Introduction

Resonance is one of the first major concepts students encounter in organic chemistry, and it is often one of the most confusing. Once you understand what resonance is, the next challenge is determining whether a structure you have drawn is actually a valid resonance contributor.

In this lesson, we'll focus on the rules for evaluating resonance contributors. This is not a lesson on how to draw resonance structures with curved arrows. Instead, we'll look at the requirements that every valid resonance contributor must satisfy. These rules provide a quick way to check your work and determine whether a proposed structure belongs in a resonance series.

Main Content

Rule 1: Only Lone Pairs and Pi Electrons Can Move

The first rule for drawing resonance contributors is that only lone pairs and pi electrons can move.

The sigma bonding network must remain unchanged.

Resonance is based on conjugation. Because of that, resonance involves the movement of electrons through adjacent p orbitals, not the breaking and forming of sigma bonds.

A structure that breaks a carbon-hydrogen sigma bond is not a valid resonance contributor.

When evaluating a resonance structure, make sure:

• Sigma bonds stay the same.
• The atom connectivity stays the same.
• Only lone pairs and pi electrons move.

Rule 2: The Number of Radical Electrons Must Stay the Same

If the original structure contains radicals, the total number of unpaired electrons must remain constant throughout the resonance series.

For example, if a structure begins with no radical electrons and a proposed resonance contributor contains two unpaired electrons, that new structure is invalid.

You can draw resonance contributors for radical species, but:

• The radical must be present from the beginning.
• The total number of radical electrons cannot change.

Resonance can move radical electrons, but it cannot create or destroy them.

Rule 3: The Structure Must Be a Valid Lewis Structure

This rule is easy to overlook.

Every resonance contributor must be a valid Lewis structure.

That means:

• Carbon cannot have more than four bonds.
• Hydrogen cannot have more than one bond.
• Normal valence rules must be followed.

If the proposed structure violates basic Lewis structure rules, it cannot be a valid resonance contributor.

A useful corollary is that the number of valence electrons must remain the same throughout the resonance series.

You cannot add electrons.

You cannot remove electrons.

You only move the electrons that already exist in the molecule.

Rule 4: The Net Charge Must Stay the Same

Every resonance contributor must have the same overall charge.

This is one of the most useful ways to check your work.

For example:

• If the starting structure has a net charge of -1,
• Every valid resonance contributor must also have a net charge of -1.

If a proposed resonance contributor changes the overall charge of the molecule, it is invalid.

In many cases, a change in net charge also indicates that electrons have been added or removed incorrectly.

A Quick Resonance Checklist

Whenever you draw a resonance contributor, ask yourself these questions:

• Did I only move lone pairs and pi electrons?
• Did I leave all sigma bonds unchanged?
• Did I keep the same number of radical electrons?
• Is the structure still a valid Lewis structure?
• Did the total number of valence electrons remain constant?
• Did the net charge remain the same?

If the answer to all of these questions is yes, then you likely have a valid resonance contributor.

What Comes Next?

These rules help you determine whether a structure is a valid resonance contributor.

However, they do not tell you how to generate the next resonance contributor.

To do that, you'll need to learn how to use curved arrows and apply electron-pushing formalism correctly. Once you understand those tools, you can generate resonance contributors and then use these rules to verify that your structures are valid.

Resonance can be challenging when you're first learning organic chemistry, but understanding these rules is an important step toward mastering the topic.

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