As the semester comes to a close, I have been reflecting on my lectures and experiences with a mechanistically organized course. The Karty text has presented many different types of reactions; from all of the reactions, I want the students to be aware of the central theme in ALL organic mechanisms. In every step, there will be a nucleophile and electrophile. Upon completion of the course, I want students to see this as part of the big picture.
Teaching the course mechanistically has helped the students develop the ability to find the nucleophile and electrophile. In Chapters 7-9, SN2, SN1, E1 and E2 reactions are presented and the students are taught how to differentiate among organic species that are electron rich and electron poor. At first, my class had some trouble identifying the kinetics of the reaction (i.e. the influence of steric hindrance of both the electrophile and nucleophile). As we worked through in-class problems, they appeared to gain more understanding for determining the reaction’s direction and products. With the help of the text, the students learned to focus on the following questions when examining a reaction:
- Where is the nucleophile and the electrophile?
- Is the nucleophile strong (charged), weak (neutral), small, bulky, etc.?
- What are the conditions? Is the reaction under acidic or basic conditions?
- Are there any catalysts?
- Will the intermediate rearrange?
Beginning with Chapter 6, and the introduction of the proton transfer reaction, I instructed the students to focus on the details and context of the movement of all electrons within the reaction. It is essential that they build upon the knowledge from the simple reactions discussed (i.e. SN2, SN1, E1 and E2 reactions), as the reactions spotlighted in Chapter 10 (“Nucleophilic Substitution and Elimination Reactions 2”) are slightly more complicated. Mechanisms help to emphasize this central theme of electron movement in many ways throughout Chapter 10. For example, there are definitions of various terms, descriptions of potential reagents, and inspections of example reactions.
As this is a mechanistically organized course, the necessity of comprehension over memorization is stressed. Students become more comfortable if they learn to apply basic concepts when looking at reactions. This seems to establish a solid background and will prepare the students for Organic Chemistry 2. I asked students to think through the following steps:
- Identify the species in the reaction that is most electron-rich as the nucleophile and the species that is most deficient in electrons as the electrophile.
- Keeping this in mind, the arrows in EACH step must be drawn from the negative to positive. Or, drawing the arrow from an electron pair for the electron-rich species to the electron-deficient species. This could be explained in many ways:
- Nucleophile to the electrophile
- Lewis Base to the Lewis Acid
- Electron rich to electron poor
I told the students to review the reactants and products to determine the “condition of the reaction.” It is helpful for them to learn to determine how the electrons are moving and, if possible, the type of reaction. I reminded them that the same conditions are maintained for both the reactants and products. In other words, if it starts positive (acidic), then the reaction will end positive (acidic). In like manner, I mentioned that in a reaction with “negatives” present, it is not possible for a positive to exist and the reaction will be occurring under a basic environment. Students started to identify and understand how catalysts help transform the reagents into products.
Furthermore, mechanisms in the text help connect the concepts of carbocation and carbanion stabilities. The students were able to grasp the concept that stability and sterics of the carbon electrophile in the SN2, SN1, E1 and E2 reactions largely direct the type and rate of reaction. I asked them to consider the question “can the intermediate rearrange to form a more stable structure?” Consequently, I want them to think about how the intermediate will rearrange to become a more stable intermediate. I really like that this concept is tied-in from Chapter 7 (“An Overview of the Most Common Elementary Steps”) with the presentation of 1,2-hydride shifts to 1,2-alkyl shifts and, lastly, ring expansions.
A mechanistically organized course can be challenging for students but it also seems to be more rewarding for them compared to other organic courses I have taught. Throughout the semester, they are given big picture lessons of electron movement in each step. The major hurdle for undergraduates is generally developing the perspective useful for evaluating a reaction scheme, depicting a reaction mechanism, and identifying each organic species. I consider this semester a success as the students have become confident in explaining the movement of electrons between the organic species of every step and are able to demonstrate electron movement from the nucleophile and electrophile.
-Kerri Taylor, Columbus State University
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