Intro to Resonance

by Kiley Lynch

A few words on resonance

When we can draw two (or more) forms of the same molecule that differ only in the placement of their electrons, these are called “resonance forms” (or “resonance structures”). Here’s some common mistakes lots of students make. Avoid these!

  • It’s NOT an equilibrium. A molecule does not go back and forth between resonance forms. Rather, each resonance form makes a contribution to the overall structure of the molecule, which is a hybrid of all of these forms. As an analogy, you’re the offspring of your mom and your dad, but you don’t go back and forth between the two – you’re a hybrid!  We have to draw resonance structures to describe electron density because drawing just one structure can’t give the full picture of where the electrons are.
  • We use a double headed arrow to denote resonance structures. Note that this is *different* than an equilibrium arrow.
  • The contribution of each resonance form to the overall hybrid will depend on its stability. How to evaluate stability is a post in itself.
  • The key value of resonance structures will be in figuring out where the electrons are. This will be extremely valuable when we start talking about reactions.

 

Two examples: the “acetate ion” and the “allyl carbocation”

 

 

One of the other cool things about the resonance forms is that these “hybrid” forms have bond lengths (and strengths) that are intermediate between these two extremes… for instance the bond length of the carbon-oxygen bonds in the “acetate ion” is about halfway between the normal lengths/strengths of C-O single bonds and C-O double bonds.

Tomorrow: OK, resonance forms are important. But how can you *use* them to make decisions? Using some simple principles, we can use them to figure out where electrons are (and aren’t!)

Thanks for reading! James

P.S. Relevant post: Introduction to Resonance