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

검증된 단계별 안내

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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이 영상 해법은 위 문제에 도움이 된다고 튜터들이 추천한 것입니다.

주요 개념

질문에 올바르게 답하기 위해 반드시 이해해야 하는 핵심 개념들은 다음과 같습니다.

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.

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