Dichloroethane and Dichloroethene

........................................................

Dichloroethane has two isomers only. Either the two chlorine atoms are attached to one carbon atom, or one chlorine atom is attached to each carbon atom. Rotation about the single bond is possible, but this does not produce a different isomer.

.................................

Dichloroethene has three isomers. Rotation about the double bond is not possible. If the chlorine atoms are on the same side of the two carbons, we have the cis- form. If the chlorine atoms are on opposite sides of the two carbons, we have the trans- form.