A number of years ago I had a student come to me at the end of Organic II and ask, “What happened to the SN2 reaction?” She wanted to know why we had spent so much time on this one reaction in order to move on to the next unit and then never discussed this reaction again. I found this to be common when I organized my course by functional group. Students viewed each reaction as a unique mechanism and did not see the common elementary steps that comprise organic reaction mechanisms.
I recently saw the Royal Birmingham ballet perform during the Virginia Arts Festival. The dancers moved with such grace in what appeared to be effortless and different motions. Having taken ballet as a child, I know the work the dancers put into their art. Ballet classes start with fundamentals at the barre, and ballet choreography incorporates these fundamental steps over and over. Like organic chemistry, in ballet, one must practice the fundamentals and understand the basics in order to perform well. I use the same “moves” over and over throughout my organic course. How do we get students to recognize these patterns and use these same moves? I believe a key component is the order that we teach the material, and by explicitly pointing out to students the common ties in the mechanisms.
One of the first reactions covered in Organic I is SN2. I spend at least a week covering all of the aspects of this reaction—leaving group and nucleophile strength, rates of the reaction, regiochemistry, and stereochemistry.
When I organized my course by functional group, the next chapter went on to discuss all properties, reactions, and synthesis of alkenes. Alkynes came next, followed by alcohols, etc. There was no emphasis on common steps between all reaction mechanisms. The material was combined in such a way that students were forced to memorize outcomes without understanding the reasoning for the outcome.
In Joel’s book, two chapters are devoted to the elementary steps (proton transfer plus nine steps) and these elementary steps are then referred back to in future multistep mechanisms, including SN2. Below are a few examples of how the SN2 elementary step is treated later in his book.
Alcohol to Akyl Halide Using PBr3:
Witting Reaction:
Enolate Alkylation:
In my course organized by functional group, students didn’t understand when and where to incorporate a proton transfer step. Like SN2, it was introduced early, but students didn’t see how to use it to predict reactions. They saw each mechanism as “stand alone” and thought they had to memorize hundreds of mechanisms, each with new steps. They did not connect their work “at the barre” with the larger routine.
Because we now spend time up-front learning about proton transfer, and then showing it throughout the course, I have found that students are now able to understand when and how a proton transfer step occurs. I no longer hear, “What happened to SN2?” as they see, and use, SN2 over and over again in multistep mechanisms.
Ballet dancers put together fundamental, elementary steps to create a beautiful routine. Learning a routine before mastering the elementary steps would almost certainly mean a poor and awkward performance. I now train my students in a similar way. They learn the fundamentals, and by organizing the course by mechanism, I am able to easily reinforce the use of those basic steps throughout the course.
It may not be as beautiful as ballet to watch, but the results have been no less impressive.
— Marie Melzer, Old Dominion University
Marie Melzer teaches a mechanistically organized course at Old Dominion University. She plans to use Joel Karty’s book in the fall (2014). Click here to learn more about Prof. Melzer.
