You can place a reaction arrow on the canvas at any time, even on a blank canvas
Only one reaction arrow can be placed on the whole canvas.
To place a reaction arrow
Select the Insert Reaction Arrow button. You will see the reaction arrow on the tip of the cursor when you move the mouse into the canvas area.
Click the location of the tail of the arrow.
Drag the mouse and release at the location of the head.
Once you have placed a reaction arrow on the canvas, MarvinSketch considers each part of the molecule in relation to the reaction.
All parts of the molecule that are on the left the arrow are considered reactants. Every molecule on the right the arrow is a product. Molecules placed along the arrow are considered agents.
You can align and/or distribute the objects of the reaction scheme by selecting the relevant option in Edit > Object menu.
The centers of the objects will be considered during the alignment/distribution of objects. Note that selected agent fragments and texts are moved together with the arrow while keeping the distance between them.
Once the reaction arrow has been placed on the canvas, MarvinSketch automatically puts plus signs between reactants and product molecules.
To avoid plus signs between specific reactants and/or products, select the relevant fragments and use the Structure > Reaction > Merge Reactants (Products) menu option.
To remove one or more molecules from such a merged item, select the relevant molecules and go to Structure > Reaction > Unmerge Reactants (Products).