Constitutional isomers, also called structural isomers, are molecules that have the same molecular formula but different structure.
In our case, the molecular formula is C2H4Cl2. All of the constitutional isomers must have two carbons, four hydrogens and two chlorines.
We can begin by drawing two carbons. There are two ways to add two chlorines. We could add both chlorines to the same carbon, or we could add one chlorine on each carbon. The two molecules are constitutional isomers. We can then add hydrogens to carbons as well. Each carbon wants to have four bonds and we add enough hydrogens to satisfy this.
Here are all of the constitutional isomers of C2H4Cl2
