Textbook Question
Show how hex-1-yne might be converted to
f. 2,2-dibromohexane.
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Ch.9 - Alkynes
Wade9th EditionOrganic ChemistryISBN: 9780135213728
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Table of contents
- Ch.1 - Structure and Bonding107
- Ch. 2 - Acids and Bases; Functional Groups115
- Ch.3 - Structure and Stereochemistry of Alkanes100
- Ch.4 - The Study of Chemical Reactions99
- Ch.5 - Stereochemistry93
- Ch.6 - Alkyl Halides; Nucleophilic Substitution118
- Ch. 7 - Structure and Synthesis of Alkenes; Elimination158
- Ch.8 - Reactions of Alkenes171
- Ch.9 - Alkynes91
- Ch.10 - Structure and Synthesis of Alcohols143
- Ch.11 - Reactions of Alcohols104
- Ch. 12 - Infrared Spectroscopy and Mass Spectrometry22
- Ch. 13 - Nuclear Magnetic Resonance Spectroscopy45
- Ch. 14 - Ethers, Epoxides, and Thioethers72
- Ch. 15 - Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy89
- Ch. 16 - Aromatic Compounds74
- Ch. 17 - Reactions of Aromatic Compounds126
- Ch. 18 - Ketones and Aldehydes193
- Ch. 19 - Amines129
- Ch. 20 - Carboxylic Acids77
- Ch. 21 - Carboxylic Acid Derivatives141
- Ch. 22 - Condensations and Alpha Substitutions of Carbonyl Compounds175
- Ch. 23 - Carbohydrates and Nucleic Acids93
- Ch. 24 - Amino Acids, Peptides, and Proteins54
Chapter 9, Problem 20a
The hydroboration–oxidation of internal alkynes produces ketones.
a. When hydroboration–oxidation is applied to but-2-yne, a single pure product is obtained. Determine the structure of this product, and show the intermediates in its formation.
Verified step by step guidance1
Step 1: Understand the reaction mechanism. Hydroboration–oxidation of alkynes involves two steps: (1) addition of borane (BH₃ or a derivative) to the alkyne, and (2) oxidation of the resulting organoborane intermediate with hydrogen peroxide (H₂O₂) in a basic medium (NaOH). For internal alkynes, this reaction typically produces ketones.
Step 2: Analyze the structure of the starting material. But-2-yne is a symmetrical internal alkyne with the structure CH₃-C≡C-CH₃. Its symmetry ensures that the hydroboration–oxidation will yield a single product.
Step 3: Describe the hydroboration step. In this step, a borane reagent (commonly disiamylborane or 9-BBN) adds across the triple bond of but-2-yne. The boron atom attaches to one of the carbons in the triple bond, while a hydrogen atom attaches to the other carbon. This step forms a trialkylborane intermediate.
Step 4: Describe the oxidation step. The trialkylborane intermediate is treated with hydrogen peroxide (H₂O₂) in a basic medium (NaOH). This step replaces the boron atom with a hydroxyl group (-OH), resulting in an enol intermediate. The enol then tautomerizes to form a ketone, as the keto form is more stable.
Step 5: Determine the final product. Since but-2-yne is symmetrical, the ketone formed will be a single product: CH₃-CO-CH₃ (2-butanone). The intermediates include the trialkylborane and the enol forms, which are crucial in the reaction pathway.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Hydroboration-Oxidation Reaction
The hydroboration-oxidation reaction is a two-step process used to convert alkenes and alkynes into alcohols. In the first step, borane (BH3) adds across the double or triple bond, forming a trialkylborane intermediate. The second step involves oxidation with hydrogen peroxide (H2O2) in the presence of a base, converting the boron atom into a hydroxyl group, resulting in an alcohol. This reaction is notable for its anti-Markovnikov selectivity.
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General properties of hydroboration-oxidation.
Alkyne Reactivity
Internal alkynes, such as but-2-yne, have a triple bond between two carbon atoms that can undergo various reactions, including hydroboration. The reactivity of alkynes is influenced by their structure; internal alkynes can form more stable intermediates compared to terminal alkynes. This stability allows for the formation of ketones upon hydroboration-oxidation, as the reaction proceeds through a more favorable pathway.
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Ketone Formation
Ketones are organic compounds characterized by a carbonyl group (C=O) flanked by two carbon atoms. In the context of hydroboration-oxidation of internal alkynes, the reaction leads to the formation of a ketone as the final product. For but-2-yne, the hydroboration step generates a trialkylborane intermediate, which upon oxidation yields 2-butanone, a simple ketone. Understanding the structure and properties of ketones is essential for predicting the outcomes of such reactions.
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Related Practice
Textbook Question
The hydroboration–oxidation of internal alkynes produces ketones.
b. When hydroboration–oxidation is applied to pent-2-yne, two products are obtained. Show why a mixture of products should be expected with any unsymmetrical internal alkyne.
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Textbook Question
Show how hex-1-yne might be converted to
e. 2-bromohexane.
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Textbook Question
For each compound, give the product(s) expected from (1) HgSO4/H2SO4 - catalyzed hydration and (2) hydroboration–oxidation.
b. hex-2-yne
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Textbook Question
For each compound, give the product(s) expected from (1) HgSO4/H2SO4 - catalyzed hydration and (2) hydroboration–oxidation.
a. hex-1-yne
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Textbook Question
When pent-2-yne reacts with mercuric sulfate in dilute sulfuric acid, the product is a mixture of two ketones. Give the structures of these products, and use mechanisms to show how they are formed.
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