t need to master all nine steps right now, much of the pressure is lifted. I’m beginning to use the chapter in ways that cause my students to ask great questions—a sign they are understanding.

On day one of Chapter 7, I start with the “Pillars of Organic Chemistry” (JCE, 2008, 83-87) and map the skills that we have already covered. Next, I introduce the general types of reactions we will consider:

  • Radical (homolytic, homogenic steps)
  • Pericyclic
  • Polar (heteorgenic-heterolytic steps)
    • Acid- Base
    • Substitution
    • Elimination
    • Addition

I explain that polar is most common and where we will focus our energy. Next I give the students a worksheet with two to four examples per elementary step. We write mechanisms and predict products in groups during class. My examples include times when one elementary step leads into another as well as when one step is sufficient.

Karty’s powerpoint scaffold on how to use the Brønsted acid-base provides examples in an efficient manner, so I won’t repeat that, but I do use this as a chance to introduce Grignards, alkyl lithiums, and cuprates as a means to talk about ionic and covalent bonds, concerns of protic solvents, and spectator ions. Finally, I use hydrides (LiH and LAH) to talk about electronegativity (yes, Virginia, H can be negative!), octets, and formal charges.

Next I move to Lewis acid-base coordination as a clear way to define nucleophiles and electrophiles more broadly.

Under bimolecular substitution reactions, we explore carbons whose octets are complete acting as electrophiles as well as what makes something a good leaving group by introducing tosylates and mesylates. This is a good time to review other weak bases. The color tagging (red for nucleophiles and blue for electrophiles) is a particularly helpful tool in sections 7.1-7.3. It mimics electrostatic potential maps as well as emphasizes that seemingly dissimilar structures can function in the same manner (i.e. as the nucleophile).

On day two of Chapter 7, I start with heterolysis, highlighting bond polarity, carbocation stability, and how activation steps can lead to heterolysis. A natural next step is carbocation rearrangement as means to increase stability by resonance or inductive effects. Bimolecular elimination can be introduced as a competitor for bimolecular substitution when there is steric hindrance on the part of the electrophile and the nucleophile.

While the remaining topics are introduced, they are covered with less intensity (though I do emphasize how to recognize addition vs. substitution vs. elimination, as well as one step vs. multisteps).

Here are the kinds of questions I’ve been getting in class: “Why do nucleophiles add to ketones but add, then eliminate with acid chlorides?” “Why do we see electrophilic addition to alkenes and alkynes but addition followed by elimination with aromatics?” And “Why doesn’t the halide act as the nucleophile in the Grignard reaction? These are big questions; it’s a credit to Karty’s straightforward approach that my students are asking (and answering) these questions already.

Good questions indicate that my students are seeing trends, are curious as to how to make choices, and are truly trying to understand concepts rather than trying to memorize. This will allow them to develop problem-solving skills and ultimately makes the mechanistic approach a more interesting and valuable way to teach organic chemistry than the usual “march of the functional groups.”

-Tara Kishbaugh, Eastern Mennonite University

Click here to learn more about Prof. Kishbaugh.

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