No doubt one of the greatest benefits of teaching a mechanistic organization is the opportunity afforded to students to see patterns among mechanisms—patterns that we experts know and value, but are challenging for students to see under a traditional functional group organization. For example, as I described in my previous post, Why a Mechanistic Organization?, the following four reactions would appear in four different chapters in a textbook organized according to functional group, simply because they involve four different functional groups. But in my textbook, they all appear together because they undergo the same mechanism.
By offering students better opportunities to see mechanistic patterns like this, students will better value mechanisms as tools that actually simplify organic chemistry, and will therefore more likely make up their minds early in the course that mechanisms are worth the while.
Organic chemistry is even further simplified when students understand reactions—something that goes beyond recognizing the above kinds of patterns among sequences of elementary steps. This is where a mechanistic organization comes in yet again to offer a tremendous benefit—what I call patterns of concepts.
Consider, for example, the following three reactions, all three of which are presented in my textbook in the context of nucleophilic addition–elimination reactions (Chapters 20 and 21):
In the first of these reactions, the base-promoted hydrolysis of an amide, students are taught that the immediate product of nucleophilic addition–elimination (the carboxylic acid produced in step 2 below) is less stable than the reactant, but the reaction is facilitated by the ensuing irreversible deprotonation that takes place under the basic conditions of the reaction (step 3).
To help make sense of this idea, students are taught that the energy profile for such a process looks something like this:
Shortly afterward, students encounter the haloform reaction (the second of the above reactions), where precisely the same idea is given to explain the formation of products. Namely, the formation of the carboxylic acid from the tribromomethyl ketone (steps 7 and 8 below) is unfavorable, but the deprotonation that follows (step 9) is irreversible, thereby facilitating the reaction.
More to the point, the above energy diagram for the base-promoted hydrolysis of an amide applies equally well to these three steps of the haloform reaction.
Finally, students encounter the Claisen condensation reaction (the third reaction above). Once again, to understand the reaction, students are taught that the immediate product of nucleophilic addition–elimination (the Beta-keto ester produced in step 3 below) is unfavorable, but the reaction succeeds as a result of the ensuing irreversible deprotonation (step 4).
And again, the above energy diagram applies.
This concept of a final irreversible proton transfer driving an otherwise unfavorable reaction appears multiple times within the context of one reaction type. Consequently, similar to how a mechanistic organization helps students see that mechanistic patterns simplify learning mechanisms, it also helps students see that patterns with concepts simplify understanding reactions—something that is much more difficult to see under a functional group organization, where the reactions don’t appear together. Thus, under a mechanistic organization, students are more encouraged not only to devote their time to studying mechanisms, but also to learning and applying concepts.
In other words, students are better able to think like chemists, and are rewarded for doing so.
-Joel Karty, Elon University, Author.
Photo credit: brewbooks/Flickr
