Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

5. Chirality

Constitutional Isomers vs. Stereoisomers

Organic Chemistry

5. Chirality

Constitutional Isomers vs. Stereoisomers

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4:59 minutes

Problem 9b

Textbook Question

Draw the six stereoisomers of octa-2,4,6-triene. Explain why there are only six stereoisomers, rather than the eight we might expect for a compound with three stereogenic double bonds.

Verified step by step guidance

Step 1: Understand the structure of octa-2,4,6-triene. It is a linear hydrocarbon with three conjugated double bonds located at positions 2, 4, and 6. Each double bond can exhibit cis (Z) or trans (E) stereochemistry, which contributes to the stereoisomer count.

Step 2: Recall that for a molecule with three stereogenic double bonds, we might initially expect 2^3 = 8 stereoisomers (since each double bond can independently be cis or trans). However, conjugation and molecular symmetry reduce the number of stereoisomers.

Step 3: Consider the conjugation of the double bonds. The π-electron system in conjugated double bonds imposes restrictions on the spatial arrangement of the molecule, limiting the possible combinations of cis and trans configurations.

Step 4: Analyze the symmetry of the molecule. Octa-2,4,6-triene has a symmetrical structure, which means some stereoisomers are identical due to reflection symmetry. This further reduces the number of unique stereoisomers.

Step 5: Draw the six stereoisomers systematically. Assign cis (Z) or trans (E) configurations to each double bond while ensuring that you account for conjugation and symmetry. Label each stereoisomer clearly and verify that no duplicates are included in your set of six.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Stereoisomerism

Stereoisomerism refers to the phenomenon where compounds have the same molecular formula and connectivity of atoms but differ in the spatial arrangement of their atoms. In the case of octa-2,4,6-triene, the presence of double bonds introduces geometric isomerism, leading to different configurations (cis/trans) around each double bond. Understanding stereoisomerism is crucial for determining the number of unique isomers a compound can have.

Stereogenic Centers

Stereogenic centers are atoms in a molecule that can lead to stereoisomerism when they are bonded to different substituents. In octa-2,4,6-triene, the double bonds can act as stereogenic centers, but not all double bonds contribute to the total count of stereoisomers due to symmetry and the presence of equivalent configurations. Recognizing which parts of a molecule are stereogenic is essential for calculating the total number of stereoisomers.

Symmetry and Equivalent Configurations

Symmetry in a molecule can reduce the number of unique stereoisomers by creating equivalent configurations that do not result in distinct isomers. In octa-2,4,6-triene, the arrangement of double bonds and their substituents leads to certain configurations being indistinguishable from one another. This concept explains why, despite having three stereogenic double bonds, only six unique stereoisomers exist instead of the expected eight.

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