Introduction

In a previous lesson, we defined resonance contributors. But if resonance contributors don't actually exist, then what does exist? The answer is the resonance hybrid.

In this lesson, we'll look at the difference between resonance contributors and resonance hybrids, using an allylic carbocation as an example. We'll discuss how electron density is distributed throughout a conjugated system, why resonance contributors are not real structures, and how bond lengths provide experimental evidence for resonance. Most importantly, you'll learn how to draw a resonance hybrid and understand what it represents chemically.

Main Content

Drawing Resonance Contributors

Consider an allylic carbocation. The positive charge is the unhappiest place in the molecule because it lacks electron density.

To stabilize that positive charge, we can push electrons toward it from a neighboring pi bond. Moving the pi electrons forms a new pi bond and places the positive charge on a different carbon atom.

This gives us two resonance contributors.

The important question is:

Which of these resonance contributors actually exists?

The answer is neither.

Neither resonance contributor actually exists. These are not two different molecules. They are not in equilibrium with one another, and they are not oscillating back and forth.

The positive charge is not sometimes on one carbon and sometimes on another. Resonance contributors are simply different ways of representing electron distribution within a single molecule.

Evidence That Resonance Contributors Are Not Real Structures

Some students wonder whether resonance contributors are really just different ways of drawing the same molecule.

One way to test this idea is by looking at bond lengths.

If one structure truly contained a single bond and a double bond, the bond lengths should be different because single bonds are longer than double bonds.

However, when chemists measure the bond lengths in resonance-stabilized systems, they find something surprising.

The bond that appears as a single bond in one contributor and the bond that appears as a double bond in the other contributor have the same bond length.

Even more interesting, that bond length is intermediate between a true single bond and a true double bond.

This observation tells us that neither resonance contributor accurately describes the actual molecule by itself.

What Is the Resonance Hybrid?

The resonance hybrid is the weighted average of all valid resonance contributors.

It is not any one contributor individually.

Instead, it is a composite that combines the electron distribution shown by all valid resonance contributors.

The resonance hybrid is the only structure that actually exists.

Drawing the Resonance Hybrid

To draw the resonance hybrid, start with the carbon skeleton that is common to all resonance contributors.

Next, identify where the pi bonds appear throughout the resonance series.

If a pi bond appears in different locations among the contributors, the resonance hybrid represents that with partial pi bonds.

These partial pi bonds are typically shown as dotted lines.

For the allylic carbocation example:

• There is a partial pi bond between carbons 1 and 2.
• There is a partial pi bond between carbons 2 and 3.

This reflects the fact that the electron density is shared across both bonds.

Partial Charges in the Resonance Hybrid

The same idea applies to charges.

In the resonance contributors, the positive charge may appear on carbon 1 in one structure and carbon 3 in another.

In the resonance hybrid, the positive charge is distributed across both atoms.

Instead of drawing a full positive charge on either carbon, we draw partial positive charges using the Greek letter δ⁺.

This indicates that:

• Carbon 1 carries part of the positive charge.
• Carbon 3 carries part of the positive charge.
• Together they account for one full positive charge.

Because no resonance contributor places charge on carbon 2, carbon 2 remains neutral in the resonance hybrid.

Weighted Averages and Resonance

The resonance hybrid is described as a weighted average because not all resonance contributors necessarily contribute equally.

Some resonance contributors are more stable than others and therefore contribute more heavily to the hybrid.

In the allylic carbocation example, the contributors are equivalent, so they contribute equally.

For more complicated resonance systems, additional rules are used to determine the relative importance of each contributor.

Why Chemists Usually Draw Contributors Instead

Although the resonance hybrid is the real structure, it can be cumbersome to draw.

As a result, chemists often draw a single resonance contributor or an entire resonance series when discussing a mechanism or structure.

This is convenient, but it is important to remember that these drawings are only representations.

The actual molecule is always the resonance hybrid.

Whenever you draw resonance contributors, keep this idea in mind:

Neither resonance contributor actually exists. The only thing that exists is the resonance hybrid.

Practice This Skill

Ready to practice?

Try OChemNinja's Match the Structure.