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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0:55 minutes

Problem 25a

Textbook Question

Predict the product of the following alkyne reductions.
(a)

Verified step by step guidance

Step 1: Identify the type of reduction being performed on the alkyne. Common reduction methods include catalytic hydrogenation, Lindlar's catalyst, and dissolving metal reduction. Each method leads to different products.

Step 2: If the reduction is catalytic hydrogenation (using H₂ and a metal catalyst like Pt, Pd, or Ni), the alkyne will be fully reduced to an alkane. This involves the addition of two equivalents of H₂ across the triple bond.

Step 3: If the reduction is performed using Lindlar's catalyst (H₂ with a poisoned catalyst like Pd/CaCO₃), the alkyne will be partially reduced to a cis-alkene. This method selectively adds one equivalent of H₂ to the triple bond, stopping at the alkene stage.

Step 4: If the reduction is a dissolving metal reduction (using Na or Li in liquid NH₃), the alkyne will be partially reduced to a trans-alkene. This method involves electron transfer and protonation steps, leading to the anti-addition of hydrogen atoms.

Step 5: Based on the specific reduction method provided in the problem, predict the product by applying the appropriate mechanism and stereochemistry (cis or trans for alkenes, or fully saturated for alkanes).

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Key Concepts

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

Alkynes

Alkynes are hydrocarbons that contain at least one carbon-carbon triple bond. They are unsaturated compounds and are generally more reactive than alkenes and alkanes due to the presence of the triple bond. Understanding the structure and reactivity of alkynes is crucial for predicting the products of their chemical reactions, including reductions.

Reduction Reactions

Reduction reactions involve the gain of electrons or hydrogen, or the loss of oxygen, resulting in a decrease in oxidation state. In organic chemistry, reducing agents such as hydrogen gas (H2) in the presence of a catalyst or metal hydrides can convert alkynes to alkenes or alkanes. Recognizing the type of reduction and the conditions used is essential for predicting the final product.

Stereochemistry of Products

Stereochemistry refers to the spatial arrangement of atoms in molecules and how this affects their chemical properties. When reducing alkynes, the stereochemistry of the resulting alkenes can vary, leading to different isomers. Understanding stereochemical outcomes is important for accurately predicting the products of alkyne reductions, especially when considering cis/trans configurations.

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Related Practice

Textbook Question

Show how you would convert

a. oct-3-yne to cis-oct-3-ene.

b. pent-2-yne to trans-pent-2-ene.

Textbook Question

For the alkynes shows here, show the product(s) expected to form when treated under the following conditions: (i) H₂ , Pd/C; If you expect two products, show both.

(c)

Textbook Question

For the alkynes shows here, show the product(s) expected to form when treated under the following conditions: (ii) H₂, Pd/C, Pb(OAc)₂ , CaCO₃ (Lindlar's catalyst). If you expect two products, show both.

(c)

Textbook Question

For the alkynes shows here, show the product(s) expected to form when treated under the following conditions: (iii) Na⁰ , NH₃ (liquid), ―33°C . If you expect two products, show both.

(c)

Textbook Question

Predict the product of the following alkyne reductions.

(b)

Textbook Question

Predict the product of the following alkyne reductions.

(c)

Textbook Question

Suggest alkynes that might be used to make the following trans-alkenes.

(a)

Textbook Question

Suggest alkynes that might be used to make the following trans-alkenes.