Draw the reaction, and then click the Mark
Bonds 
icon.
Click the bond(s) to mark the reaction site.
Mark bonds that are changed (broken, formed, or change bond order) in the reaction.
Draw the reaction, and then click the Mark
Bonds
icon.
Click the bond(s) to mark the reaction site.
The bonds are marked with a double line.
To remove the reaction site, click the marked bond again.
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Drawing and Editing Structures