Proton Transfer: Well Begun is Half Done

Of all the chapters in Joel’s mechanistically organized textbook, my favorite is Chapter 6: The Proton Transfer Reaction. Acid-base chemistry might seem like an odd topic to pick in an organic chemistry textbook. It seems almost…inorganic, a throwback to general chemistry of sorts. So why do I like it so much? It accomplishes two vitally important tasks that have big pay offs for the rest of the year.

The biggest and most general benefit is the way that Joel uses proton transfer to introduce the mechanistic use of curved arrows. Of all the problems I have seen students struggle with over the years, using the curved arrows correctly when drawing out reaction mechanisms is at the top of the list. Acid-base chemistry is familiar enough to students at this point, so it doesn’t get in the way of learning about curved arrows.

I have previously used textbooks that introduce mechanisms and curved arrows with alkene additions or substitution reactions. In these cases, I found that a large fraction of the class never really understood what the arrows were trying to show. Some students would use them as stage directions for atom movements. Others would confuse the curved arrows with reaction arrows or, worse, resonance arrows. Still others would just show half a dozen or more arrows all at once to magically get to the product. In contrast, acid-base chemistry allows students to more easily grasp the meaning of the curved arrows as the flow electrons. Once they get that down, drawing the more complicated mechanisms is just the logical extension of those basic steps.

The second huge benefit of the proton transfer chapter is the many connections it allows students to make to other aspects of organic chemistry. Joel has previously written about some of these connections in Chapter 7. The insights students gain from acid-base chemistry clearly connect to other aspects of writing good mechanisms and understanding substitution and elimination reactions in Chapters 8 and 9. I find, however, that as the material gets even more complicated, these connections become even more important to avoid falling into the memorization trap. Nucleophilic Addition-Elimination reactions in Chapters 20 and 21 provide a great illustration of this.

When I point out that the stability of carboxylic acid derivatives (in Chapter 20) or conversely that their reactivity toward hydrolysis (in Chapter 21) is all about acidity or conjugate base stability, the light bulb goes on for students. Who would forget that H-Cl is a strong acid? Students immediately grasp the higher reactivity of acid chlorides. When I reprise a whole series of acids we learned about in Chapter 6 right next to a chart of acid derivatives, it is as if the fog has lifted and everyone can see clearly. There is simply no need to memorize anything. It all makes sense. I tell students that what you memorize you can forget, but what you understand you can recreate when needed.

So it all starts with the transfer of the simple proton. It’s not so simple of course, but well begun is half done. And getting off on the right foot with curved arrows and making connections with other aspects of chemistry  is a huge step in a mechanistically organized course.

-Rick Bunt, Middlebury College

Click here to learn more about Prof. Bunt.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: