Pollen sticks to this bee due to electrostatic attraction. When it flies, a bee loses electrons and builds up a positive electrical charge. The flower’s pollen, on the other hand, is negatively charged, so when the bee lands, the pollen experiences a driving force from the flower to the bee. Similarly, the elementary steps we examine here in Chapter 7 are driven by the flow of electrons from a site with excess negative charge toward a site with excess positive charge.

On completing Chapter 7 you should be able to:

1. Describe how the curved arrow notation for a proton transfer step reflects the flow of electrons from an electron-rich site to an electron-poor site.
2. Simplify metal-containing species in terms of their electron-rich behavior.
3. Identify and draw the curved arrows for the following common elementary steps:
   • Proton transfer
   • Bimolecular nucleophilic substitution (S_N2)
   • Coordination
   • Heterolysis
   • Nucleophilic addition
   • Nucleophile elimination
   • Bimolecular elimination (E2)
   • Electrophilic addition
   • Electrophile elimination
   • Carbocation rearrangement
4. Describe the electron flow in each of the above steps in terms of electron-rich and electron-poor sites.
5. Rationalize the driving force for each of the above elementary steps in terms of charge stability and bond energies.
6. Draw the keto and enol forms of a keto–enol tautomerization, and explain the driving force for such a reaction.

In Chapter 6, we learned that a proton transfer is an elementary step: it occurs as a single event. If a proton transfer takes place in isolation from other steps, as we saw in the examples in Chapter 6, then it constitutes an overall reaction. Frequently, however, a proton transfer makes up an individual step of a multistep mechanism—something we will discuss more extensively in Chapter 8.

There are a handful of other quite common elementary steps as well, which can be combined in various ways to produce mechanisms for numerous reactions. Chapter 7 provides an overview of nine of these elementary steps:

1. Bimolecular nucleophilic substitution (S_N2, Section 7.2)

2. Coordination ( Section 7.3)

3. Heterolysis ( Section 7.3)

4. Nucleophilic addition ( Section 7.4)

5. Nucleophile elimination to form a polar π bond ( Section 7.4)

6. Bimolecular elimination (E2, Section 7.5)

7. Electrophilic addition (Section 7.6)

8. Electrophile elimination to form a nonpolar π bond (Section 7.6)

9. Carbocation rearrangements (Section 7.7)

Coordination and heterolysis (Section 7.3), nucleophilic addition and nucleophile elimination (Section 7.4), and electrophilic addition and electrophile elimination (Section 7.6) are discussed in pairs in their respective sections because, in each case, one is simply the reverse of the other. This means both are governed by the same factors.

Each of these nine elementary steps can be depicted using curved arrow notation, much as proton transfer steps were depicted in Chapter 6. However, beyond simply describing how each step takes place, we need to know why it would (or would not) take place. Therefore, we devote Section 7.8 to the driving force for elementary steps. Understanding the driving force for each step will ultimately enable us to use mechanisms to make predictions about the outcomes of reactions.

The nine new elementary steps we learn here in Chapter 7, along with proton transfer steps (10 in all), make up nearly all of the reaction mechanisms you will encounter through Chapter 23. This knowledge, that the same 10 elementary steps constitute the mechanisms for a large number of reactions, can be very powerful. It allows you to be confident that mechanisms simplify organic chemistry. The time and effort you spend mastering these elementary steps will be rewarded as you continue to learn reactions throughout the rest of this book.

To help you see the similarities and differences among the various elementary steps:

You should visit those pages frequently. As you do, challenge yourself to become familiar with three aspects of each elementary step: (1) the types of species that appear as reactants and products, (2) the curved arrows that describe the electron movement, and (3) the driving force. You will especially benefit in later chapters when we study reactions that have mechanisms constructed from these steps.