E/Z isomerism
Rotation about a C=C double bond is blocked, so groups on each side stay fixed. Cis-trans (E/Z) isomers differ in which side groups sit. Rank the two groups on each carbon by priority: if the higher ones are on the same side it is Z, opposite sides is E.
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Analogy
Two people holding a rope between them cannot swap sides without letting go.
Example
Same-side CH₃ groups: (Z) / cisOpposite-side CH₃ groups: (E) / transBoiling points: cis 3.7 °C,trans 0.9 °C
In practice
Trans fats are unsaturated fats with trans double bonds, which pack together straight and behave more like saturated fats.
Practice
Flashcards
- What does Z mean in an alkene name?The higher-priority groups are on the same side of the double bond.
Questions people ask
Why can't double bonds rotate?
A double bond has a sigma bond and a pi bond. The pi bond is made by sideways overlap of p orbitals, and twisting would pull them apart and break it. That costs a large amount of energy, so at room temperature the groups stay fixed in place.
What is the difference between cis-trans and E/Z?
Cis-trans names work when each carbon has one identical or obvious group, such as the two methyl groups in but-2-ene. E/Z uses priority rules for any alkene and always gives an answer. When both can be used, cis usually matches Z and trans matches E.