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
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Ch. 10 - Alkynes: Electrophilic Addition and Redox Reactions
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471
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Table of contents
- Ch. 2 - General Chemistry Translated: Finding the Electrons114
- Ch. 3 - Alkanes and Cycloalkanes: Properties and Conformational Analysis109
- Ch. 4 - Acids and Bases: Electron Flow144
- Ch. 5 - Chemical Reaction Analysis: Thermodynamics and Kinetics118
- Ch. 6 - Stereoisomerism: Arrangement of Atoms in Space112
- Ch. 7 - Principles of Green (Organic) Chemistry3
- Ch. 8 - Alkenes I: Properties and Electrophilic Additions158
- Ch. 9 - Alkenes II: Oxidation and Reduction150
- Ch. 10 - Alkynes: Electrophilic Addition and Redox Reactions101
- Ch. 11 - Properties and Synthesis of Alkyl Halides: Radical Reactions108
- Ch. 12 - Substitution and Elimination: Reactions of Haloalkanes172
- Ch. 13 - Alcohols, Ethers and Related Compounds: Substitution and Elimination239
- Ch. 14 - Structural Identification I: Infrared Spectroscopy and Mass Spectrometry54
- Ch. 15 - Structural Identification II: Nuclear Magnetic Resonance Spectroscopy93
- Ch. 16 - Metals in Organic Chemistry48
- Ch. 17 - Carbonyl Addition Reactions: Aldehydes and Ketones45
- Ch. 18 - Nucleophilic Acyl Substitution I: Carboxylic Acids18
- Ch. 19 - Nucleophilic Acyl Substitution II: Carboxylic Acid Derivatives39
- Ch. 20 - Enolates: Carbonyl Addition and Substitution13
- Ch. 21 - Conjugated Systems I: Stability and Addition Reactions56
- Ch. 22 - Conjugated Systems II: Pericyclic Reactions54
- Ch. 23 - Benzene I: Aromatic Stability and Substitution Reactions64
- Ch. 24 - Benzene II: Reactions Influenced by the Aromatic Ring62
- Ch. 25 - Amines: Structure, Reactions, and Synthesis27
- Ch. 26 - Amino Acids, Proteins, and Peptide Synthesis9
- Ch. 27 - Carbohydrates, Nucleic Acids, and Lipids54
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
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Mullins 1st EditionCh. 10 - Alkynes: Electrophilic Addition and Redox ReactionsProblem 15c
Mullins 1st EditionCh. 10 - Alkynes: Electrophilic Addition and Redox ReactionsProblem 15cChapter 9, Problem 15c
Calculate for the following acid–base reactions. Which is the best base to use to deprotonate acetylene and make the acetylide anion?
(c) 
Verified step by step guidance1
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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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
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.
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Identifying pKa values
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).
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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.
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Related Practice
Textbook Question
Using pKa values, calculate an approximate Keq value for the following substitution reaction.
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Textbook Question
Which of the following would be expected to give a hotter flame during combustion? Explain.
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Textbook Question
Estimate the Keq for the following reactions based on the stability of the anions involved.
(a)
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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
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Textbook Question
Estimate the Keq for the following reactions based on the stability of the anions involved.
(b)
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