Question 11.3: Two isomers of 2-butene are shown below. Which is the cis is...

As we saw with cycloalkanes, the prefixes cis and trans refer to the placement of the substituents attached to a bond that cannot undergo free rotation. In the case of alkenes, it is the groups attached to the carbon-carbon double bond (in this example, the H and CH_3 groups). When the groups are on the same side of the double bond, as in the structure on the left, the prefix cis is used. When the groups are on the opposite sides of the double bond, as in the structure on the right, trans is the appropriate prefix.