Reaction and mechanism arrows

Implementation notes and historical checks. Open the visual guide →.

Arrows are editable drawing objects for reactions and mechanisms. Curved arrows can attach their ends to atoms, bonds and lone pairs and follow them when they move. Arrows never change the chemistry of the drawing.

Use it

Draw an arrow

1. Choose Arrow (A). Hold the tool or click its lower-right triangle to choose another preset.
2. Click empty space to place a fixed-length arrow pointing right, or drag to set its length and direction. Option/Alt allows any angle.
3. With an arrow tool active, click an existing arrow to give it that style. Repeated clicks cycle its direction, half-head side or equilibrium variant.

The palette explains each preset when you hover over it:

• Forward shows a net reaction.
• Equilibrium (⇌) shows a reversible reaction or interconversion of tautomers.
• Resonance (↔) links contributing structures of one species, not an equilibrium.
• Retrosynthesis (⇒) means the target is made from the indicated precursors, following Corey’s retrosynthetic arrow convention.
• Curved / electron pair shows an electron pair moving.
• Bent / elbow is a reaction arrow with a corner.
• Single electron shows one electron moving, including homolysis and radical steps.
• Dipole marks a bond dipole.
• No reaction means the reaction does not take place, following ChemDraw’s convention.

Bold, dashed, hollow, angled and half-arrow variants have the meaning of a Forward arrow; unequal equilibrium has the meaning of an Equilibrium arrow. Help also finds these meanings by preset name, such as “resonance” or “retrosynthesis”.

Only Forward, Equilibrium and Bent / elbow arrows accept new reaction roles. The Reaction roles list shows each arrow’s kind and explains when roles cannot be assigned. Older files retain existing roles on other arrow kinds and show a warning when opened.

Bend and aim an arrow

1. Select the arrow with Select or the arrow tool.
2. Drag a round end handle to move that end. Drag the diamond to bend the arrow.
3. On a curved arrow, drag a square handle to change how that end leaves or arrives. The endpoints stay attached; an attached head may move a little back or sideways to keep it clear of bond ink.

Each drag is one Undo step; Escape cancels it. In Properties, Reverse changes the direction, Flip bend mirrors the curve and Straighten removes the bend. Reset arrow style restores the preset’s look.

Optional target attachments

1. Choose Curved / electron pair or Single electron. Attach ends to atoms and bonds in Properties is on by default.
2. Click the source: an atom, a bond or a lone pair. It is highlighted, and the drawing does not change yet.
3. Click the destination. ReShiki adds one curved arrow in one Undo step.

For a free arrow, Option/Alt-click, drag, or turn off Attach ends to atoms and bonds. Escape cancels a picked source. An attached arrow follows its targets when they move. Detach start and Detach end in Properties release one end. Dragging an end also detaches it; dragging a square handle keeps the link.

Write conditions on an arrow

1. Choose Text (T) and click above or below a straight arrow.
2. Type the conditions and apply them. The text attaches to the arrow, centred on that side.
3. Text you drag onto an arrow’s upper or lower side attaches the same way.

The arrow grows to fit its widest label, with padding at each end. An arrow fitted to its text follows when you edit the text, and shrinks back to its shortest length when the text shrinks. An arrow you lengthened by hand keeps its length unless the text outgrows it. Dragging an end handle can lengthen the arrow freely; shortening it stops at the label width. Moving, rotating or reversing the arrow carries its labels.

Each side holds one label; use line breaks for several lines of conditions. Drag the text across the arrow to move it to the other side. Dragging perpendicular to the shaft adjusts its gap; dragging sideways beyond the arrow span detaches it. Vertical arrows take labels on their left and right. Copy, Cut and Delete of an arrow include its condition text; copying or deleting the text alone leaves the arrow. The eraser removes only what it touches.

Adjust label spacing

Select an attached condition label. In Properties → ARROW LABEL SPACING, enter a nonnegative Arrow gap and press Enter. The field uses the current ruler unit: mm, cm, in or pt. Zero is allowed. The gap is the clearance between the arrow ink and the text box, measured perpendicular to the shaft, including on diagonal arrows.

Drag the label perpendicular to the shaft to adjust the gap while keeping it attached and centred. Drag sideways beyond the arrow span, or choose Detach, to make it ordinary text. Auto restores the default spacing. Select several attached labels to apply one gap to all of them in one Undo step; selecting only the arrow does not show these label controls.

Magenta dimensions show the actual gap of selected labels, including the final position during a drag. They are editing guides and do not appear in SVG, PDF or PNG exports. See the native examples and saved drawing.

Custom spacing is stored in native .rsk format version 31. Drawings using only Auto continue to save as version 30. Older builds that cannot read the file’s version report that an update is required.

Style an arrow

Properties shows a preview and sets each end’s head (full, half or none), head shape, color, line style and width, head length, half-width and notch. Equilibrium arrows also set Reverse length (0.2–1) and Shaft separation (pt). Number and color fields apply when you press Enter.

