Textbook Question
1. Determine which of the following compounds show cis-trans isomerism.
2. Draw and name the cis and trans (or Z and E) isomers of those that do.
e. 2,3-dimethylpent-2-ene
f. 3,4-dibromocyclopentene
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4. Alkanes and Cycloalkanes
Cis vs Trans
4:04 minutesProblem 8g,h
Textbook Question
Some of the following examples can show geometric isomerism, and some cannot. For the ones that can, draw all the geometric isomers, and assign complete names using the E-Z system.
(g) 
(h) 
Verified step by step guidance1
Identify the presence of a double bond in the given compounds. Geometric isomerism occurs in alkenes due to restricted rotation around the double bond.
For cyclohexene, note that it is a six-membered ring with a double bond. In small rings like cyclohexene, geometric isomerism is not possible because the ring size restricts the necessary spatial arrangement for E-Z isomerism.
For cyclodecene, which is a ten-membered ring with a double bond, the larger ring size allows for the possibility of geometric isomerism. The double bond can have substituents that are positioned differently in space, leading to E-Z isomers.
Draw the structure of cyclodecene, ensuring to represent the double bond and the substituents on either side of the double bond. Consider the spatial arrangement of these substituents to determine the E (entgegen) and Z (zusammen) configurations.
Assign the E-Z configuration to each isomer of cyclodecene by comparing the priority of the substituents on each carbon of the double bond according to the Cahn-Ingold-Prelog priority rules. The isomer with higher priority groups on opposite sides of the double bond is the E isomer, while the one with higher priority groups on the same side is the Z isomer.
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Video duration:
4mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Geometric Isomerism
Geometric isomerism, also known as cis-trans isomerism, occurs due to restricted rotation around a double bond or a ring structure, leading to different spatial arrangements of substituents. In cyclic compounds, the rigidity of the ring can allow for geometric isomers if substituents are positioned differently relative to the plane of the ring.
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E-Z Nomenclature System
The E-Z system is a method for naming geometric isomers based on the priority of substituents attached to the double bond. 'E' (entgegen) indicates that the highest priority groups are on opposite sides, while 'Z' (zusammen) means they are on the same side. This system is particularly useful for compounds with complex substituents where cis-trans nomenclature is insufficient.
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Ring Size and Isomerism
The size of a ring in cyclic compounds affects the possibility of geometric isomerism. Smaller rings, like cyclohexene, often do not exhibit geometric isomerism due to steric constraints, while larger rings, such as cyclodecene, can allow for different spatial arrangements of substituents, making geometric isomerism possible.
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