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Ch. 7 - The Reactions of Alkynes • An Introduction to Multistep Synthesis
Bruice - Organic Chemistry 8th Edition
Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook
All textbooksBruice 8th EditionCh. 7 - The Reactions of Alkynes • An Introduction to Multistep SynthesisProblem 54
Chapter 8, Problem 54

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.

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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.
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Acid-catalyzed hydroboration-oxidation mechanism

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.
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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

A chemist is planning to synthesize 3-octyne by adding 1-bromobutane to the product obtained from the reaction of 1-butyne with sodium amide. Unfortunately, however, he forgot to order 1-butyne. How else can he prepare 3-octyne?

1005
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Textbook Question

What stereoisomers are obtained from the following reactions?

b.

1834
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Textbook Question

Explain why, in hydroboration–oxidation, HO− and HOOH cannot be added until after the hydroboration reaction is over.

1745
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Textbook Question

Show how each of the following compounds can be prepared using the given starting material, any needed inorganic reagents, and any ­organic compound that has no more than four carbons:

f.

793
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Textbook Question

What stereoisomers are obtained from the following reactions?

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Textbook Question

Show how each of the following compounds can be prepared using the given starting material, any needed inorganic reagents, and any ­organic compound that has no more than four carbons:

e.

832
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