Introduction

Resonance is one of the most important concepts in organic chemistry, but it is also one of the most misunderstood. Many students learn how to draw resonance contributors without ever stopping to ask what resonance actually is. In this lesson, we'll go back to a familiar Lewis structure example and use it to build a precise definition of resonance.

You'll see how multiple valid Lewis structures can exist for the same molecular formula, what makes two structures resonance contributors, and why the location of lone pairs and pi electrons is the key to understanding resonance. We'll also introduce one of the most important ideas in organic chemistry: resonance contributors are not separate molecules.

Starting with a Lewis Structure

To define resonance, let's start with the molecular formula C₂H₄.

The first step in drawing a Lewis structure is counting valence electrons.

• Each carbon contributes 4 valence electrons.
• There are 2 carbons, giving 8 electrons.
• Each hydrogen contributes 1 valence electron.
• There are 4 hydrogens, giving 4 electrons.

That gives a total of 12 valence electrons.

Next, place the carbon atoms in the center and connect the hydrogens to create the sigma bonding framework.

At this point there are five sigma bonds, accounting for 10 electrons. That leaves 2 electrons unaccounted for.

Following the Lewis Structure Rules

Many students immediately use those remaining electrons to form a pi bond between the carbons. While that eventually leads to the most stable structure, it is not technically the next Lewis structure rule.

The next rule is to place remaining electrons as lone pairs on the most electronegative available atom. In this molecule, that could place a lone pair on one of the carbon atoms.

This gives a structure with:

• A lone pair on one carbon
• A positive charge on the other carbon
• A complete accounting of all 12 valence electrons

While this structure is not very stable, there is nothing illegal about it. It follows the rules of Lewis structures and is therefore a valid Lewis structure.

A Second Valid Lewis Structure

We can also use that lone pair to form a pi bond between the two carbon atoms.

Doing so creates the familiar alkene structure with a carbon-carbon double bond.

Now we have two valid Lewis structures for the same molecular formula:

• One containing a lone pair and formal charges
• One containing a carbon-carbon pi bond

Both structures are valid representations of the same molecule.

What Is Resonance?

This leads to the definition of resonance.

Resonance occurs when we can draw multiple valid Lewis structures for the same molecular formula where:

• The sigma bonding network remains identical.
• The only differences are the locations of lone pairs and pi electrons.

The atoms stay connected in the same way.

Only the electron placement changes.

That is the defining feature of resonance contributors.

Resonance Arrows

Resonance contributors are connected by a special arrow.

This arrow has points on both ends and is called a resonance arrow.

The resonance arrow indicates that:

• The structures are related by resonance.
• The structures have the same connectivity.
• Only electron placement differs between them.

By convention, resonance contributors are also enclosed in square brackets to indicate that they belong to a related resonance set.

A Common Misunderstanding

One of the biggest misconceptions about resonance is that the contributors are real molecules that rapidly convert back and forth between one another.

That is not correct.

The resonance arrow is not an equilibrium arrow.

The molecule is not switching between one contributor and another.

It is not sometimes one structure and sometimes the other.

The Key Truth About Resonance

Neither resonance contributor actually exists.

This is one of the most important ideas in organic chemistry.

The resonance contributors are useful drawings that help us represent electron distribution, but neither contributor is a real, isolated structure.

When you draw resonance contributors, you should remember:

• They are not different molecules.
• They are not in equilibrium.
• They are not oscillating back and forth.

The only thing that actually exists is the resonance hybrid, which combines the features of all valid resonance contributors.

The resonance hybrid is discussed separately, but understanding that resonance contributors themselves do not exist is essential to understanding what resonance really means.

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