Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

10. Addition Reactions

Alkyne Hydroboration

Organic Chemistry

10. Addition Reactions

Alkyne Hydroboration

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5:27 minutes

Problem 54

Textbook Question

a. Explain why a single pure product is obtained from hydroboration–oxidation of 2-butyne, whereas two products are obtained from hydroboration–oxidation of 2-pentyne.
b. Name two other internal alkynes that yield only one product upon hydroboration–oxidation.

Verified step by step guidance

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 solution to form an alcohol. This reaction proceeds with anti-Markovnikov regioselectivity and syn addition.

For 2-butyne, the molecule is symmetrical, meaning the two carbons in the triple bond are equivalent. As a result, the hydroboration step adds boron to one carbon and hydrogen to the other in a single, uniform manner. Upon oxidation, this leads to the formation of a single enol intermediate, which tautomerizes to a single ketone product (butan-2-one).

In contrast, 2-pentyne is an asymmetrical alkyne. The two carbons in the triple bond are not equivalent, so hydroboration can occur in two different ways, depending on which carbon the boron adds to. This results in two different enol intermediates, which tautomerize to two different ketones (pentan-2-one and pentan-3-one).

To identify other internal alkynes that yield only one product upon hydroboration–oxidation, we need to consider symmetrical alkynes. Symmetry ensures that the two carbons in the triple bond are equivalent, leading to a single product. Examples include 3-hexyne and 4-octyne, both of which are symmetrical internal alkynes.

In summary, the key factor determining whether one or two products are formed is the symmetry of the alkyne. Symmetrical alkynes like 2-butyne, 3-hexyne, and 4-octyne yield a single product, while asymmetrical alkynes like 2-pentyne yield two products due to the possibility of regioisomeric additions.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Hydroboration-Oxidation Mechanism

Hydroboration-oxidation is a two-step reaction involving the addition of borane (BH3) to an alkyne, followed by oxidation with hydrogen peroxide (H2O2) in a basic medium. This process results in the formation of alcohols, where the boron atom adds to the less substituted carbon of the alkyne, leading to syn-addition. The regioselectivity of this reaction is crucial in determining the number of products formed.

Regioselectivity in Alkynes

Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others. In the case of 2-butyne, the symmetrical nature of the molecule allows for only one product after hydroboration-oxidation, while 2-pentyne, being asymmetrical, can lead to two different products depending on which carbon the boron adds to. Understanding the structure of the alkyne is essential for predicting the outcome of the reaction.

Internal Alkynes and Product Formation

Internal alkynes are those with the triple bond located between two carbon atoms, as opposed to terminal alkynes, which have the triple bond at the end. Certain internal alkynes, like 3-heptyne and 3-octyne, can yield only one product upon hydroboration-oxidation due to their symmetrical structure, which allows for a single regioselective addition of boron, resulting in a uniform product after oxidation.

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Related Practice

Textbook Question

What are products of the following reactions?

Textbook Question

The following functional-group interchange is a useful synthesis of aldehydes.

(a) What reagents were used in this chapter for this transformation? Give an example to illustrate this method.

(b) This functional-group interchange can also be accomplished using the following sequence.

Propose mechanisms for these steps.

Textbook Question

For each of the following alkynes, draw the products obtained from (1) the acid-catalyzed addition of water (mercuric ion is added for part a) and from (2) hydroboration–oxidation:

a. 1-butyne

Textbook Question

What is the major product of the reaction of 1 mol of propyne with each of the following reagents?

e. aqueous H₂SO₄, HgSO₄

f. R₂BH in THF followed by H₂O₂/HO⁻/H₂O

Textbook Question

What are products of the following reactions?

Textbook Question

What hydrocarbon would you use to prepare the organoboron compound in Problem 14?

Multiple Choice

Predict the major, organic product for the following reaction.

Textbook Question

Answer Problem 39, parts a–h, using 2-butyne as the starting material instead of propyne.

f. R₂BH in THF followed by H₂O₂/HO⁻ /H₂O

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a. Explain why a single pure product is obtained from hydroboration–oxidation of 2-butyne, whereas two products are obtained from ­hydroboration–oxidation of 2-pentyne.b. Name two other internal alkynes that yield only one product upon hydroboration–oxidation.

Key Concepts

Hydroboration-Oxidation Mechanism

Regioselectivity in Alkynes

Internal Alkynes and Product Formation

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a. Explain why a single pure product is obtained from hydroboration–oxidation of 2-butyne, whereas two products are obtained from hydroboration–oxidation of 2-pentyne.
b. Name two other internal alkynes that yield only one product upon hydroboration–oxidation.