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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Problem 15b

Textbook Question

Calculate for the following acid–base reactions. Which is the best base to use to deprotonate acetylene and make the acetylide anion?
(b) H₂N^- + H―C ≡ C―H ⇌ ^-C ≡ C―H + H₃N

Verified step by step guidance

Step 1: Understand the problem. The goal is to determine which base is best suited to deprotonate acetylene (H―C≡C―H) and form the acetylide anion (⁻C≡C―H). This involves comparing the acid–base equilibrium and analyzing the relative strengths of the acids and bases involved.

Step 2: Write the acid–base reaction. The given reaction is:

H₂N⁻ + H―C≡C―H ⇌ ⁻C≡C―H + H₃N

. Here, H₂N⁻ is the base, H―C≡C―H is the acid, ⁻C≡C―H is the conjugate base, and H₃N is the conjugate acid.

Step 3: Compare the pKa values of the acids. The pKa of acetylene (H―C≡C―H) is approximately 25, while the pKa of ammonia (H₃N) is approximately 38. Since a stronger acid has a lower pKa, acetylene is the stronger acid compared to ammonia.

Step 4: Analyze the equilibrium direction. The equilibrium of the reaction will favor the formation of the weaker acid and weaker base. Since ammonia (H₃N) is a weaker acid than acetylene, the equilibrium will favor the products (⁻C≡C―H and H₃N). This indicates that H₂N⁻ is a suitable base for deprotonating acetylene.

Step 5: Conclude the reasoning. Based on the pKa comparison and equilibrium analysis, H₂N⁻ is a strong enough base to deprotonate acetylene and form the acetylide anion (⁻C≡C―H). The reaction proceeds because the conjugate acid (H₃N) is weaker than the original acid (H―C≡C―H).

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Key Concepts

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

Acidity and Basicity

Acidity refers to the tendency of a compound to donate protons (H⁺), while basicity is the ability to accept protons. In organic chemistry, the strength of an acid is often measured by its pKa value; lower pKa values indicate stronger acids. Understanding the relative strengths of acids and bases is crucial for predicting the outcome of acid-base reactions.

Acetylide Anion

The acetylide anion (⁻C≡C―H) is formed when acetylene (H―C≡C―H) is deprotonated. This anion is a strong nucleophile and can participate in various reactions, including nucleophilic substitutions. Recognizing the structure and reactivity of the acetylide anion is essential for understanding its role in organic synthesis.

Strong Bases for Deprotonation

To deprotonate a weak acid like acetylene, a strong base is required. Common strong bases include alkyl lithium reagents and amides like H₂N⁻ (amide ion). The choice of base affects the efficiency of the deprotonation process, making it important to select a base that is stronger than the conjugate acid of the target compound.

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Related Practice

Textbook Question

Show the products of the following acetylide alkylation reactions. [Make sure your product has the correct number of carbons.]

(d)

Textbook Question

The introduction of elimination reactions provides a second way to synthesize alkynes in a two-step process starting with an alkene. Suggest a mechanism for both steps of this process.

Textbook Question

When the reaction scheme in Assessment 12.63 is done on a monosubstituted alkene, at least three equivalents of base are needed. Reacting the product of step 2 with D–Cl (D is an isotope of H) incorporates deuterium at the terminal carbon. Explain these two observations.

Textbook Question

Calculate K_eq for the following acid–base reactions. Which is the best base to use to deprotonate acetylene and make the acetylide anion?

(a) HO^- + H―C ≡ C―H ⇌ ^-C ≡ C―H + H₂O

Textbook Question

Show the products of the following acetylide alkylation reactions. [Make sure your product has the correct number of carbons.]

(b)

Textbook Question

Calculate for the following acid–base reactions. Which is the best base to use to deprotonate acetylene and make the acetylide anion?

(c)

Textbook Question

Suggest reagents you might use to generate the product from the given reactant.

(b)

Textbook Question

A chemist is planning to synthesize 3-octyne by adding 1-bromobutane to the product obtained from the reaction of 1-butyne with sodium amide. Unfortunately, however, he forgot to order 1-butyne. How else can he prepare 3-octyne?

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Calculate for the following acid–base reactions. Which is the best base to use to deprotonate acetylene and make the acetylide anion?(b) H2N- + H―C ≡ C―H ⇌ -C ≡ C―H + H3N

Key Concepts

Acidity and Basicity

Acetylide Anion

Strong Bases for Deprotonation

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Calculate for the following acid–base reactions. Which is the best base to use to deprotonate acetylene and make the acetylide anion?
(b) H₂N^- + H―C ≡ C―H ⇌ ^-C ≡ C―H + H₃N