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

9. Alkenes and Alkynes

Hydrogenation of Alkynes

Organic Chemistry

9. Alkenes and Alkynes

Hydrogenation of Alkynes

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3:31 minutes

Problem 38b

Textbook Question

What reagents would you use for the following syntheses?
b. (E)-3-hexene from 3-hexyne

Verified step by step guidance

Step 1: Begin by understanding the transformation required. The goal is to convert 3-hexyne (a triple bond) into (E)-3-hexene (a double bond with trans stereochemistry). This involves selective reduction of the alkyne to an alkene with control over stereochemistry.

Step 2: To achieve the (E)-configuration (trans alkene), use a dissolving metal reduction. This method typically involves reagents such as sodium (Na) in liquid ammonia (NH₃). The dissolving metal reduction selectively produces the trans alkene due to the mechanism of electron transfer and protonation.

Step 3: Write the reaction mechanism. The dissolving metal reduction involves the addition of electrons from sodium to the alkyne, forming a radical anion intermediate. Protonation from ammonia then leads to the trans alkene.

Step 4: Ensure stereochemical control. The dissolving metal reduction is preferred for forming the (E)-alkene because it minimizes steric hindrance and stabilizes the trans configuration during the reaction.

Step 5: Verify the product. After the reaction, confirm that the product is (E)-3-hexene by analyzing its stereochemistry using techniques such as NMR spectroscopy or IR spectroscopy to ensure the double bond is in the trans configuration.

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Key Concepts

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

Alkyne to Alkene Conversion

The conversion of alkynes to alkenes typically involves a process called hydrogenation or partial hydrogenation. In this case, 3-hexyne, a terminal alkyne, can be converted to (E)-3-hexene by using a reagent that selectively adds hydrogen across the triple bond, resulting in the formation of a double bond while maintaining the desired stereochemistry.

Stereochemistry of Alkenes

Stereochemistry refers to the spatial arrangement of atoms in molecules and is crucial in determining the properties of alkenes. The (E) designation indicates that the higher priority substituents on either side of the double bond are on opposite sides, which can be achieved through careful selection of reagents during the synthesis process to ensure the correct geometric isomer is formed.

Reagents for Selective Hydrogenation

To achieve the selective hydrogenation of 3-hexyne to (E)-3-hexene, specific catalysts such as Lindlar's catalyst (palladium on calcium carbonate) can be used. This catalyst allows for the partial hydrogenation of alkynes to alkenes without fully saturating the double bond, thus preserving the desired alkene configuration.

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Related Practice

Textbook Question

We have studied a number of pericyclic reactions previously. Draw the mechanism of the steps shown. The section number where this material was first studied is given for your review.

(a)