Delocalization and Resonance

This guide is an early version — the text is complete, and a few figures are still being redrawn. Spotted something unclear? Let us know.

The question this page answers: How do we approximate the sharing of electrons between more than two atoms 1) in our mind and 2) in line-angle drawings?

Deeper reading: Clayden 2e: Chapter 7 Page 141–156 — see our chapter-by-chapter practice map for Clayden.

Delocalization of π electrons

When do p orbitals conjugate?

A combination of valence bond and MO theory can be used to describe delocalization of π electrons through multiple atomic p orbitals that are conjugated together to form extended π-systems.

These p orbitals will only conjugate if they are aligned and parallel with each other.

Here are some experimental observations that tell us valence bond theory is insufficient:

Simplified MO diagrams

What does the MO picture reveal?

To describe π delocalization, we draw simplified MO diagrams of only the valence p orbitals and electrons of the molecule. Here is an example for 1,3-butadiene, where there are four p orbitals and four electrons:

Figure coming soon — being redrawn for this guide.

Notice how this MO diagram reveals there is π bonding between C2–C3 in the π1 MO that is not reflected in the line-angle drawing.

We can explain the observed bond lengths of 1,3-butadiene by saying that there is a reduction in double-bond character of C1–C2 and C3–C4 that is compensated by gaining electron delocalization through increased double-bond character of C2–C3.

The concept of resonance

Do molecules flip between forms?

To visually and mentally “see” this delocalization that is present in conjugated systems, we rely on the concept of resonance.

Resonance requires us to

draw a few of the most reasonable Lewis structures for a given molecule (these are called resonance forms or resonance contributors)
determine their relative importance to the molecule
mix these resonance forms together in our mind, weighted by their relative importance, to create a resonance hybrid, which is the more accurate understanding of electron distribution.

It is crucial to remember that resonance forms are fictitious drawings that help us imagine the resonance hybrid. In other words, molecules do not exist as interchanging resonance forms, but rather exclusively as the resonance hybrid.

Here is how resonance would work for 1,3-butadiene:

Thus, much like the MO picture above, Resonance form 1 and 3 tell us that C2–C3 has some double bond character in the resonance hybrid, and that C1–C2 and C3–C4 have a little less double bond character.

Common conjugation and resonance motifs

Which patterns show up frequently?

There are several conjugation and resonance motifs that show up frequently.

Here are the most common patterns:

π bonds between atoms of different electronegativity

π bonds between atoms of different electronegativity

π bonds separated by a single bond

atom with lone pair adjacent to an empty atomic orbital

atom with lone pair adjacent to an empty atomic orbital

“3-atom” delocalization where π bonds are conjugated to…
a lone pair

an empty orbital

an unpaired electron

Relative importance of resonance forms

What makes one form more important?

The relative importance of resonance forms is determined by their relative stability. The most reasonable resonance forms will be the most stable Lewis structures.

Here is a general guideline of factors to consider, from most to least important, with the more important resonance forms depicted in red.