Organic chemistry can be challenging. Often, students might be cautious or reluctant to attempt the material. However, I use “starbursts” in my classroom when teaching organic chemistry through a mechanistic organization to keep my students motivated. Starbursts compile a range of reaction schemes and conditions to provide comparisons among key reactions, which allow students to apply their knowledge and utility of mechanisms in a “big-picture” perspective.
For example, in Figure 1, a diol is treated against five different transformations. For each of the questions, students are asked to fill in the blanks that correspond to the major product that is generated in the highest yield possible. This educational tool effectively compliments a mechanistic organization of the material.
Figure 1. Transformations of a Diol Starting Material
Figure 2 displays the products for the five different transformations into halohydrins, ethers, and racemic alkyl halides. For each of the reactions, the major product shown is generated in the highest yield possible and in the appropriate regiochemistry. A mechanistic organization of this material provides students with the ability to clearly execute and compare these particular products. Furthermore, the starbursts in Figures 1 and 2 help demonstrate the necessity of being aware of how and when to use reagents to obtain a desired organic product.
Figure 2. Transformations and Products of a Diol
Mechanistically, the comparisons in Figures 1 and 2 allow students to note the specificity and the regioselectivity of the reagents. Students can observe how the mechanism drives the production of the type and the number of products in each reaction. By comparing the halogenation reactions of Figures 1 and 2, the variation and the specificity of reagents are supported by the respective products of each reaction. These starburst activities are incredibly useful, especially for instructors, because they demonstrate the level of students’ knowledge of the material while also helping students gauge their own awareness of the utility of these organic mechanisms. Additionally, these starburst activities prove to students that although mixtures can be generated, they can also be easily avoided. For example, let’s look at the three types of transformations in Figure 2:
- Halogenation using Lewis acids (PBr3, PCl3, SOCl2): They are specifically meant to react with primary and secondary alcohols. Keep in mind that if, and when, the tertiary alcohol reacts, it is in a negligible amount.
- Halogenation using mineral acids (HBr): They react with all types of alcohols, which lead to mixtures and multiple products.
- Williamson ether synthesis: This requires a strong base and small alkyl halides with a favorable leaving group. The sterics of the reaction should be kept with the nucleophilic portion of the reaction (i.e. the alkoxide).
Instructors can provide one blank box for each reaction to push their students’ abilities by assessing their background knowledge of organic reactions. In doing so, these starburst activities reinforce the style of teaching organic chemistry through mechanisms, as they support students’ comprehension by prompting them to consider the differences between varied conditions. To review the scheme in Figure 2:
- Lewis acids (PBr3, PCl3, SOCl2): These electron acceptors react via an SN2 pathway with a regioselectivity for the interconversion of the stereocenters of secondary alcohols.
- Mineral acids (HBr): These species react via an SN1 pathway with multiple steps, which yield multiple products and many enantiomeric pairs.
- Williamson ether synthesis: This reacts via an SN2 pathway to provide one product in high yield when the nucleophilic oxygen reaction with an electrophilic carbon is small and has a good leaving group.
As another learning bonus, these starburst activities do not have to be restricted to one type of transformation. In order to best present organic chemistry in a mechanistic perspective, I try to incorporate both acidic and basic conditions with one blank box per reaction.
In Figure 3, an epoxide is treated against eight different transformations. As before, students are asked to fill in the blanks that correspond to the major product that is generated in the highest yield possible. However, these bases and electrophiles range in strength as well as in sterics. This variety enables students to consider how to maneuver through the assignment. I also add isotopes to assess their critical-thinking skills. In select reactions, the reaction conditions have deuterated workup (D3O+) to assist me in monitoring the overall attention of my students. This helps me gauge my students’ awareness and see if they are truly reading and working through these schemes properly.
Figure 3. Transformations of a Mono-Substituted Epoxide Starting Material
Ultimately, organic chemistry instructors can use these starburst activities to provide their students with clear, visual schematics that break down complex reactions into smaller and more manageable chunks, which prompt students to use these exercises to strengthen their recognition of the relationship between any and all reagents. In my own classroom, I try to encourage students to view these reactants as essential “tools in their toolbox,” which they can repeatedly use to generate one desired product in many different ways. As such, these starburst activities serve as valuable educational tools that can help instructors guide their students towards reaching a deeper understanding of the “bigger picture” when it comes to viewing organic chemistry through a mechanistic lens.
-Kerri Taylor, Columbus State University