Textbook Question
What are products of the following reactions?
e.
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10. Addition Reactions
Alkyne Hydroboration
5:27 minutesProblem 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 guidance1
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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5mKey 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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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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