Fundamentals · 7 min read

Curved-arrow notation: rules and the mistakes students make

A curved arrow shows the movement of an electron pair (or a single electron). It is a bookkeeping device: if you draw the arrows correctly, the structures before and after the arrow are automatically correct too.

The two kinds of arrow

Full-headed arrow (⟶, curved): moves a pair of electrons. Used in polar (ionic) mechanisms.
Half-headed "fish-hook" arrow (⇀): moves a single electron. Used in radical mechanisms, for example in radical chain reactions.

Do not mix them up. A full-headed arrow in a radical mechanism implies two electrons moving, which is a different reaction.

The golden rule: tail at the source, head at the destination

The tail of the arrow starts where the electrons are: a lone pair, or the middle of a bond. The head points to where they end up: an atom (forming a new lone pair or a new bond) or the middle of a bond (forming a π bond).

Lone pair to a new bond: in the SN2 reaction, the arrow goes from the nucleophile's lone pair to the carbon.
Bond to an atom: the C–Br bond arrow goes onto the bromine as it leaves with the bonding pair, becoming bromide.
Bond to a bond: the π bond of an alkene attacking a proton, as in acid-catalysed hydration.

Common mistakes

1. Arrows drawn the wrong way round

The most frequent error. Arrows run from electron-rich to electron-poor, never the reverse. If your arrow starts on a positive charge, it is almost certainly backwards.

2. Starting an arrow on an atom instead of on the electrons

An arrow should start from a lone pair or a bond, not from the nucleus label. In an acid–base step, the arrow starts on the base's lone pair, not on the base atom in general, and ends on the hydrogen.

3. Showing a proton moving as if it were an electron pair

A proton moves because the electrons in the O–H or C–H bond are pulled away. Show the arrow from the base's lone pair to H, and then a second arrow from the H–X bond to X.

4. Forgetting that a new bond needs a broken bond

Carbon has a maximum of four bonds. If an arrow forms a fifth bond, another arrow must remove one. In SN2 the C–Br bond breaks as the C–O bond forms. In E2, three things happen together: the base takes H, the C–H electrons form the C=C bond, and the C–Br electrons leave with bromide.

5. Charges that do not balance

Add up formal charges before and after. A negative ion attacking a neutral molecule gives a species with a net negative charge on the product side, either as a new anion or as a leaving group. If the charge total changes, an arrow is missing.

6. Drawing "impossible" intermediates

Primary carbocations, carbanions next to nothing stabilising them, or oxygen with three bonds and no charge. If your intermediate looks very unstable, step back and ask what the base or acid in the solution would really do.

Resonance arrows are different

Arrows that show resonance are not showing a reaction. They show different ways of drawing the same molecule. Use the double-headed straight arrow (↔) between resonance forms and keep the curved arrows only to show how to get from one form to the next.

A quick self-check routine

Say aloud where each arrow starts and ends.
Count electrons and formal charges after each step.
Check that every atom has a full set of bonds and lone pairs for its charge.
Ask whether the step is plausible in the given solvent and conditions.

Practise on the three-step mechanisms of SN1, ester hydrolysis and nucleophilic addition to a carbonyl, where the arrow patterns are clean and repeatable.

Practise in the interactive app Browse all 345 reactions