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Answer 1 · 2017-03-28T01:34:00

1,2-Dichlorocyclopropane has cis and trans isomers because there is no free rotation about the #"C-C"# bonds.

The structures of the two isomers are shown below.

12Cl2

In one, both #"Cl"# atoms are on wedges.

They are both facing "up" and are on the same side of the molecule.

This is the cis isomer.

The second structure has one #"Cl"# atom on a dash and the other on a wedge.

They are on opposite sides of the ring, so this is the trans isomer.

It is impossible to convert one isomer into the other without breaking a #"C-C"# bond and rotating the atoms into place.

However, we are not allowed to break bonds to interconvert isomers.

Hence, the restricted rotation caused by the ring is responsible for the existence of the cis and trans isomers.