How to draw curly arrows in organic mechanisms

The rules

1. Start from electrons: a lone pair, or the middle of a bond (σ or π). Never start from a positive charge or a hydrogen atom.
2. End where the electrons go: at an atom (to make a new lone pair or negative charge), or between two atoms (to make a new bond).
3. A full arrowhead moves two electrons. A half-headed "fish-hook" arrow moves one electron, for radical reactions.
4. Keep charge balanced. The total charge is the same before and after each step.
5. Don't break the octet rule. If a new bond would give carbon five bonds, another bond must break in the same step.

Five patterns behind most mechanisms

1. Nucleophilic attack

A lone pair on a nucleophile attacks an electron-poor (δ+) atom. In SN2, a second arrow shows the leaving group's bond breaking at the same time.

2. A π bond attacks an electrophile

The C=C electrons attack H⁺ (or another electrophile) and a carbocation forms.

3. Attack on a carbocation

4. Addition to a C=O

The nucleophile attacks the carbonyl carbon and the C=O π electrons move onto oxygen.

5. Proton transfer

A base uses a lone pair to take H⁺, and the old bond's electrons stay behind.

Common mistakes

• Drawing the arrow backwards, from the electrophile to the nucleophile.
• Starting an arrow from H⁺ in "H⁺ attacks the alkene". The electrons come from the alkene, so the arrow starts at the C=C.
• Forgetting the second arrow when a bond to the leaving group breaks.
• Leaving out lone pairs and charges, which examiners mark.