Textbook Question
Suggest an acetylide ion and a carbonyl that might be used to make the following products.
(c) 5-phenylhex-2-yn-1-ol
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9. Alkenes and Alkynes
Acetylide
Problem 15a
Textbook Question
Calculate Keq 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
Verified step by step guidance1
Step 1: Understand the problem. The goal is to determine whether hydroxide (HO⁻) is a strong enough base to deprotonate acetylene (H―C≡C―H) and form the acetylide anion (⁻C≡C―H). This involves comparing the acidities of the reactants and products using their pKa values.
Step 2: Recall the concept of pKa. The pKa value is a measure of the acidity of a compound. A lower pKa indicates a stronger acid. The equilibrium of an acid-base reaction favors the side with the weaker acid (higher pKa).
Step 3: Identify the pKa values of the relevant species. Acetylene (H―C≡C―H) has a pKa of approximately 25, while water (H₂O), the conjugate acid of hydroxide (HO⁻), has a pKa of approximately 15.7.
Step 4: Compare the pKa values. Since acetylene has a higher pKa (25) than water (15.7), acetylene is a weaker acid than water. This means the equilibrium will not favor the formation of the acetylide anion (⁻C≡C―H) when hydroxide is used as the base.
Step 5: Conclude that hydroxide (HO⁻) is not a strong enough base to deprotonate acetylene effectively. A stronger base, such as sodium amide (NaNH₂, conjugate acid NH₃ with pKa ~38), would be required to deprotonate acetylene and form the acetylide anion.
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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.
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Acetylide Anion
The acetylide anion (C≡C⁻) is formed when acetylene (H―C≡C―H) is deprotonated. This anion is a strong nucleophile and is important in organic synthesis, particularly in reactions involving carbon-carbon bond formation. Identifying a suitable base to generate the acetylide anion is essential for various synthetic applications.
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Strong Bases for Deprotonation
To deprotonate a weak acid like acetylene, a strong base is required. Common strong bases include alkoxides (like NaO⁻) and hydrides (like NaH). The choice of base affects the efficiency of the deprotonation process, and understanding the relative strength of potential bases is key to selecting the best one for generating the acetylide anion.
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