Cis-trans isomerism is a type of geometric isomerism in which compounds have the same molecular formula and connectivity but different spatial arrangements. It most often occurs around a carbon-carbon double bond, where rotation is restricted by the pi bond. This difference in shape can change physical properties such as boiling point, melting point, polarity, and solubility.
It also matters in biology, materials science, and medicine because molecular shape affects how molecules interact.
Key Facts
- Cis isomers have similar or identical groups on the same side of a double bond or ring.
- Trans isomers have similar or identical groups on opposite sides of a double bond or ring.
- Restricted rotation around a C=C bond occurs because the pi bond would have to break for rotation to happen.
- Cis-trans isomerism requires each carbon of the double bond to have two different substituents.
- E and Z naming uses priority rules: Z means higher-priority groups are on the same side, and E means they are on opposite sides.
- Alkenes have the general formula CnH2n when they are acyclic and contain one C=C bond.
Vocabulary
- Geometric isomer
- A geometric isomer is a compound with the same atom connections as another compound but a different fixed arrangement in space.
- Cis isomer
- A cis isomer has matching or comparable groups positioned on the same side of a double bond or ring.
- Trans isomer
- A trans isomer has matching or comparable groups positioned on opposite sides of a double bond or ring.
- Pi bond
- A pi bond is a covalent bond formed by sideways overlap of p orbitals, which restricts rotation in a double bond.
- E/Z nomenclature
- E/Z nomenclature is a system for naming alkene stereochemistry using priority rules for the groups attached to each double-bond carbon.
Common Mistakes to Avoid
- Calling every alkene cis or trans is wrong because cis-trans naming only works when each double-bond carbon has two different substituents and there are comparable groups to compare.
- Assuming single bonds show cis-trans isomerism is wrong because most single bonds rotate freely, so the positions are not fixed in the same way as around a double bond.
- Treating E/Z and cis/trans as always identical is wrong because E/Z uses priority rules and can describe more substituted alkenes where cis/trans labels are unclear.
- Ignoring three-dimensional arrangement in drawings is wrong because the same formula and atom connections can represent different stereoisomers with different properties.
Practice Questions
- 1 Draw cis-2-butene and trans-2-butene. Label which structure has the two CH3 groups on the same side of the C=C bond.
- 2 For 1,2-dichloroethene, draw the cis and trans isomers and predict which one is more polar based on the direction of the C-Cl bond dipoles.
- 3 Explain why 1-butene does not have cis-trans isomers, but 2-butene does.