Ch. 10 - Alkynes: Electrophilic Addition and Redox Reactions

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Mullins 1st Edition

Ch. 10 - Alkynes: Electrophilic Addition and Redox Reactions

Problem 37

Chapter 9, Problem 37

Draw the ketone(s) you would expect to form by reacting the following alkynes under the conditions of oxymercuration: (a) 6-methyloct-1-yne, (b) 1,10-dicyclohexyldec-5-yne, and (c) 5-phenylhex-2-yne.

Verified step by step guidance

Understand the reaction: Oxymercuration of alkynes involves the addition of water across the triple bond in the presence of mercuric sulfate (HgSO₄) and sulfuric acid (H₂SO₄), leading to the formation of ketones.

Identify the alkyne structure: For each alkyne, locate the triple bond and determine the carbon atoms involved. This will help in predicting the site of ketone formation.

Apply Markovnikov's rule: In oxymercuration, the addition of water follows Markovnikov's rule, where the hydroxyl group (OH) adds to the more substituted carbon atom of the triple bond, leading to the formation of an enol intermediate.

Tautomerization: The enol intermediate formed will undergo tautomerization, a process where the enol rearranges to form a more stable ketone. This involves the migration of a hydrogen atom and the shift of a double bond.

Draw the ketone structures: For each alkyne, draw the resulting ketone structure after tautomerization. Ensure that the ketone carbonyl group (C=O) is correctly positioned according to the Markovnikov addition.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Oxymercuration of Alkynes

Oxymercuration is a reaction that converts alkynes into ketones using mercuric sulfate (HgSO₄) and sulfuric acid (H₂SO₄) in the presence of water. The reaction proceeds via the formation of an enol intermediate, which tautomerizes to form a ketone. This process is regioselective, typically resulting in the Markovnikov addition of water across the triple bond.

Markovnikov's Rule

Markovnikov's Rule states that in the addition of HX to an unsymmetrical alkene or alkyne, the hydrogen atom will attach to the carbon with more hydrogen atoms, while the halide or other group will attach to the carbon with fewer hydrogen atoms. In the context of oxymercuration, this rule helps predict the major product by determining the position of the hydroxyl group before tautomerization to the ketone.

Tautomerization

Tautomerization is the chemical process where an enol (a compound with a hydroxyl group attached to a carbon-carbon double bond) rearranges to form a ketone or aldehyde. This is a crucial step in the oxymercuration of alkynes, where the initially formed enol intermediate rapidly converts to a more stable ketone, completing the transformation of the alkyne to the ketone product.

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Tautomerization Mechanisms

Related Practice

Textbook Question

For each of the following ketones/aldehydes, indicate whether it is possible to synthesize it from an alkyne as the only compound in good ( > 50%) yield. If so, how would you do it?

(a)

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Textbook Question

For each of the following ketones/aldehydes, indicate whether it is possible to synthesize it from an alkyne as the only compound in good (> 50%) yield. If so, how would you do it?

(d)

928

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Textbook Question

Predict the major products resulting from the addition of one equivalent of HX to the following alkynes.

(b)

1059

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Textbook Question

Predict the alkyne and reactants you might use to make the following haloalkenes. [Providing the reactant and the reagent is how we start thinking about synthesis.]

(a)

1051

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Textbook Question

Predict the alkyne and reactants you might use to make the following haloalkenes. [Providing the reactant and the reagent is how we start thinking about synthesis.]

(b)

646

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Textbook Question

Draw the ketone(s) you would expect to form by treating the following alkynes under the conditions of hydroboration–oxidation: (a) 6-methyloct-1-yne, (b) 1,10-dicyclohexyldec-5-yne, and (c) 5-phenylhex-2-yne.

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1033

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