The Second Most Important Question to Ask When Learning A New Reaction

by James

in Chemical Bonds, Understanding Electron Flow, Where Electrons Are

So once you can recognize the key bonds that are forming and breaking in a chemical reaction, the second most important question to ask yourself when learning a new reaction is “how do the electrons move?” I have a friendly disagreement with a lot of instructors on this point, since it’s common to claim that this is the actually the most important question. I stress bond forming and breaking first because I’ve seen so many students first-hand fail to recognize changes in bonding, and I found that if I tried to talk about electrons to them without establishing the changes in bonding that were occuring, I never got very far.

"Your lips are moving but I have no fricking idea what you are talking about"

That being said,  understanding electron flow is really where the rubber meets the road in organic chemistry.In a chemical reaction, we’re forming and breaking chemical bonds, which are  pairs of electrons shared between two atoms. The process of breaking and forming bonds is going to change the charges around each atom, at least momentarily. So if we want to have a deeper understanding of chemical reactions, we need to understand how this happens.

Remember: chemistry is all about  “opposite charges attract, like charges repel”. The nucleus, which is positively charged, is composed of protons. Surrounding the nucleus are negatively charged electrons. It’s important to note that chemical reactions don’t affect the number of protons (i.e. the nucleus). This ain’t nuclear physics!  Instead, chemical reactions affect the number of electrons around an atom. Just like economic transactions are a transfer of currency between two entities, you can kind of think of electrons as the “currency” of chemistry. Think of it like this:

Every chemical reaction is a transaction of electrons between atoms. 

If you understand how to recognize where the electrons are (and aren’t), and can draw a series of diagrams that show how the electrons flow from atom to atom, depicting bond-forming and bond-breaking events in detail,  then you’ll have a much better understanding of What Is Going On.

It’s the difference between this:

and this – the same reaction, but in more detail:

So what I’m going to talk about in the next series of posts is understanding electron flow. Using the analogy of “electrons” to currency, hopefully it will be made clear how using these principles: 

  • opposite charges attract, like charges repel
  • negative charge (high electron density) flows to positive charge (low electron density)
  • electronegativity is an atom’s ability to stabilize negative charge. In other words, it’s an atom’s “greediness” for electrons.

We can start to figure out answers to the following questions:

  • How can we look at a molecule and tell where the electrons are (or aren’t)? Also, why is “formal charge” not the same as “electron density”?
  • How can we use electron densities to tell plausible (or implausible) reactions?
  • Resonance. What the hell is it?
  • How do we tell which resonance forms are significant and which are not?
  •  Since electrons are the “currency” of chemistry, it makes sense to have an accounting system, right? So we’ll see how to apply the accounting system of organic chemistry (the curved arrow formalism) to show electron flow in detail. EDIT: Jess rightly points out that “formal charge” is also a form of accounting. That’s absolutely true – and very important!

Finally we can then apply all of these tools toward understanding simple reactions, especially acid-base, substitution, addition and elimination reactions.

This is going to open up a whole new set of questions! But it will at least lay the foundations for a deeper understanding of organic chemistry.

Next Post: How to use electronegativity to figure out electron density (and why NOT to trust formal charge)

 

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{ 4 comments… read them below or add one }

jess

Great stuff! I’ve been perusing your site for the past few weeks, since your comment on my post (thanks for the link, btw)

In your final bullet point, I would argue that curved arrow notation is not the only form of accounting we use, since I’ve always considered the concept of “formal charge” as a form of accounting myself. :)

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james

Oh yeah, that’s absolutely true. Editing the post to reflect that. Thanks !

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Patrick

Check the bottom right structure on “How can we tell which resonance form is best?” The nitrogen should be *doubly* cationic, no?

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james

Crap you are right. Fixed! thanks for the spot.

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