Resonance Between Three Atoms

Resonance: Electron Redistribution Between Three Atoms (Allylic Systems)

This section explores the next set of common and important resonance patterns you will encounter in organic chemistry, often involving electron delocalization over three adjacent atoms. There are two primary types of three-atom resonance systems:

1. Atom with an Incomplete Octet (e.g., Carbocation) Adjacent to a π Bond

This pattern involves an atom with an incomplete octet (most commonly a carbocation, which has only 6 valence electrons) immediately adjacent to a π bond. The electron redistribution in this pattern involves depicting the shifting of the 𝜋 bond towards the electron-deficient atom to form a new 𝜋 bond and delocalize the positive charge.

For example, consider the propene cation (also known as an allylic cation):

Propene

 The two resonance structures of the allylic cation, depicting the delocalization of the positive charge.

Notice how the positive charge is effectively shared between the two terminal carbons. This delocalization is captured by the resonance hybrid, which shows the partial positive charge distributed over these carbons:

The resonance hybrid of the allylic cation, showing delocalized π electrons and partial positive charges on the terminal carbons.

In the resonance hybrid, the charge is shared between each terminal carbon. Each C-C bond is also found to be the exact same length experimentally, a length that is intermediate between a normal C-C single and a C=C double bond.

The propene cation's electrostatic potential map further supports this delocalization:

Electrostatic potential map of the allylic cation, with blue regions indicating shared positive charge on the terminal carbons.

If you were a nucleophile (Nu^-, an electron-rich species), which carbon would you preferentially attack? The electrostatic potential map clearly shows that the positive charge is distributed equally across the two terminal carbons, making both sites susceptible to nucleophilic attack.

2. Atom with a Lone Pair of Electrons Adjacent to a π Bond

When an atom possessing a lone pair of electrons (e.g., an anion, or a neutral atom like oxygen or nitrogen in an alcohol or amine) is immediately adjacent to a π bond, a different resonance pattern emerges. Here, the lone pair is depicted as shifting to form a new π bond, simultaneously causing the existing π bond to shift onto the adjacent atom.

General pattern for a lone pair next to a π bond, showing the lone pair's electrons forming a new π bond and pushing the existing π bond.

The allylic anion shown below is a good example of this type of resonance:

The two resonance structures of the allylic anion, illustrating the delocalization of the negative charge.

Your Turn!