Chapter 11 of the Karty text focuses on electrophilic addition along pi bonds. This can be a tough topic to tackle. The terms themselves can scare students, which is why I’ve found that it is key to break down the words into smaller chunks, especially for elementary steps.
In the text, a variety of electrophilic additions are discussed in Sections 11.6-11.10. I like to look at these reactions and find the key players first: where are the electrophile and the nucleophile? I also like how Section 11.6 is introduced with the reaction of 1-butene and water, and its result of NR (no reaction). I mention to my students that both alkenes and alkynes are robust and stable compounds that are unreactive to weak acids, such as water. Thus, the electron-rich compounds need a “push” to promote the movement of electrons.
As I prepare my lectures, I like to think about what worked well for me and how different concepts connected together for me in the past (while also keeping in mind that no two people learn in the same way). When I was a student, I had a great organic chemistry professor, and he had all of his notes written on notebook paper and treated them as if they were works of art.
In channeling a similar kind of unique energy, I chose to use molecular orbitals to show the transformation of a pi bond in an alkene and the subsequent formation of two sigma bonds with the electrophile and nucleophile. This link was really helpful for me, because it appeals to visual learners, and organic chemistry can be quite visual when grasping new material.
In the illustration above, the solid lines signify the bonds coming into the foreground while the hashed lines signify the bonds oriented towards the back of the page. The image clearly demonstrates the difference between pi bonds and p orbitals when compared to single bonds and spn orbitals. Because organic chemistry is best learned through visuals, if I can make students more accustomed to MO theory through my notes, examples from the textbook, and additional resources, then great!
Circling back to Karty’s coverage of this material, on behalf of all my students, I can say that we really appreciate the straightforward discussion about how electrophiles (E+) are first added to the electron-rich species, and then they generate the carbocation as an intermediate afterwards. To further explain this challenging concept, I suggested that my students visualize a teeter-totter—when one side goes down, the other side will go up. This means that when you add a hydrogen to one carbon, then the other carbon on the pi bond needs the positive charge. This promotes the idea of addition as one in which two systems combine to become one, resulting in the formation of two new bonds. Check out this post as an additional resource for your class.
In general, I also like for my students to notice trends. For instance, in an acid-catalyzed addition, I want them to understand that the acid is only a catalyst: it makes a slow reaction faster! This means that anything can be added along the double bond by switching out the neutral species that contains the electrophile and the nucleophile.
This type of addition is also consistent with the electrophilic addition of mineral acids. The hydrogen adds to one carbon, and then the nucleophile (X-) adds to the other carbon on the pi bond, which holds the positive charge. This reinforces the idea of addition as one in which two systems combine to become one, resulting in the formation of two new bonds. However, for each addition, there needs to be one equivalent of the mineral acid (HX). This means that if the alkyne needs to be completely converted into the alkane, then there needs to be a minimum of two equivalents. This follows Markovnikov’s rule: the hydrogen atom adds to the carbon with the most hydrogen, and the charge goes to the carbon that best stabilizes the charge.
Overall, teaching with visuals, while adapting both course materials and outside resources, enables me to meet the diverse needs of my students and their different learning styles. As instructors, we tend to be more efficient at explaining topics in either textual or visual manners. However, I’ve grown to recognize that it is essential to work past these pedagogical limitations so we can continually sharpen our skills for the benefit of our students.
-Kerri Taylor, Columbus State University
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