Fundamentals · 8 min read

Resonance and electron pushing: rules that stop mistakes

Resonance explains why some charges are stable and others are not, which in turn explains which pathway a mechanism takes. The rules are short, and most mistakes come from breaking one of them.

What resonance structures are

Resonance structures are different ways of drawing the same molecule. Only electrons move: the atoms stay exactly where they are. The real molecule is a single hybrid that is more stable than any one drawing suggests. The double-headed arrow ↔ means "contributes to the same hybrid". It is not an equilibrium, and the molecule does not flip between the structures.

The five rules for drawing them

1. Move only π electrons and lone pairs. σ bonds stay where they are.
2. Never move atoms. If an atom moves, you have drawn a different compound (an isomer), and the connection between them is a reaction, not resonance.
3. Keep the total charge the same on every structure, and the same number of electrons.
4. Respect the octet for second-row elements: carbon, nitrogen, oxygen and fluorine cannot have more than eight electrons. A carbon with five bonds is always wrong.
5. Each arrow starts at electrons (a lone pair or a bond) and ends where a bond or a lone pair forms.

The three arrow patterns

Almost all resonance (and most mechanism steps) are one of these three moves.

• Lone pair → adjacent π bond. A lone pair on an atom next to a double bond becomes a new π bond and pushes the old π bond onto the far atom as a lone pair. This is how an enolate or an allylic anion delocalises: ⁻CH₂–CH=O ↔ CH₂=CH–O⁻.
• π bond → adjacent empty orbital or cation. A π bond shifts toward a neighbouring positive charge, moving the charge along. This is how an allyl cation or a benzylic cation spreads out.
• π bond → adjacent more electronegative atom. A π bond moves onto an electronegative atom in a conjugated polar bond, as in the C=C–C=O system, where the β carbon becomes electron-poor. This explains the reactivity of enones in the Michael addition.

Which structure matters most?

Structures are not equal contributors. A better contributor looks more like the real molecule. Rank them with these guidelines, in order.

1. Full octets beat incomplete octets. The structure where every second-row atom has eight electrons usually dominates. That is why an oxocarbenium ion R₂C=O⁺R is better described with the C=O⁺ form than with C⁺–O.
2. Fewer formal charges are better, and no charge separation is best.
3. Negative charge on the more electronegative atom, positive charge on the less electronegative atom. An enolate is better drawn with the negative charge on oxygen than on carbon, but the carbon form still matters because carbon is where the enolate usually reacts.
4. More bonds is better. Structures with more covalent bonds are lower in energy.
5. More equivalent structures means more stabilisation. A carboxylate has two equivalent forms and a benzene ring has two Kekulé forms, which is why both are especially stable.

Why it matters for reactivity

Acidity. The more places a charge can be delocalised, the more stable the conjugate base. That is why carboxylic acids (pKa about 5) are so much more acidic than alcohols (about 16): see the acids and bases guide.

Nucleophilicity at two sites. An enolate has charge on both carbon and oxygen, so it can react at either. Most reactions of enolates with alkyl halides and carbonyl compounds go through carbon, as in the Base-catalyzed aldol addition of acetaldehyde and Claisen reactions. Hard electrophiles such as silyl chlorides prefer oxygen.

Stability of intermediates. A carbocation next to a double bond or an aromatic ring is stabilised by resonance, so allylic and benzylic substrates react faster by SN1. A radical is stabilised in the same way, which is why allylic and benzylic positions are the ones that brominate in a radical chain (propagation steps).

Directing effects in aromatic substitution. Drawing the resonance forms of the intermediate shows why an –OH or –OMe group directs substitution to the ortho and para positions: only there can the oxygen lone pair stabilise the positive charge. See electrophilic aromatic substitution.

Common errors

• Moving a σ bond or an atom. If an H moves, it is a tautomerisation or a proton transfer, not resonance.
• Arrows that start at an atom or a charge. Arrows always start at electrons, even though they often end at an atom.
• Forgetting a lone pair. If you remove a lone pair to make a bond, redraw the formal charge it leaves.
• Hypervalent carbon. Five bonds on carbon is never correct. Break one bond as you form another.
• Confusing resonance with equilibrium. Use ↔ for resonance and ⇌ for a real equilibrium between different species.

Practice

Take the allyl anion CH₂=CH–CH₂⁻ and draw the arrow that gives its second structure (the lone pair forms a π bond and the old π bond becomes the new lone pair). Then do the same for acetate, phenoxide and an enone. Once these feel automatic, the arrows in a full mechanism, such as the one in the guide to curved-arrow notation, are just resonance moves that happen to break a σ bond at the end.

Practise in the interactive app   Browse all 345 reactions

More study guides

• How to learn reaction mechanisms (without memorising everything)A practical method for learning organic reaction mechanisms: understand electron flow, group reactions into a few patterns, and practise by drawing.
• Curved-arrow notation: rules and the mistakes students makeHow to read and draw curved arrows in organic mechanisms: where they start and end, full-headed versus fish-hook arrows, and the most common errors.
• SN1, SN2, E1 or E2? How to decide which mechanism winsA clear decision guide for substitution versus elimination: substrate, nucleophile or base strength, solvent and temperature, with examples.
• Nucleophiles, electrophiles and leaving groups: what makes a reaction goWhat decides how reactive a nucleophile, an electrophile or a leaving group is, and how solvent, charge, size and resonance change reactivity.