What are cis and trans isomers?
Cis-trans isomers are stereoisomers that exist because there is restricted rotation about certain bonds.
There are two types of compounds that can exist as cis-trans isomers: (a) compounds with double bonds and (b) cyclic compounds.
(a) Compounds with double bonds
An alkene such as but-2-ene has two different groups on each alkene carbon.
The double bond prevents rotation about the C=C axis.
It can exist as cis-trans isomers that differ in the positions of the substituents on the double-bonded atoms.
One isomer has substituents on the same side (cis) of the double bond. The other has substituents on the opposite side (trans).
(b) Cyclic compounds
Cyclic compounds can exist as cis-trans because there is restricted rotation about single bonds.
For example, consider 1,2-dimethylcyclohexane.
The methyl groups are both "up" — on the same side of the ring or cis.
The cyclohexane ring is flexible but, no matter how much you twist it, the methyl groups will always be on the same side of the ring.
In the same way, methyl groups on opposite sides of the ring (trans) will always stay there.
So 1,2-dimethylhexane exists as a pair of cis-trans isomers.