Resonance

Organic chemistry students often dread resonance, but it's a key concept that unlocks many properties of molecules. Your exams will frequently touch on resonance, so mastering it becomes a practice-makes-perfect situation.  Resonance is a fundamental concept in understanding the stability and behavior of molecules with delocalized electrons.

Resonance Example (Acetate Anion)

In general chemistry, you are taught that triple bonds are shorter and stronger than double bonds, and double bonds are shorter and stronger than single bonds.  However, when chemists examined the structure of the acetate ion, they noticed that each C-O bond was the same length.

Acetate anion

The C-O bonds are the same, and the -1 charge is shared by each oxygen atom.  We would need to use Quantum Mechanics to determine the exact charge on each oxygen, but they would be equal.  What would you guess the charge to be close to on each O atom?

Using MOPAC the following electrostatic potential map for the acetate anion was calculated.  Red regions have more negative charge than blue regions.  Note that each oxygen atom is the same hue of red (i.e. they have the same charge).  The charge on each O atom was determined to be -0.64. 

We use curved arrow notation to obtain the different resonance structures.  Recall our previous discussion of Curved Arrows that when we move two electrons we use double-headed arrows as follows. 

It is very important to understand that this does not imply that the two structures are in equilibrium.  Furthermore, the electrons are not moving as depicted by the arrows.  When we say that the electrons aren't moving, that's not necessarily the entire story.  The electrons are constantly in motion.  However, the electron distribution does not change or fluctuate as the resonance structures do.  You must imagine the real electron distribution is a combination of all these, but the electrons are not moving in the sense of the arrows. When a molecule has multiple valid Lewis structures, the true structure is a resonance hybrid—a blend of these resonance forms.

So in summary for the acetate anion, resonance tells us

1. The C-O bonds are indistinguishable
2. The negative charge is shared between both oxygen atoms, each with a charge somewhere between 0 and -1.

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