Drawing Mechanisms: Curly Arrow Conventions That Earn Marks
A mechanism is marked on the arrows, not on the products. Getting the conventions right is most of the score.
What an arrow means
A double-headed curly arrow shows the movement of an electron pair. A single-headed arrow, or fishhook, shows the movement of a single electron and is used for radical mechanisms only.
Where arrows start
- A lone pair on a nucleophile.
- A pi bond, when the alkene or carbonyl acts as the electron source.
- A sigma bond, when a bond breaks and its electrons move elsewhere.
- A negative charge, which implies a lone pair.
Where arrows end
- At an atom, forming a new bond to it.
- At a bond, when that bond is broken and the electrons go to one atom.
- At a position between two atoms, forming a new pi bond.
The checks that catch errors
- Count electrons. Each arrow moves two, so charges must balance across the step.
- Check formal charges on every intermediate. A carbon with four bonds is neutral; three bonds and no lone pair is positive; three bonds and a lone pair is negative.
- Check valency. Carbon never has five bonds. An arrow that would produce one requires a simultaneous arrow breaking another bond.
- Check the step is reasonable. A primary carbocation is rarely acceptable; a mechanism requiring one is usually wrong.
Point three is the most useful diagnostic. If drawing an arrow gives carbon five bonds, another arrow is missing — and finding it usually reveals the correct mechanism.
Common step types
| Step | Arrow |
|---|---|
| Nucleophilic attack | From the nucleophile's lone pair to the electrophilic atom |
| Leaving group departure | From the bond to the leaving group, onto the leaving group |
| Protonation | From a lone pair or pi bond to the hydrogen of the acid |
| Deprotonation | From the base to the hydrogen, and from the C–H bond to the carbon |
| Pi bond formation | From a lone pair or sigma bond into the space between two atoms |
| Carbonyl attack | From nucleophile to carbon, and from the C=O pi bond onto oxygen |
The last row is worth learning as a pair, since attacking a carbonyl always requires two arrows — one forming the new bond and one moving the pi electrons onto oxygen. Drawing only the first gives carbon five bonds.
Errors that lose marks
- Arrows starting at a positive charge instead of at the electrons.
- Missing the second arrow when a carbonyl is attacked.
- Omitting formal charges on intermediates.
- Skipping steps — each elementary step needs its own arrows.
- Using double-headed arrows for radical mechanisms.
Skipping steps is particularly costly, because marking schemes allocate marks per step. Two steps compressed into one lose the mark for the omitted one even where the final product is correct.
Frequently asked questions
Which way should the arrow curve?
The direction of curvature carries no meaning; only the start and end points matter. Curve it however keeps the diagram clear.
Do I need to show every proton transfer?
Yes in a full mechanism. Protonation and deprotonation are genuine steps and usually carry marks of their own.
How do I know how many steps a mechanism has?
Each elementary step involves one bond-forming or bond-breaking event, or a concerted set of them. Any step that would produce an impossible valency must be split.
When should fishhook arrows be used?
Only for radical mechanisms, where single electrons move. Using them in an ionic mechanism, or double-headed arrows in a radical one, is a convention error that is penalised.
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