Ten Elementary Steps Are Better Than Four

Mechanisms can greatly simplify learning organic chemistry because the hundreds of reactions that students need to know have mechanisms that are constructed from just a handful of distinct elementary steps. This is easy for us professors to see—after all, we’ve been through the year’s reactions and mechanisms multiple times. Students, on the other hand, must be convinced of this at the outset if we want them to commit to learning mechanisms, at a point when memorizing reactions might seem so attractive. Hence, one of the main purposes of Chapter 7 in my textbook, which breaks down the most common elementary steps into these ten:

• Proton transfer
• S_N2
• Bond forming (coordination) and its reverse, bond breaking (heterolysis)
• Nucleophilic addition and its reverse, nucleophile elimination
• E2
• Electrophilic addition and its reverse, electrophile elimination
• Carbocation rearrangement

The above system is not the only way to distinguish the common elementary steps. Another popular system is to condense them to the following four:

• Proton transfer
• Nucleophilic attack
• Loss of a leaving group
• Rearrangement

This system of four elementary steps is more streamlined, certainly, but for students in an introductory organic chemistry course, I believe it is much better to keep the common elementary steps divided into ten distinct ones rather than four. Why?

The answer is concreteness. In Chapter 7 of my textbook, students learn that each of the ten elementary steps: (a) involves characteristic “major players” as reactants, and (b) has a specific way in which the curved arrow notation should be drawn. With this in mind, consider the coordination, nucleophilic addition, and electrophilic addition steps shown below.

Students learn that, on the reactant side of a coordination step, the electron rich species has an atom with a lone pair and the electron-poor species has an atom lacking an octet. Students further learn that a single curved arrow is drawn from the lone pair to the atom lacking an octet. In a nucleophilic addition step, the electron-poor site is at the less electronegative atom of a polar  bond rather than an atom lacking an octet, and two curved arrows are drawn: the first from the lone pair to the less electronegative atom of the polar  bond, and the second from the center of the polar  bond to its more electronegative atom. In an electrophilic addition step, the electron-rich species has a nonpolar  bond rather than an atom with a lone pair, and the curved arrow originates from the center of that  bond.

The concreteness in these distinctions is important because it gives students something to hang their hats on when deciding the next step of a multistep mechanism. When a student next encounters a scenario in which a species that has either an atom with a lone pair or a nonpolar  bond is present alongside a species that has either an atom lacking an octet or a polar  bond, he or she can quickly determine which of the ten steps is available by first characterizing the “major players” present. Then the student can add the curved arrow notation associated with that elementary step and follow the curved arrows to arrive at the products.

Now consider the ambiguity that students would have to deal with in this situation if they learned just four types of elementary steps. Coordination, nucleophilic addition, and electrophilic addition steps (three distinct steps in my book) would be indistinct under that system, all treated as nucleophilic attack. Therefore, the student would first have to ponder which type of nucleophile is present—one having an atom with a lone pair or a nonpolar  bond. The student would also have to ponder what type of electrophile is present—one having an atom lacking an octet or a polar  bond. Then, students would need to decide how to add curved arrows because it differs from one type of nucleophilic attack to another: How many curved arrows; where should each curved arrow begin and where should it point?

Under the system of four distinct elementary steps, another problem arises: some elementary steps are described as a combination of two steps taking place simultaneously. The S_N2 step, for example, is described as a simultaneous nucleophilic attack and loss of a leaving group. The E2 step is described as a simultaneous proton transfer and loss of a leaving group. Not only does this add to the ambiguity that already exists, but it also sends a dangerous message to students that it’s okay to combine elementary steps to arrive at new, more complex ones. Indeed, combining elementary steps is sometimes reasonable (we can find a good number of other examples), but I don’t think it’s a good idea to give this kind of license to students at the time they are just beginning to learn about elementary steps and mechanisms. Early in the course, students don’t have the judgment to determine when it is reasonable to combine elementary steps, so if we give students that liberty, we can expect them all too frequently to make up elementary steps that are beyond reasonable.

Students by and large enter organic chemistry equating learning with memorizing, so they are at a crossroads when they first see mechanisms alongside reactions. Mechanisms will at first appear to be extra information that can be ignored, which makes it really important for us, as educators, to convince students very early on that mechanisms do indeed simplify learning organic chemistry, and that a commitment to learning mechanisms is worth it. Providing an overview of the small number of common elementary steps up front is key, particularly in a way that removes ambiguity—as ten distinct elementary steps rather than four.

-Joel Karty

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