Instructor Card #5

September 13, 2018
Flash PointsDane DeSutter

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Diagrams and representations can often be used to express the same chemical idea in very different ways. Instruction in the classroom should focus on helping students bridge between representations that are equivalent informationally, but may not appear to contain the same information.

This Flash Point activity is designed to help students connect between their learning in Mechanisms with more traditional ways of representing organic mechanisms using arrow pushing motation.

Learning Objectives

  1. Students should be able to explain how Mechanisms puzzles and bond line diagrams represent the same underlying bond breaking and formation process
  2. Students should be able to contrast the relative affordances of the Mechanisms puzzles with respect to their diagrammatic equivalent.

Learning Activity

Note to Instructor

  • Assign students Addition Puzzle #5
  • Mechanisms can be downloaded in the Itunes App store or Google Play. Web version is also available upon request.

Scaffold Learning

Explain to students that they will be connecting what they see in Mechanisms to standard arrow pushing notation and exploring how both convey the same information. Remind your students that arrow pushing specifically denotes the movement of electron density and that they can also see the movement of electrons in Mechanisms as dots on each bond.

Model for students how to connect the causal bond breaking and bond formation in Mechanisms with arrow pushing. In Addition Puzzle #5, we know that the dipole of hydrobromic acid yields an electrophilic hydrogen that will be attracted to the pi system. Demonstrate the shift of electrons from the pi system to form a transition complex with HBr. Next, draw the equivalent move in diagram notation using the curved arrow formalism.

Note: Students may not place the hydrogen on the less substituted carbon, producing a higher energy carbocation. This is a good moment to demonstrate that the way the bond breaks matters! Pulling at the electron attached to the more substituted carbon produces the Markovnikov product, whereas pulling electrons from the less substituted carbon produces the energetically less stable 2º carbocation.

Emphasize that when the screen dims, this indicates a “decision point” — a point where a concerted step occurs in a mechanism. At this point demonstrate how to shift the excess electrons from hydrogen onto bromine.

Decision point in addition reaction

Encourage students to work through the next steps of this reaction. Let students choose whether to work out the steps in Mechanisms or on paper, but inform them they should ultimately check their solution in the app to confirm that they have proposed chemically valid operations. Students should be able to propose that the negatively charged bromine is attracted to the carbocation.

Emphasize along the way how various steps are equivalent and also draw attention to the affordances of each representation. Specifically, note how Mechanisms lets students causally act out bond breaking and bond formation process and depicts how electrons move to do so.

Explain how both Mechanisms and arrow pushing ultimately convey the same information, but that they do so in different ways. Diagrams show the structure of a molecule at various phases and how electron density moves in order to explain the creation of a product under the given conditions. However, most of this motion must be inferred by the student mentally and cannot be determined without instruction. Mechanisms, on the other hand, allows a student to readily see how electron density can be rearranged in the molecule to arrive at the product. Moreover, implicit feedback cues let the learner know when something they are trying is permitted or not in a way that is not clear when using diagrams.

Challenge Your Students

  • Ask your students to work a mechanism first out on paper, but ask them to explicitly label electrons in a manner consistent with Mechanisms using dots on bonds.

How this Activity Targets Learning

  1. This activity contrasts two different representations of the same reaction mechanism and demonstrate that both diagram formats can ultimately represent the same underlying chemical information.
  2. It also shows that each representation conveys information in different ways: Mechanisms affords insight into the dynamic nature of these rearrangements that must be mentally imagined by the student when working with static diagrams. Diagrams, by contrast, give a bird's-eye view of the entire reaction process.

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