MolScript

Short answer: MolScript is a short text language for reaction schemes and curly arrow mechanisms. Open Templates in the left rail and pick the MolScript tab. Write species as SMILES and one move line per arrow, then click Draw on canvas. MolDraw draws the scheme and checks every arrow.

Last updated 20 Oct 2026 · By the MolDraw team at LabCanvas

You write a few lines of text. MolDraw draws the structures, plus signs, reaction arrows, conditions, lone pairs, charges and curly arrows. The result is normal canvas content. You can edit it, export it or save it as chemfig. The AI chat also writes MolScript when you ask it for a mechanism.

For a long tutorial, read MolScript: a text language for mechanisms. For finished examples, see the MolScript gallery.

Draw your first script#

The scheme is placed beside your drawing as one undo step. On a phone, the editor opens as a bottom sheet.

A complete example#

This is the SN2 script from the examples list:

title: SN2 of bromomethane
[OH-:1] "hydroxide" + [CH3:2][Br:3] "bromomethane" ->[SN2][acetone] CO "methanol" + [Br-]
move :1 -> 1..2
move 2-3 -> 3

• title: puts a heading above the scheme.
• Species are SMILES. :1, :2, :3 are atom map numbers on the atoms the arrows touch.
• "hydroxide" is a label drawn under the species.
• ->[SN2][acetone] is a reaction arrow with text above and below it.
• move :1 -> 1..2 is a curly arrow from the lone pair on atom 1 to a new bond between 1 and 2.
• move 2-3 -> 3 moves the 2 to 3 bond onto atom 3.

Scheme operators#

Put spaces around operators. A line that starts with an operator continues the scheme, so a multi-step mechanism reads one step per line.

Curly arrow ends#

Use -> in a move for a two-electron arrow and ~> for a one-electron fish-hook. This example from the gallery splits bromine with light:

[Br:1][Br:2] ->[hν] [Br] + [Br]
move 1-2 ~> 1; 1-2 ~> 2

push writes a resonance chain once. push :3 > 1 > 2 is the same as move :3 -> 3-1; 1-2 -> 2:

C[C:1](=[O:2])[O-:3] <-> CC([O-])=O
push :3 > 1 > 2

Options and directives#

• Species options go in braces after a species. Examples: {ts} for a transition state bracket, {highlight=2/3-4:blue}, {ox=1:II} for an oxidation state, {flip}.
• Names: let enolate = [CH2-:1]C(C)=O "enolate", then use $enolate.
• Abbreviations: [Ph], [Me], [tBu], [Boc], [R1], [Ar] and others.
• Directives: title:, per-row: (2 to 8, default 4) and layout: (row, grid, cycle or branch). Also number: species, check: (warn, strict or off) and # for a comment.

Electron checking#

MolDraw replays every arrow on the electrons. The structures it gets must match the products you wrote. Problems show under the editor with a line number, for example C (map 2) has no lone pair. Click a problem to jump to its line. It also reports an atom over its octet and an arrow from an atom with no electrons left. Stereocentres are followed too, so a missed SN2 inversion is reported. Draw on canvas stays off while there are errors.

Editor help#

• Words, options and map numbers are suggested as you type. Press Tab to take the first suggestion.
• Language guide opens the full tutorial.
• Bad lines are marked in the editor.

More examples#

The gallery shows the same mechanisms that are in the Example... list. For curly arrow drawing by hand, see curved arrow mechanisms. For arrow pushing theory, read the curly arrows guide.

Last updated: October 20, 2026