Questions and Answers
What are geometric isomers?
Molecules with the same structural formula, but which are different as a result of groups being locked into their spatial positions with respect to one another.
What structural features give rise to geometric isomerism?
A C=C double bond, ring, or other rigid structure.
What aspect of the C=C double bond gives rise to geometric isomerism?
The C=C double bond cannot rotate.
Which of these alkenes shows stereoisomerism?
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Draw the Cis and Trans isomers of but-2-ene.
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When to use cis/trans rather than E/Z?
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How are E/Z isomers distinguished?
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For E/Z how are the priority groups positioned?
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Do cis/trans and E/Z always agree?
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Draw the structure of (Z)-1-bromo-2-chloro-1-fluoroethane.
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Identify the Z geometric isomer and explain your reasoning.
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Identify the E geometric isomer and explain your reasoning.
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How many geometric isomers are there of hexa-2,4-diene?
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Study Notes
Geometric Isomerism Overview
- Geometric isomers have identical structural formulas, differing due to fixed spatial arrangements of groups.
- Structural features that lead to geometric isomerism include C=C double bonds, rings, or other rigid structures.
C=C Double Bond Characteristics
- The C=C double bond is rigid and does not allow rotation, which is crucial for the existence of geometric isomers.
Stereoisomerism in Alkenes
- C₃H₆ (propene) and C₄H₈ (butene) structures that exhibit stereoisomerism are C₃H₆ and C₄H₈, specifically CH₃CH=C(CH₃)₂ and CHBr=CHBr.
Cis and Trans Isomers
- But-2-ene (CH₃CH=CHCH₃) can exist as cis (same side) and trans (opposite sides) isomers.
Cis/Trans vs. E/Z Notation
- Use cis/trans notation when a C=C bond has one non-hydrogen group attached to each carbon.
- E/Z notation relies on Cahn-Ingold-Prelog (CIP) priority rules for differentiating isomers.
E/Z Isomer Distinctions
- In E/Z isomers, "Z" stands for together (German: zusammen) and "E" stands for opposite (German: entgegen).
Agreement Between Cis/Trans and E/Z
- Cis/trans and E/Z designations can diverge, as seen in 2-bromobut-2-ene, where cis has CH₃ groups on the same side, but is represented as E due to priority rules.
Specific Isomer Structures
- For (Z)-1-bromo-2-chloro-1-fluoroethane, Br and Cl are identified as the priority atoms.
- In molecular identification, the Z isomer has higher priority groups together, while the E isomer has them apart, often assessed by atomic mass in isotopes.
Geometric Isomers of Hexa-2,4-diene
- Hexa-2,4-diene can exhibit three distinct geometric isomers, illustrating combinations similar to flipping coins for outcomes.
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Test your understanding of geometric isomerism with these flashcards. Explore key concepts such as the definition of geometric isomers and the structural features that lead to their formation. Perfect for chemistry students looking to reinforce their knowledge.
