Introduction

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.

What is Resonance?

Resonance occurs when a molecule or polyatomic ion has two or more possible valid Lewis structures that differ only in the placement of electrons (not atoms). These individual Lewis structures are called resonance structures (or resonance contributors). It is crucial to understand that resonance structures are hypothetical; they do not exist independently, but rather collectively describe the actual electronic structure of the molecule.

The true structure is a composite, or resonance hybrid, of all contributing resonance structures. This concept is vital for describing delocalized electrons – electrons that are not confined to a single bond or atom, but are spread out over several atoms. A single Lewis formula, which assigns an integer number of covalent bonds, cannot adequately express this delocalization.

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 (CH₃COO^-), 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.

Summary for the Acetate Anion:

• The C-O bonds are indistinguishable.
• The negative charge is shared between both oxygen atoms, each with a charge somewhere between 0 and -1 (specifically, approximately -0.64).

Key Takeaways on Resonance:

• Resonance describes molecules where electrons are delocalized over multiple atoms, rather than fixed in single positions.
• The true structure is a resonance hybrid, an average of all valid contributing Lewis structures.
• Resonance significantly influences a molecule's stability, bond lengths, and reactivity.
• Curved arrows are used to show electron movement between hypothetical resonance forms.

As you progress through this chapter, you will learn to identify and draw resonance structures for the three most common patterns of electron delocalization: a π bond between two atoms of different electronegativity, an atom with a lone pair or incomplete octet adjacent to a π bond, and extended conjugated systems.

Practice Time