Question

How do cis and trans isomers arise?

Answer 1

When we have a C=C with each carbon of the C=C having two different groups attached to it.

Explanation:

Because there is restricted rotation around C=C bonds, a consequence of the formation of the pi-bond being above and below the plane of the molecule, we can have molecules with the same structural formula but different spatial arrangement.

cis-isomers refer to alkenes where there are two groups the same, both on the same side of the double bond.

trans-isomers refer to alkenes where there are two groups the same but on either side of the double bond.

Image: http://www.chemguide.co.uk/basicorg/isomerism/geometric.html