Nearly every semester, I am asked a question that I’ve never been asked before. I have found that by giving students some rules of thumb (ROTs), I can help them understand the basics while also giving them specific strategies for solving organic chemistry problems. More recently, I’ve recognized that the mechanistic organization helps enforce my ROTs in ways that a functional-group organization cannot.
This semester, I was asked …
“Where do I draw the line for substitution versus elimination? What amount of sterics causes the reaction to generate the major product as the substitution product versus the elimination product? What is the point of no return to produce the Zaitsev and Hofmann products?”
Here are the ROTs that I used to answer these questions …
ROT1: Ethyl groups tend to be a “point of no return” for deciphering between substitution and elimination reactions. When nucleophiles include a propyl chain (i.e. having a minimum of 3 carbons), I suggest that students think more about elimination as the mechanistic pathway (Figure 1).
Figure 1. Substitution versus Elimination
As the sterics of the nucleophile increase, the distribution of elimination products increases, and the percentage of substitution products would decrease. However, as the alkyl center goes from a primary to a secondary, I tell my students to be aware that the Zaitsev product is likely to compete with the substitution product.
During the semester, to help my students recognize the competition between Zaitsev and Hofmann, we do countless organic problem solving. We survey all types of alkyl centers, conditions, and nucleophiles for my students to build their backgrounds in organic chemistry. In doing so, I make sure to maintain one variable while changing all the other variables. The mechanistic organization of Karty’s text enables students to generate additional ROTs through a variety of questions that help students view reactions as algorithms.
- How would you characterize the alkyl halide?
- Is the nucleophile charged or neutral?
- Is the nucleophile bulky or small?
- Based on a second review of the alkyl halide and the nucleophile, are there any sterics to direct the reaction?
- What is the solvent? Is it polar protic or polar aprotic?
While reviewing these questions, the mechanistic text supports my students as they decipher between SN1/E1 versus SN2/E2, all by answering Questions 1 and 2. Questions 3 and 4 accompany the organic mechanisms and enable students to rationalize “why substitution” or “why elimination.” Question 5 provides confirmation and confidence that the correct mechanistic pathway was chosen.
Figure 2. Competition of Organic Species
Competition in organic synthesis lies between the sterics of the alkyl groups on the electrophilic carbon and the nucleophile. However, this is a fine line for students. Why? This leads me to the next few ROTs.
ROT2: Both the number and the placement of alkyl groups dramatically dictate how the reaction proceeds and what products are generated. The mechanistic organization of Chapters 8 and 9 helps to make these topics clearer. Chapter 8 gives a transparent presentation of substitution versus elimination, while Chapter 9 goes more into detail about the variables, such as the strength of the nucleophile, the temperature, and the solvent. A real difference is made when the text is used to present substitution and elimination in a simultaneous fashion. Likewise, I try to help my students gain a solid understanding of substituion and elimination as we apply ROTs to Chapter 9.
Figure 3. Substitution versus Elimination of a Secondary Center
ROT3: As the nucleophile becomes bulkier, the percentage of the Hofmann product is favored as the elimination product.
Figure 4. Elimination Products from Two Very Bulky Reagents
ROT4: Low temperatures will favor substitution: 50°C will give a mixture of substitution products, while 100°C and greater will favor elimination. I remind my students that RT (room temperature) reactions generally favor substitution. I have noticed that my students can get tripped up on the variable of temperature. The heat will speed up the reactions and limit the potency of substitution reactions.
ROT5: Solvent has little to do with deciding the mechanistic route. (This is why it is discussed last in the text.) Nucleophilic strength is reversed when put in the opposite solvent.
The placement of details in Chapter 9 helps to eliminate this confusion for students. Solvent is a variable that is superfluous. Traditionally, solvent is discussed along with the “bigger topics,” like the nucleophile or alkyl halide. At times, the solvent might be addressed with one or two sentences. The Karty text does a great job of explicitly stating each role and each use of the variables. It benefits students and instructors greatly as its organization demonstartes the importance of subtopics. Once my students have the basics, I try to change their perspective further with the topics of heat, solvents, and the sterics of substrates and nucleophiles.
Overall, the mechanistic organization of Karty’s book supports my ROTs by looking at each reaction, with the help of the previously stated questions, as an algorithm. The mechanistic text directs the students to focus on the most important factors first: the characterization of the alkyl halide, followed by the nucleophile. Once the proper conditions are met, students can review the sterics of the alkyl halide and nucleophile to design the products through the specific mechanistic movements of substitution or elimination. Ultimately, these never-been-asked-before questions keep me on my toes, help me prepare for the next time I teach a concept, and are one of the reasons I love teaching organic chemistry!
-Kerri Taylor, Columbus State University
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