Textbook Question
Predict the product(s) that would result when the alkenes shown here are allowed to react under the following conditions: (v) 1. Hg(OAc)2 , H2O 2. NaBH4
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10. Addition Reactions
Oxymercuration
5:19 minutesProblem 44c(vi)
Textbook Question
For the alkynes shows here, show the product(s) expected to form when treated under the following conditions: (vi) H2SO4, HgSO4, H2O. If you expect two products, show both.
(c) 
Verified step by step guidance1
Identify the alkyne structure provided in the problem. Determine whether it is a terminal alkyne (triple bond at the end of the chain) or an internal alkyne (triple bond within the chain). This distinction is important for predicting the product.
Understand the reaction conditions: H₂SO₄ (sulfuric acid), HgSO₄ (mercuric sulfate), and H₂O (water). These conditions promote the hydration of alkynes, leading to the formation of a ketone via the Markovnikov addition of water across the triple bond.
Apply the mechanism: The alkyne reacts with H₂O in the presence of H₂SO₄ and HgSO₄. First, the alkyne is protonated to form a carbocation intermediate. The Hg²⁺ ion stabilizes the carbocation, and water adds to the more substituted carbon (Markovnikov addition).
Recognize the enol intermediate: The addition of water forms an enol (a compound with a hydroxyl group attached to a carbon-carbon double bond). This enol is unstable and undergoes tautomerization to form a ketone. Tautomerization involves the migration of a proton and the rearrangement of the double bond.
If the alkyne is symmetrical, a single ketone product will form. If the alkyne is asymmetrical, two possible ketone products may form due to the different substituents on either side of the triple bond. Analyze the structure of the alkyne to determine whether one or two products are expected.
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5mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Alkyne Hydration
Alkyne hydration is a chemical reaction where an alkyne reacts with water in the presence of an acid catalyst, typically sulfuric acid (H₂SO₄) and a mercury(II) salt (HgSO₄). This process leads to the formation of an enol, which can tautomerize to a more stable ketone. Understanding this reaction is crucial for predicting the products formed when alkynes are treated under these conditions.
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Markovnikov's Rule
Markovnikov's Rule states that in the addition of HX (where X is a halogen or OH) 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 regioselectivity of the reaction, guiding the formation of the major product when alkynes undergo hydration.
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Tautomerization
Tautomerization is a chemical process where a compound exists in two interconvertible forms, typically involving the relocation of a proton and a shift of a double bond. In the context of alkyne hydration, the initial enol product can convert to a ketone through tautomerization, which is often the more stable and predominant form. Recognizing this process is essential for identifying all possible products of the reaction.
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