Curved arrow mechanisms
Short answer: Pick the Reaction arrow tool, open the menu beside it and choose Electron flow (2e⁻) or Fishhook (1e⁻) under Mechanism. Drag from a lone pair, bond or atom to the atom or bond that receives the electrons. The arrow snaps to the structure and moves with it.
Curved arrows show where electrons go in a mechanism. MolDraw has two kinds. A curved arrow with a full head moves two electrons. A fishhook with a half head moves one electron, as in radical steps. For straight scheme arrows and reagent text, see Reaction arrows.
- Click the small menu beside Reaction Arrow in the left tool rail. Its name is Reaction arrow style.
- Under Mechanism, pick Electron flow (2e⁻) or Fishhook (1e⁻).
- Point at the electron source. A highlight shows the lone pair, bond or atom the tail will sit on.
- Drag to the target atom or bond and release. You can also click the start, then click the end.
- To change the curve, click the arrow and drag its middle handle.
Where the ends go#
| End | Snaps to | Tip |
|---|---|---|
| Tail (start) | Lone pair, bond or atom | Point at O, N or a halide and the tail starts on its nearest lone pair. This works even when the pairs are not drawn. |
| Head (end) | Atom or bond | The head never lands on the tail's own atom or bond. On an atom, it sits on the side that faces the tail. |
A bond as the tail means the bonding pair moves. A bond to one of its own atoms, like C=O to O, gives a clear arc beside the bond. Away from any structure, an end stays where you release it.
Tip
Hold Alt while you drag to place an end freely, without snapping.
Adjust an arrow#
- Curve: a new arrow bulges away from the structure. Click the arrow, then drag the middle handle to bend it more, less or to the other side.
- Ends: drag an end handle to move it. It snaps again to a new lone pair, bond or atom.
- Follow the structure: snapped ends stay attached. Drag the molecule and its arrows move with it.
- Style: pick the arrow tool with nothing selected to see the Electron flow panel. It sets Head (Filled, Open, Fishhook, None), Tail (None, Bar, Reverse, Circle) and Head size. For an arrow you already drew, select it and open Color & style.
The snap setting#
Open Settings, then General. In the Canvas group, Snap curved arrows is on by default. Turn it off to place every end freely. See the settings reference for the other canvas options.
Multistep mechanisms#
- Draw each species from left to right: starting material, intermediates, product.
- Join the steps with a Straight arrow from the Scheme group of the same menu.
- Add the curved arrows on the structures before the step they explain.
- Turn on Snap to grid in Settings, General, Canvas to line the steps up.
Curved and fishhook arrows are display only. They do not split the drawing into reactants and products, so reaction SMILES, Automap and the stoichiometry table ignore them.
To start faster, place a ready mechanism from mechanism templates, or write it as text in MolScript. The AI assistant can also draw a mechanism from a request.
Common problems#
| Problem | Fix |
|---|---|
| The tail starts on the wrong pair | Move the pointer closer to the pair you want before you press. |
| The arrow will not attach | Check that Snap curved arrows is on, and that you are not holding Alt. |
| You drew a plain curved arrow | Curved in the Scheme group is a bent reaction arrow. Pick Electron flow (2e⁻) under Mechanism instead. |
For the rules of arrow pushing, read the curly arrow guide. To practise, try the arrow pushing practice tool.
Questions and answers
How do I draw a fishhook arrow in MolDraw?
Open the menu beside the Reaction Arrow tool and pick Fishhook (1e⁻) under Mechanism. Drag from the bond or electron that moves to the atom that receives it.
Do I need to draw lone pairs before a curved arrow?
No. Point at a heteroatom and the tail starts on its nearest lone pair position. Draw the pairs if you want students to see the dots.
How do I stop curved arrows snapping to atoms?
Hold Alt while you drag to place one end freely. To turn snapping off for good, switch off Snap curved arrows in Settings, General, Canvas.
Why do my curved arrows move when I move the molecule?
Snapped ends are attached to the lone pair, bond or atom. This keeps the mechanism correct when you rearrange the drawing.
Can I change a curved arrow into a fishhook later?
Yes. Select the arrow, open Color & style and set Head to Fishhook.
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