How to draw resonance structures

The rules

1. Only move electrons: lone pairs, π bonds, and the charges that go with them. Atoms stay exactly where they are, and σ bonds never break.
2. Move electrons with curly arrows, from a lone pair or π bond to an adjacent atom or bond.
3. Never give an atom more than its octet. Carbon, nitrogen and oxygen can have at most eight electrons (four bonds for C).
4. Keep the overall charge the same in every contributor.
5. Connect contributors with a double-headed arrow (↔), not an equilibrium arrow. The real molecule is a hybrid of all of them. It doesn't switch between them.

Patterns to look for

Resonance needs a π bond next to something that can give or take electrons. These patterns cover most cases:

• Lone pair next to a π bond: the lone pair forms a new π bond, and the old π bond moves onto the far atom (carboxylates, amides, enolates).
• Positive charge next to a π bond: the π bond moves towards the charge, and the charge moves along (allyl cation).
• π bond next to a π bond, in a ring: the π bonds shift around the ring (benzene).

Examples

Which contributor matters most?

Contributors aren't equal. The major one is usually the structure with:

1. the most atoms with a full octet;
2. the fewest charges, or charges as close together as possible;
3. any negative charge on the more electronegative atom (O rather than C).

In the enolate, the form with the negative charge on oxygen is the major contributor. But enolates still react at carbon, which is why they make C–C bonds in aldol reactions.

Why resonance matters

Spreading out charge stabilises a species. That explains why carboxylic acids are more acidic than alcohols (the carboxylate is resonance-stabilised), why amides are much less basic than amines, and why benzene doesn't react like an alkene.

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