Geometric Isomerism

AQA AS-level ChemistryAlkenesLesson 5 of 23

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:

More in Alkenes

• General Formula
• Naming Alkenes
• Bonding
• Position Isomerism
• Nomenclature (Cahn-Ingold-Prelog)
• Physical Properties of Alkenes
• Combustion
• Electrophiles

All 23 lessons in Alkenes · All AQA AS-level Chemistry topics