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.

Accepted Answer

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.

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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Concetti chiave

Hydroboration-Oxidation Mechanism

Regioselectivity in Alkynes

Internal Alkynes and Product Formation

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.

Risposta video verificata per un problema simile:

Concetti chiave

Hydroboration-Oxidation Mechanism

Regioselectivity in Alkynes

Internal Alkynes and Product Formation

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.