There are two different structural isomers for 2-butene. Single bonded carbon atoms allow for rotation around the C-C bond, however, placing a second bond (producing a double bond) between two carbon atoms stops the rotation around these atoms. How does this restriction of rotation give rise to two different structural isomers of 2-butene (cis/trans isomers). (Hint: consider cyclo compounds which also have cis/trans isomers.)