Overview
This lecture explains cis and trans isomerism in alkenes and rings, how to identify and name geometric isomers, and outlines when the cis/trans system does not apply.
Alkene Structures and Isomerism
- Alkenes are named by identifying the longest carbon chain and the position of the pi bond (e.g., 2-butene).
- 2-butene can exist as two geometric isomers: cis and trans, based on the relative position of substituents.
- Isomers have the same molecular formula but different arrangements; geometric (cis/trans) isomers differ in spatial arrangement due to restricted rotation.
- Free rotation around single bonds (alkanes) prevents geometric isomerism, while pi bonds (alkenes) restrict rotation, leading to locked configurations.
Criteria for Geometric Isomerism
- Geometric isomers occur when rotation is restricted, either by a double bond (pi bond) or a ring structure.
- For alkenes, only atoms directly attached to the sp2 carbons of the double bond are considered for cis/trans assignment.
- In rings, substituents can also be locked into cis or trans positions due to restricted rotation.
Identifying Cis and Trans Isomers
- Identify the pi bond and compare the groups attached to each double-bonded carbon.
- Cis: Substituents on the same side of the double bond or ring (“cisters”).
- Trans: Substituents on opposite sides (“transferred away”).
- Drawings may show both substituents up or down; the relative position is what matters.
Naming Cis and Trans Isomers
- Add “cis-” or “trans-” before the alkene name, positioning the number of the double bond appropriately (e.g., cis-2-butene, trans-2-butene).
- For compounds with multiple double bonds, specify configuration for each (e.g., 2-cis-4-trans-2,4-heptadiene).
Rings and Substituent Orientation
- In cyclohexane, wedges mean “up” and dashes mean “down”; both up = cis, one up and one down = trans.
- Cis/trans applies to any substituents (not just methyl groups), e.g., two halogens on the same side are cis.
Limitations of Cis/Trans System
- If only hydrogens are attached on one side of a double bond, cis/trans does not apply.
- When different types of substituents are present on the same double bond, cis/trans may not be applicable; use E/Z notation instead.
Key Terms & Definitions
- Isomer — Compounds with the same molecular formula but different structures.
- Geometric (cis/trans) isomer — Isomers differing in spatial arrangement due to restricted rotation.
- Cis isomer — Substituents on the same side of a double bond or ring.
- Trans isomer — Substituents on opposite sides of a double bond or ring.
- Pi bond — A bond formed by side-by-side overlap of p orbitals, restricting rotation.
- sp2 carbon — Carbon atom with one double bond (planar, 120° bond angles).
Action Items / Next Steps
- Review E/Z configuration naming for cases where cis/trans does not apply.
- Practice identifying and naming cis and trans isomers in alkenes and rings.
- Read about chair conformations for cyclohexane substituent orientation.