Work in groups on these problems. You should try to answer the questions without referring to your textbook. If you get stuck, try asking another group for help.
1. For each of the following reactions, designate the acids and the bases and use curved arrows to show the flow of electrons as the reaction proceeds from left to right. Circle the major species at equilibrium. Start by drawing complete Kekulé structures showing all bonding and nonbonding electron pairs at the reaction centers.
2. Draw complete Kekulé structures of reactants and products using lines for bonds, show nonbonding electron pairs, and indicate formal charges on atoms. Identify the acid and the base for each reaction. Use curved arrows to show the movement of electron pairs as the reactions proceed from left to right. Where appropriate identify and name the reactive species: carbocation, cabanion or free radical.
3. Predict the product(s) for the following reactions. The ideas that you used in the previous problems will be useful.
4. For each of the following pairs, one reaction proceeds to the specified products and the other does not. Predict which is which and clearly explain your choice. It will help to start with curved arrows to show which bonds are made and which bonds are broken in each reaction.
Structure: Properties: Acids and Bases
This Workshop is an example of a carefully structured development from the familiar (proton transfer) to the unfamiliar (predicting products of new reactions). There is also a structured progression from equilibrium concepts in Problem 1 to rate concepts in Problem 4. The switch from equilibrium (first-year chemistry) to rate (second-year chemistry) is a stumbling block for many students. Drawing complete Kekulé structures, as directed in Problem 1, is the proper starting point for all four of these problems. Using curved arrows to show the flow of electron pairs is an important aid to understanding reactions and is an important prelude to their use in complex mechanistic problems.
- Problem 1: If students have a problem getting started, the leader might ask them to identify which species are giving up (donating) protons (acid) and which are taking on (accepting) protons (base).
- Take-Home Point. Proton transfer occurs from one electron pair to another. Think of the equilibrium as a competition for the proton; the weaker acid wins the competition (predominates at equilibrium).
- Problems 2 and 3: These problems are collections of Bronsted and Lewis acid-base reactions. The big idea is to help students see that proton transfers belong to a subset of Lewis donor-acceptor interactions. Work with them to find the electron pair donor and acceptor character of all the reactions. That is, of course, the basis for predictions in Problem 3. Problem 3 is a very big intellectual and psychological jump for students because they must make predictions (go out on limbs).
- Problem 4: For Problem 4, the big idea is that the more favorable reaction involves transferring negative charge to the more electronegative atom. Problem 4 introduces the idea of competitive reactions without saying much about it. The problem looks ahead to the transition from thinking about equilibrium to thinking about rate. We will return to this issue in full form in Workshop 5.
- Take-Home Points. This Workshop addresses a major dilemma for students: How does one know what reacts with what? How can one predict products?