How to draw curly arrows (without losing marks)

What a curly arrow actually means

A curly arrow shows the movement of a pair of electrons from where they are now to where they end up. A full, double-headed arrow means two electrons move together; a single-headed “ fishhook” arrow means one electron moves on its own — you only need those for free-radical mechanisms like the chlorination of methane.

The three rules

1. 1. Start from the electrons

   Every curly arrow begins at a pair of electrons — either a lone pair or the middle of a bond. Never start an arrow from a positive charge or from an atom itself.

2. 2. Point to where they go

   The arrowhead shows the destination: onto an atom (making a lone pair / negative charge) or into the gap between two atoms (making a new bond).

3. 3. One arrow per electron pair

   A full (double-headed) curly arrow moves two electrons. If two pairs move in a step, you draw two arrows. A single-headed 'fishhook' arrow moves just one electron — only in free-radical mechanisms.

Nucleophiles vs electrophiles

Almost every arrow is one of these two ideas. A nucleophile is an electron-pair donor — it has a lone pair or negative charge, and arrows start from it (think OH-, CN-, NH₃). An electrophile is an electron-pair acceptor — it's electron-deficient (often a δ+ carbon), and arrows point towards it.

A worked example: hydroxide + bromoethane

Nucleophilic substitution of CH₃CH₂Br by hydroxide takes exactly two arrows in one step:

1. Arrow 1: from a lone pair on the OH- oxygen to the δ+ carbon bonded to bromine — this forms the new C–O bond.
2. Arrow 2: from the C-Br bond onto the bromine atom — the bond breaks heterolytically and bromine leaves as Br-.

The product is ethanol and a bromide ion. Notice the carbon never has five bonds at once — as the new bond forms, the old one breaks. That's the detail examiners look for.

Mistakes that lose marks

• Starting an arrow from a + charge instead of from a bond or lone pair.
• Arrows that start from an atom rather than from its electrons.
• Forgetting the arrow from the C–Br bond to Br when the leaving group leaves.
• Drawing an arrowhead onto an atom that already has a full octet.
• Using a fishhook (single-headed) arrow when a full pair is moving.

The fix for all of them: check every arrow starts on electrons and ends where a bond or lone pair can actually form.