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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4:13 minutes

Problem 38a

Textbook Question

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

Verified step by step guidance

Step 1: Recognize that the problem involves converting 3-hexyne (an alkyne) into (Z)-3-hexene (a cis-alkene). This requires selective reduction of the triple bond to a double bond while ensuring the cis configuration.

Step 2: Understand that a common method for achieving cis-selective reduction of alkynes is using Lindlar's catalyst. Lindlar's catalyst is a palladium catalyst modified with lead and quinoline, which selectively reduces alkynes to cis-alkenes without further reducing them to alkanes.

Step 3: Write the reaction mechanism conceptually: 3-hexyne reacts with hydrogen gas (H₂) in the presence of Lindlar's catalyst. The catalyst facilitates the addition of hydrogen atoms to the triple bond, forming a double bond with the cis configuration.

Step 4: Confirm that the product is (Z)-3-hexene. The 'Z' designation indicates that the two substituents on the double bond are on the same side, which is consistent with the cis configuration achieved using Lindlar's catalyst.

Step 5: Summarize the reagents needed for this synthesis: 3-hexyne, hydrogen gas (H₂), and Lindlar's catalyst. These will selectively reduce the alkyne to the desired cis-alkene.

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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 the use of reagents that can selectively reduce the triple bond to a double bond. Common methods include catalytic hydrogenation or the use of Lindlar's catalyst, which allows for the formation of cis-alkenes from alkynes. Understanding this process is crucial for synthesizing (Z)-3-hexene from 3-hexyne.

Stereochemistry of Alkenes

Stereochemistry refers to the spatial arrangement of atoms in molecules and is essential in determining the properties of alkenes. In the case of (Z)-3-hexene, the 'Z' configuration indicates that the highest priority substituents on the double bond are on the same side. Recognizing how to manipulate stereochemistry during synthesis is vital for achieving the desired product.

Reagents for Selective Reduction

Choosing the right reagents for selective reduction is key in organic synthesis. For converting 3-hexyne to (Z)-3-hexene, one might use hydrogen gas with a catalyst like Lindlar's or sodium in liquid ammonia, which selectively reduces the alkyne to the cis-alkene. Familiarity with these reagents and their mechanisms is essential for successful synthesis.

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Related Practice

Textbook Question

Describe the alkyne you should start with and the reagents you should use if you want to synthesize

a. pentane.

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)

Textbook Question

What alkene would you start with if you wanted to synthesize

b. ethylcyclopentane?

Textbook Question

a. How many alkenes could you treat with H₂, Pd/C to prepare methylcyclopentane?

b. Which of the alkenes is the most stable?

c. Which of the alkenes has the smallest heat of hydrogenation?

Textbook Question

What reagents would you use for the following syntheses?

b. (E)-3-hexene from 3-hexyne

Textbook Question