Question

How are cis and trans isomers formed?

Answer 1

cis and trans isomers are formed by two functional groups across a double bond which does not allow for twisting

Explanation:

If the double bond has the same functional groups at all four positions no isomers are formed. But if there are two functional groups on carbons on either side of a double bond isomers can form.

Both functional groups could be on the same side of the double bond forming a cis isomer. ( both "up", or " down")

Both function groups could be on opposite sides of the double bond forming a trans isomer ( "one up", " one down" )