Textbook Question
Draw and name all stereoisomers of 3-chlorohepta-2,4-diene
a. using the cis-trans nomenclature.
4. Alkanes and Cycloalkanes
Cis vs Trans
2:13 minutesProblem 42a,b
Textbook Question
Determine which compounds show cis-trans isomerism. Draw and label the isomers, using both the cis-trans and E-Z nomenclatures where applicable.
a. pent-1-ene
b. pent-2-ene
Verified step by step guidance1
Identify the type of isomerism: Cis-trans isomerism occurs in alkenes where each carbon of the double bond has two different substituents. This is also known as geometric isomerism.
Analyze compound a (pent-1-ene): Draw the structure of pent-1-ene. Note that the double bond is between the first and second carbon atoms. Since one of the carbons in the double bond is attached to two hydrogen atoms, cis-trans isomerism is not possible for pent-1-ene.
Analyze compound b (pent-2-ene): Draw the structure of pent-2-ene. The double bond is between the second and third carbon atoms. Check the substituents on each carbon of the double bond. If each carbon has two different groups, cis-trans isomerism is possible.
Draw and label the isomers for pent-2-ene: If cis-trans isomerism is possible, draw the 'cis' isomer where similar groups are on the same side of the double bond, and the 'trans' isomer where similar groups are on opposite sides.
Apply E-Z nomenclature: Assign priorities to the substituents on each carbon of the double bond using Cahn-Ingold-Prelog rules. Label the isomer as 'E' if the higher priority groups are on opposite sides, and 'Z' if they are on the same side.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Cis-Trans Isomerism
Cis-trans isomerism, also known as geometric isomerism, occurs in alkenes where two substituents are attached to each carbon of the double bond. If the substituents are on the same side, it's a 'cis' isomer; if on opposite sides, it's a 'trans' isomer. This type of isomerism is possible when there are different groups attached to the carbons of the double bond.
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E-Z Nomenclature
E-Z nomenclature is a more comprehensive system for naming geometric isomers, especially when there are more than two substituents. 'E' (from German 'entgegen') indicates that the highest priority groups are on opposite sides, while 'Z' (from German 'zusammen') indicates they are on the same side. Priority is determined using Cahn-Ingold-Prelog rules based on atomic number and connectivity.
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Alkene Structure
Understanding alkene structure is crucial for identifying isomerism. Alkenes are hydrocarbons with at least one carbon-carbon double bond, which restricts rotation and allows for isomerism. In pent-1-ene, the double bond is at the first carbon, limiting isomerism, while in pent-2-ene, the double bond at the second carbon allows for cis-trans isomerism due to different substituents on each carbon.
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