Textbook Question
How can the following compounds be prepared using ethyne as the starting material?
a.
10. Addition Reactions
Alkyne Hydroboration
3:07 minutesProblem 39e,f
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
Verified step by step guidance1
Step 1: Understand the reaction conditions for part (e). The reaction involves propyne (CH≡C−CH3) with aqueous H2SO4 and HgSO4. This is an example of an acid-catalyzed hydration of an alkyne, which follows Markovnikov's rule. The reaction proceeds via the formation of an enol intermediate, which tautomerizes to a ketone.
Step 2: Write the mechanism for part (e). The π-electrons of the triple bond attack the proton (H⁺) from H2SO4, forming a carbocation intermediate. Hg²⁺ from HgSO4 stabilizes the intermediate, and water (H2O) adds to the more substituted carbon (Markovnikov addition). This forms an enol intermediate (a compound with a hydroxyl group attached to a carbon-carbon double bond).
Step 3: Explain tautomerization for part (e). The enol intermediate undergoes keto-enol tautomerization, where the hydroxyl group (−OH) and the adjacent double bond rearrange to form a ketone. The major product is methyl ketone (CH3−CO−CH3).
Step 4: Understand the reaction conditions for part (f). The reaction involves propyne with R2BH in THF, followed by H2O2/HO⁻/H2O. This is an example of hydroboration-oxidation of an alkyne, which follows anti-Markovnikov's rule. The reaction produces an aldehyde as the major product.
Step 5: Write the mechanism for part (f). In the first step, R2BH adds to the triple bond of propyne in an anti-Markovnikov fashion, with the boron atom attaching to the less substituted carbon. In the second step, oxidation with H2O2/HO⁻ replaces the boron atom with a hydroxyl group, forming an enol intermediate. This enol undergoes tautomerization to form an aldehyde (CH3−CH2−CHO).
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Hydration of Alkynes
The hydration of alkynes involves the addition of water across the triple bond, typically catalyzed by acids. In the presence of aqueous H2SO4 and HgSO4, propyne undergoes Markovnikov addition, leading to the formation of an enol that tautomerizes to a ketone. This process is crucial for understanding how alkynes can be converted into carbonyl compounds.
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Alkyne Hydration
Hydroboration-Oxidation
Hydroboration-oxidation is a two-step reaction that converts alkynes into alcohols. The first step involves the addition of borane (R2BH) to the alkyne, resulting in a trialkylborane intermediate. The subsequent oxidation with H2O2 and hydroxide leads to the formation of an alcohol, following anti-Markovnikov selectivity, which is essential for predicting the product of the reaction.
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Markovnikov's Rule
Markovnikov's Rule states that in the addition of HX to an alkene or alkyne, the hydrogen atom will attach to the carbon with the greater number of hydrogen atoms already attached. This principle helps predict the major product of reactions involving unsymmetrical reagents, such as the hydration of propyne, where the more substituted carbon becomes the site of the new functional group.
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