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

검증된 단계별 안내

Step 1: Understand the problem. The goal is to determine the best base to deprotonate acetylene (H-C≡C-H) and form the acetylide anion (C≡C⁻). This involves comparing the acid-base properties of the reactants and products.

Step 2: Analyze the reaction provided. The reaction shows phenoxide ion (C₆H₅O⁻) acting as a base to deprotonate acetylene, forming phenol (C₆H₅OH) and the acetylide anion. The pKa of phenol is given as 10.

Step 3: Use the pKa values to assess the acid-base equilibrium. Acetylene has a pKa of approximately 25, which is much higher than phenol's pKa of 10. A lower pKa indicates a stronger acid, so phenol is a stronger acid than acetylene.

Step 4: Evaluate the equilibrium direction. Since phenol is a stronger acid than acetylene, the equilibrium will favor the formation of phenol and the acetylide anion. This suggests that phenoxide ion (C₆H₅O⁻) is a suitable base for deprotonating acetylene.

Step 5: Consider other bases. To determine the 'best' base, compare the basicity of phenoxide ion with other potential bases, such as hydroxide ion (OH⁻) or amide ion (NH₂⁻). Stronger bases (with conjugate acids having higher pKa values) are more effective at deprotonating acetylene.

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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 pKa

Acidity is a measure of how readily a compound donates protons (H+ ions) in a reaction. The pKa value quantifies this tendency; lower pKa values indicate stronger acids. In the context of the question, understanding the pKa of acetylene (approximately 25) and the phenoxide ion (pKa around 10) is crucial for determining which base can effectively deprotonate acetylene to form the acetylide anion.

Acetylide Anion Formation

The acetylide anion is formed when acetylene (C≡C-H) loses a proton. This reaction is significant in organic synthesis, as acetylide anions are strong nucleophiles that can react with electrophiles. Identifying a suitable base to deprotonate acetylene involves selecting a base that is stronger than the conjugate acid formed, which in this case is the phenol (pKa 10).

Base Strength and Conjugate Acids

The strength of a base is determined by its ability to accept protons, which is inversely related to the strength of its conjugate acid. A strong base will have a weak conjugate acid, making it effective for deprotonation. In this scenario, the base used must have a pKa greater than that of the conjugate acid formed (phenol) to ensure that the reaction proceeds favorably towards the formation of the acetylide anion.

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