Limits

• Arrows do not check electron counts, map atoms or generate mechanisms.
• Check structure adds informational hints for a resonance or equilibrium arrow with one complete molecule at each end: a resonance arrow between structures whose atoms or H differ suggests ⇌ for tautomers, and an equilibrium arrow between structures that differ only in π bonds or charge placement suggests ↔. Abbreviations, partial structures and multi-molecule sides are skipped.
• Only .rsk files keep the links between an arrow and its targets. Other formats keep the arrow’s shape.
• Multi-segment arrow splines, circular or elliptical arcs, broad hollow arrow bodies and automatic routing are not available.
• Some arrow styles cannot be exported to CDXML and give an export error; see CDXML exchange.
• Only straight arrows without target attachments carry condition text. Bending an arrow or attaching its ends releases its labels as ordinary text. Text that was already beside an arrow attaches only when you move it; editing it in place leaves it unattached.
• Only .rsk files keep the link between an arrow and its condition text. CDXML/CDX, SVG, PDF and PNG keep the text and the arrow where they are.

Implementation notes

Every new document restores JACS / ACS defaults: Arial 10 pt, black drawing colors, 14.4 pt bonds and 0.6 pt lines. Arrow overrides stay with the drawing. Selecting a template or storing a mixed-object template preserves arrow bends and styling. Arrow placement does not inherit the fixed bond length; the fixed-angles setting snaps arrows to 15°. The live preview uses the same geometry and style as the committed object. Cross/hash no-reaction marks and a dipole cross are available. Reverse keeps the curve’s shape. The diamond lies on the curve, and moving a cubic endpoint carries its adjacent direction control with it.

Attachment targets are a source atom, a visible bond or a positioned lone pair. Moving a target carries its endpoint and adjacent control. Target deletion detaches at the last resolved position, and Undo restores the reference. The attachment record has the desktop comparison, saved files and limits.

The assistant can lay out branching schemes with one shared reactant; this does not add chemical electron-flow semantics. See the feature status.

Condition labels

A document lists each label as an arrow ID, a text ID and a side (above or below); document version 27 added the list. An optional gap, stored in world units, requires version 31. The text stays an ordinary caption. Every edit re-checks the labels in the same Undo step:

1. Text dragged on its own re-centres in its slot. Perpendicular movement changes the saved gap or side; sideways movement beyond the arrow span detaches it.
2. New or moved text attaches when its centre lies within the arrow span (or the text spans the arrow midpoint) and it is no more than one bond length beyond the attached position. The nearest free side wins. Arrows that the same edit created or moved, such as pasted schemes, do not capture text.
3. The arrow is lengthened to at least the widest label plus 0.2 bond lengths at each end. A dragged end handle keeps the other end fixed; otherwise the midpoint stays. An arrow fitted to its text follows text edits to max(label width, 2.8 bond lengths) + 0.4 bond lengths, the spacing that assistant schemes use.
4. Auto places labels 0.45 bond lengths beyond the arrow ink, including heads and equilibrium shafts. A custom gap overrides that default and stays with the label when the arrow or its style changes.

Copying or duplicating an arrow copies its labels, and Delete or Cut removes them with it; copying the text alone gives ordinary text. Dragging one end onto the other keeps the arrow’s direction and stops at the label width. Molecule-name captions never attach. CDXML/CDX export reports that the links are omitted. ChemDraw records similar links as ReactionStepObjectsAboveArrow/BelowArrow in reaction steps; ReShiki does not yet read or write them.

Geometry and persistence

.rsk files keep arrow data from earlier versions, optional target references and stable lone-pair IDs. An arrow keeps its endpoints, either an optional quadratic control or two independent cubic controls, and appearance. Existing free quadratic curves keep their exact saved and rendered shape until a direction control is edited. Older ReShiki builds report that an update is required instead of discarding the links. Legacy curved arrows materialize their implicit bend before reflection, copying or transformation. Translation, rotation, resizing, grouping and template placement carry the controls with the endpoints. Stroke widths and head dimensions keep their publication point sizes when the geometry is resized. Attached endpoints use final-ink clearance; shrinking into fixed-size ink can require the bounded outward correction documented in the attachment record.

The same paths draw the canvas, the placement preview, SVG, PDF and PNG. Hit testing follows the curve and heads; selection bounds, lasso and alignment include the visible arrow paths and stroke extent. Hollow full heads leave an opening in the shaft instead of drawing through the hollow center.

CDXML exchange

Supported exchange includes straight full/half-headed arrows, straight retrosynthesis arrows with editable heads and style, head shape/dimensions/notches, color, solid/dashed strokes, resonance, dipole/no-go marks, equal-length equilibrium with half heads, and single cubic or quadratic Bézier arrows with solid heads. Cubic arrows keep both independently editable controls; quadratic arrows use their exact equivalent cubic control points. Mixed-object groups keep supported arrow children.

SVG/PDF/PNG and supported CDXML/CDX export the resolved attached curve and report that editing attachment links are omitted. .rsk files keep those links. This drawing metadata does not change molecular connectivity or electron counts.

The importer rejects circular/elliptical arrow arcs, multi-segment arrow splines, unknown head/stroke types and unsupported decoration. The exporter rejects dotted arrows, curved retrosynthesis arrows, bent/unequal equilibrium, equilibrium with full/absent heads, dipoles with a tail head, and curved arrows with hollow/angled heads or dipole/no-go decoration. Those combinations remain editable and exportable in .rsk/SVG/PDF/PNG. Interchange checks found that external saves can remove a dipole tail head or convert an angled Bézier head to solid; those combinations therefore return an explicit export error. Head dimensions are quantized to CDXML’s hundredths of a point; external applications can render strokes differently.

Records

The original arrow checks record the first build’s fixtures, desktop checks and tests.