E/Z Absolute Configuration of Alkenes
TLDRIn this educational video, Professor Dave explains the use of E and Z nomenclature for alkenes, particularly when cis and trans terminology becomes ambiguous in tri- or tetrasubstituted alkenes. He introduces the Cahn-Ingold-Prelog convention for prioritizing substituents and demonstrates how to determine if an alkene is E or Z based on the relative positions of the highest priority groups. With a simple mnemonic 'zee same side' for Z, the tutorial aims to clarify the absolute configuration of alkenes for better understanding and consistent labeling.
Takeaways
- 🧪 Cis and trans terminology is ambiguous for tri- or tetrasubstituted alkenes and requires an absolute configuration system.
- 🇩🇪 E and Z are German terms for alkenes, standing for 'entgegen' and 'zusammen', which refer to the relative positions of the highest priority groups on the double bond.
- 🔍 The Cahn-Ingold-Prelog (CIP) convention is used to prioritize substituents on the carbons of an alkene, similar to R/S assignment for chiral centers.
- ⚖️ The priority of substituents is determined by comparing atomic masses, starting with the first atom directly attached to the double-bonded carbons.
- 👀 When using CIP, it's important to focus on one atom at a time and not to consider the overall bulkiness or mass of the substituent.
- 🔄 In case of a tie in priority, continue the comparison to the next atoms until a difference is found.
- 📐 E alkenes have the two highest priority groups on opposite sides of the double bond, while Z alkenes have them on the same side.
- 🔑 The mnemonic 'zee zame side' can be used to remember that Z alkenes have the highest priority groups on the same side of the double bond.
- 🚫 Cis and trans cannot be used for tri- or tetrasubstituted alkenes without ambiguity, but E and Z can be universally applied.
- 📚 Understanding E and Z nomenclature is crucial for unambiguously describing the configuration of alkenes with multiple substituents.
- 📧 Professor Dave encourages viewers to subscribe for more tutorials and to reach out with questions, promoting engagement with the audience.
Q & A
What is the difference between cis and trans alkenes?
-Cis and trans are terms used to describe the spatial arrangement of substituents on disubstituted alkenes, where 'cis' means both substituents are on the same side of the double bond, and 'trans' means they are on opposite sides. However, these terms become ambiguous in tri- or tetrasubstituted alkenes, necessitating the use of E and Z terminology.
Why is E and Z terminology necessary for tri- or tetrasubstituted alkenes?
-E and Z terminology is necessary for tri- or tetrasubstituted alkenes because cis and trans become ambiguous when there are more than two substituents, as it's unclear what is 'cis' or 'trans' to which group.
What do E and Z stand for in the context of alkenes?
-E stands for 'entgegen' and Z stands for 'zusammen' in German, referring to whether the highest priority groups on each side of the double bond are on the same side (Zusammen - Z) or opposite sides (Entgegen - E).
How is the priority of substituents determined on the carbons of an alkene?
-The priority of substituents is determined using the Cahn-Ingold-Prelog convention, which assigns priority based on atomic mass, comparing substituents one atom at a time until a difference is found.
What is the Cahn-Ingold-Prelog convention used for?
-The Cahn-Ingold-Prelog convention is used for assigning R and S configurations to chiral centers and is also used to determine the priority of substituents on the carbons of an alkene for E and Z determination.
Can you explain the process of determining if an alkene is E or Z?
-To determine if an alkene is E or Z, first assign the priority of substituents on each carbon of the double bond using the Cahn-Ingold-Prelog convention. Then, look at the plane of the double bond and see if the two highest priority groups are on the same side (Z) or opposite sides (E).
What is the mnemonic for remembering the difference between E and Z alkenes?
-The mnemonic 'zee same side' can be used to remember that Z alkenes have the two highest priority groups on the same side of the double bond.
How does the presence of a tie in substituent priority affect the determination of E or Z configuration?
-In the case of a tie in substituent priority, you must continue to compare the next atoms in the substituents until a point of difference is found to determine the correct priority.
What is the importance of going one atom at a time when using the Cahn-Ingold-Prelog convention?
-Going one atom at a time ensures that the priority is determined accurately without being misled by the overall size or bulkiness of a substituent, which could incorrectly suggest its priority.
Can the terms cis and trans be used interchangeably with E and Z?
-No, cis and trans are specific to disubstituted alkenes and describe relative configurations, while E and Z are absolute configurations that can be used for any number of substituents on the double bond.
Why is it important to have an absolute configuration terminology like E and Z for alkenes?
-An absolute configuration terminology like E and Z is important because it provides a clear and unambiguous way to describe the spatial arrangement of substituents on alkenes, regardless of the number of substituents present.
Outlines
🧪 Understanding E and Z Notation for Alkenes
Professor Dave introduces the concepts of E and Z alkenes, explaining that while cis and trans terminology is clear for disubstituted alkenes, it becomes ambiguous for tri or tetrasubstituted alkenes. To resolve this, he discusses the absolute configuration terminology of E and Z, derived from the German words 'entgegen' and 'zusammen', which refer to the relative positioning of the highest priority groups on the double bond. The Cahn-Ingold-Prelog convention is used to assign priority to substituents based on atomic mass, and the method of determining E or Z configuration is explained through step-by-step analysis of example molecules.
📚 Mastering the E and Z Configuration with a Mnemonic
In the second paragraph, Professor Dave continues the discussion on E and Z alkenes, focusing on how to determine if the configuration is E or Z by examining the priority groups' positions relative to the double bond. He provides a mnemonic, 'zee zame side', to help remember that Z alkenes have the two priority groups on the same side of the double bond. The summary also includes a brief explanation of how to use the Cahn-Ingold-Prelog convention to avoid common pitfalls when identifying the priority of substituents, ensuring a clear understanding of the E and Z nomenclature.
Mindmap
Keywords
💡E and Z alkenes
💡cis and trans
💡tri- and tetrasubstituted alkenes
💡Cahn-Ingold-Prelog convention
💡R and S
💡Atomic mass
💡Stereochemistry
💡Double bond
💡Methyl group
💡Zee zame side
Highlights
Professor Dave introduces the concept of E and Z alkenes, explaining the limitations of cis and trans terminology for disubstituted alkenes.
The need for an absolute configuration system like E and Z to label tri and tetrasubstituted alkenes is discussed.
E and Z terminology originates from German, with E meaning 'entgegen' and Z meaning 'zusammen', referring to the relative positioning of substituents.
The Cahn-Ingold-Prelog convention is used to prioritize substituents on the carbons of an alkene.
The process of assigning priority to substituents involves comparing atomic masses one atom at a time.
An example is given where a carbon atom with a methyl group and hydrogen is prioritized over an implied hydrogen due to atomic mass.
The importance of not considering the overall bulkiness or weight of substituents, but rather their atomic mass, is emphasized.
A detailed example demonstrates the step-by-step process of determining the priority of substituents using the Cahn-Ingold-Prelog convention.
The concept of 'E' alkenes is explained as having the two highest priority groups on opposite sides of the double bond.
An example of an E alkene is provided, illustrating the application of the Cahn-Ingold-Prelog convention.
The ambiguity of cis and trans terminology in tri and tetrasubstituted alkenes is contrasted with the clarity provided by E and Z labeling.
A method to remember the difference between E and Z alkenes is introduced with the phrase 'zee zame side'.
An additional example is given to demonstrate the determination of Z alkene configuration using the same prioritization method.
The video concludes with a reminder of the importance of the Cahn-Ingold-Prelog convention in unambiguously labeling alkenes.
Professor Dave invites viewers to subscribe for more tutorials and encourages questions via email.
Transcripts
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