Find the Next Flash Point in this series here
One theme that comes up over and over in the research literature on why students struggle to learn science is that they often draw superficial connections between diagrams and other representations. Students tend to gravitate toward irrelevant features of representations rather than viewing those representations through the conceptual structure of the discipline. Addressing this skill in the classroom can take the form of any number of individual lessons, but each lesson should share a commitment to guiding students as they identify and analyze features of a particular diagram or representation. Teaching should focus on helping students find patterns of features in representations and modeling how reading these representations can serve as evidence to support claims, explain processes, support inferences, and justify predictions.
The following Flash Point activity uses Mechanisms to teach students how they can make sense of common representations found in the organic curriculum. As you review this activity, think about how you could use Mechanisms and Card No. 1 to foster learning in your classroom!
- Students should be able to identify which reaction pathway corresponds to SN1 vs. E1.
- Students should be able to connect discrete (loss of leaving group, formation of carbocation, formation of pi-bond) to energy maxima/minima in the reaction coordinate diagram.
- Students should be able to justify why SN1 is thermodynamically favored over E1.
Note to Instructor
- Assign Substitution Puzzle #12 & Elimination Puzzle #13.
- For this activity, treat the preparation of the leaving group as a pre-activity step.
This learning activity asks students to work with two Mechanisms puzzles that begin with the same molecular substrate, but then work out the mechanisms for competing SN1 and E1 reaction pathways and reason about why these two pathways are different. SN1 and E1 compete with one another because, chiefly, they require the same precursor — the formation of a carbocation — under the same solvent conditions. Students may (or may not) know at this point that the yield of the SN1/E1 is driven by thermodynamic considerations.
Begin working with your students to use Mechanisms to solve Substitution Puzzle #12 and Elimination Puzzle #13. Model for them how they can identify states (beginning materials, transition states, intermediates) and emphasize how these states coincide with energy transitions.
Use either a qualitative or quantitative approach to reason about how the actions that students perform in Mechanisms represent the transition between states and what the energy consequences of those moves would be.
For example, to get from starting materials (State 1) to the loss of a leaving group (State 2, transition state), energy must be supplied from the system to reach this level (↑E). Having reached the first activation energy, the leaving group cleaves off and releases energy and forming a carbocation intermediate that is stabilized by hyperconjugation (State 2 to State 3). This transition lowers the overall energy of the system.
Continue showing moves in Mechanisms and write a running tally of steps on the board:
- State 1 (initial products) → State 2 (transition state): Loss of leaving group (breaking sigma bond: ↑E)
- State 2 (transition state) → State 3 (intermediate): Formation of carbocation (sigma bond broken, hyperconjugation: ↓E)
Prompt students to continue writing down all the discrete steps they take in Mechanisms, identifying all of the bond breaking/formation steps that occur, whether those steps absorbed or released energy, and what transition states and intermediates arise.
Note: Students may need extra guidance as they go from State 3 → State 4 where the SN1 and E1 pathways diverge.
Provide students with a blank plot of a generic SN1 and E1 reaction coordinate diagram (see figure below for a completed example). Have students work in small groups using Mechanisms and have them sketch appropriate structures at the following states:
- Initial state (State 1)
- Local energy maxima (States 2, 4)
- Local energy minimum (State 3)
- Final products (State 5)
Students should be able to justify why both mechanisms have the same energy at the transition state (the loss of leaving group is the same in both). They should then be able to reason why going from the carbocation to the final product is energetically lower for SN1 (forming sigma bond to nucleophile) than E1 (abstraction of H+ and formation of pi bond) and use this insight to label the different energy pathways.
- Ask students: "Which pathway is favored at low temperature vs. high temperature?" Students should be able to engage a discussion about the relative activation energy of extracting a beta proton and forming a pi bond versus simply forming a new sigma bond. This should lead them to propose that the E1 pathway is higher in energy. Check that students are appealing to energy absorption and release, that they are connecting it to each step they take in Mechanisms, and that this serves as evidence that low temperature favors SN1.
- Ask students: "How would the energy diagram need to change to reflect the proton transfer steps?" Students should consider that the proton transfer steps are another example of bond breaking and formation. Check that students are reasoning appropriately about the relative magnitude of the bond breaking and formation relative to the leaving group.
How this Activity Targets Learning
This activity is designed to:
- Connect actions that occur during a reaction and how those actions relate to states as represented by a reaction coordinate diagram, a new representation. Students must directly connect features of the coordinate diagram to regular behaviors in Mechanisms (bond breaking/formation)
- Highlight for students the regularities between the two mechanisms and their energetics (forming the carbocation intermediate) as well as their differences (diverging activation energies for the second step).
- Challenge students to interpret the two representations of SN1 and E1 in light of other disciplinary principles (i.e., temperature and thermodynamic vs. kinetic product formation).