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

Acetylide

Organic Chemistry

9. Alkenes and Alkynes

Acetylide

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6:30 minutes

Problem 58

Textbook Question

A chemist attempted to do the following acetylide alkylation reaction but was unsuccessful in several attempts, producing only the original starting materials in each case. Explain why the reaction didn't work.

Verified step by step guidance

Understand the reaction: Acetylide alkylation involves the reaction of a terminal alkyne with a strong base to form an acetylide ion, which then reacts with an alkyl halide to form a new carbon-carbon bond. The failure of this reaction suggests an issue with one or more of these steps.

Step 1: Analyze the base used. For the formation of the acetylide ion, a sufficiently strong base (e.g., sodium amide \( \text{NaNH}_2 \)) is required to deprotonate the terminal alkyne. If the base is not strong enough, the acetylide ion will not form, and the reaction will fail.

Step 2: Examine the alkyl halide. Acetylide ions are strong nucleophiles, and they react best with primary alkyl halides via an \( S_N2 \) mechanism. If the alkyl halide is secondary or tertiary, steric hindrance will prevent the \( S_N2 \) reaction, and elimination (\( E2 \)) may occur instead, leading to no desired product.

Step 3: Consider solvent effects. The reaction typically requires an aprotic solvent (e.g., tetrahydrofuran or dimethyl sulfoxide) to stabilize the acetylide ion and facilitate the \( S_N2 \) reaction. If a protic solvent is used, it may interfere with the nucleophile or promote side reactions.

Step 4: Check for competing reactions. If the alkyl halide is prone to elimination (e.g., due to the presence of a strong base or a bulky alkyl halide), the reaction may favor \( E2 \) elimination over \( S_N2 \) substitution, resulting in no formation of the desired product.

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Key Concepts

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

Acetylide Ion

An acetylide ion is a negatively charged species formed by deprotonating a terminal alkyne. It is a strong nucleophile, capable of attacking electrophiles in nucleophilic substitution reactions. Understanding the stability and reactivity of acetylide ions is crucial for predicting their behavior in alkylation reactions.

Nucleophilic Substitution Mechanisms

Nucleophilic substitution mechanisms, such as SN2 and SN1, describe how nucleophiles replace leaving groups in organic molecules. The success of an alkylation reaction involving acetylide ions depends on the nature of the substrate and the mechanism involved. Factors like steric hindrance and the stability of the leaving group can significantly influence the reaction outcome.

Steric Hindrance

Steric hindrance refers to the prevention of chemical reactions due to the spatial arrangement of atoms within a molecule. In the context of acetylide alkylation, if the electrophile is sterically hindered (e.g., tertiary alkyl halides), the acetylide ion may struggle to effectively attack, leading to a lack of reaction and the persistence of starting materials.

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Related Practice

Textbook Question

Draw the product of each of the following reactions:

Textbook Question

Show how you might synthesize the following compounds, using acetylene and any suitable alkyl halides as your starting materials. If the compound given cannot be synthesized by this method, explain why.

d. 4-methylhex-2-yne

e. 5-methylhex-2-yne

f. cyclodecyne

Textbook Question

Show how the following compounds can be prepared, using ethyne as one of the starting materials:

3. 2-methyl-3-hexyn-2-ol

Textbook Question

Why is a cumulated diene not formed in the reaction shown above?

Textbook Question

Using the given starting material, any necessary inorganic reagents and catalysts, and any carbon-containing compounds with no more than three carbons, indicate how each of the following compounds can be prepared:

Textbook Question

Suggest a method for synthesizing the following alkynes using an alkyne and an alkyl halide. [There are two correct answers for each product.]

(a)

Textbook Question

Show how you would accomplish the following synthetic transformations. Show all intermediates.

c. but-1-yne → oct-3-yne

Multiple Choice

Predict the major, organic product of the following reaction.

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