Geometric Isomerism

Stereoisomers have the same structural formula but a different arrangement of their atoms in space. Alkenes can show E/Z isomerism, a type of stereoisomerism (it used to be called geometric or cis-trans isomerism).Two conditions are needed:
1. There is restricted rotation about the C=C bond, because rotating would break the π bond. The groups on each carbon are locked in place.
2. Each carbon of the C=C has two different groups attached.But-2-ene, CH₃CH=CHCH₃, meets both conditions: each C=C carbon carries one H and one CH₃. It exists as two isomers that cannot be turned into one another at room temperature:In Z-but-2-ene the two CH₃ groups are on the same side of the double bond (Z from German zusammen, together). In E-but-2-ene they are on opposite sides (E from entgegen, opposite). When each carbon has one H, as here, Z is the same as cis and E is the same as trans.But-1-ene, CH₂=CHCH₂CH₃, has no E/Z isomers. Carbon 1 has two identical hydrogen atoms, so swapping the groups on that carbon gives the same molecule.The E/Z labels are formally decided by ranking the groups on each carbon by priority; that rule is covered in the next lesson. In but-2-ene the CH₃ groups are the higher-priority groups.