Ch. 18 - Ketones and Aldehydes

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 18 - Ketones and Aldehydes

Problem 64c

Chapter 18, Problem 64c

Hydration of alkynes (via oxymercuration) gives good yields of single compounds only with symmetrical or terminal alkynes. Show what the products would be from hydration of each compound.
c. hex-1-yne

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Identify the type of alkyne: Hex-1-yne is a terminal alkyne because the triple bond is located at the first carbon of the chain.

Understand the reaction mechanism: Hydration of alkynes via oxymercuration involves the addition of water (H₂O) across the triple bond in the presence of a mercuric ion catalyst (Hg²⁺) and an acid (H₂SO₄). This reaction follows Markovnikov's rule, where the hydroxyl group (-OH) attaches to the more substituted carbon.

Predict the intermediate: The initial addition of water forms an enol intermediate (a compound with a hydroxyl group attached to a carbon-carbon double bond). For hex-1-yne, the enol will have the hydroxyl group on the second carbon.

Account for tautomerization: The enol intermediate is unstable and undergoes keto-enol tautomerization, where the hydroxyl group is converted into a carbonyl group (C=O). This results in the formation of a ketone. For hex-1-yne, the ketone formed will be 2-hexanone.

Summarize the product: The hydration of hex-1-yne via oxymercuration yields 2-hexanone as the final product, following the Markovnikov addition and subsequent tautomerization.

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Key Concepts

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

Hydration of Alkynes

Hydration of alkynes involves the addition of water (H2O) across the triple bond of an alkyne, typically in the presence of a catalyst. This reaction can lead to the formation of enols, which can subsequently tautomerize to form ketones or aldehydes. The regioselectivity of the reaction is influenced by the structure of the alkyne, particularly whether it is symmetrical or terminal.

Oxymercuration-Demercuration

Oxymercuration-demercuration is a two-step reaction used to hydrate alkenes and alkynes. In the first step, mercuric acetate reacts with the alkyne to form a mercurial intermediate, which is then treated with water to yield an alcohol. The second step involves the removal of the mercury group, typically using sodium borohydride, resulting in the formation of the final alcohol product.

Terminal vs. Internal Alkynes

Terminal alkynes have a triple bond at the end of the carbon chain, while internal alkynes have the triple bond between two carbon atoms. The distinction is crucial in hydration reactions, as terminal alkynes typically yield aldehydes or ketones upon hydration, while internal alkynes can lead to more complex products. In the case of hex-1-yne, being a terminal alkyne, the hydration will produce a specific ketone or aldehyde based on the reaction conditions.

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Anti-Markovnikov addition of alcohols to terminal alkynes yields aldehydes

Related Practice

Textbook Question

Hydration of alkynes (via oxymercuration) gives good yields of single compounds only with symmetrical or terminal alkynes. Show what the products would be from hydration of each compound.

a. hex-3-yne

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Textbook Question

Hydration of alkynes (via oxymercuration) gives good yields of single compounds only with symmetrical or terminal alkynes. Show what the products would be from hydration of each compound.

d. cyclodecyne

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Textbook Question

Hydration of alkynes (via oxymercuration) gives good yields of single compounds only with symmetrical or terminal alkynes. Show what the products would be from hydration of each compound.

e. 3-methylcyclodecyne

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Textbook Question

Hydration of alkynes (via oxymercuration) gives good yields of single compounds only with symmetrical or terminal alkynes. Show what the products would be from hydration of each compound.

b. hex-2-yne

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Textbook Question

Two structures of the sugar fructose are shown next. The cyclic structure predominates in aqueous solution.

(a) Number the carbon atoms in the cyclic structure. What is the functional group at C2 in the cyclic form?

(b) Propose a mechanism for the cyclization, assuming a trace of acid is present.

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Textbook Question

Which of the following compounds would give a positive Tollens test? (Remember that the Tollens test involves mild basic aqueous conditions.)

(a) CH₃CH₂CH₂COCH₃

(b) CH₃CH₂CH₂CH₂CHO

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Hydration of alkynes (via oxymercuration) gives good yields of single compounds only with symmetrical or terminal alkynes. Show what the products would be from hydration of each compound.c. hex-1-yne

Verified video answer for a similar problem:

Key Concepts

Hydration of Alkynes

Oxymercuration-Demercuration

Terminal vs. Internal Alkynes

Hydration of alkynes (via oxymercuration) gives good yields of single compounds only with symmetrical or terminal alkynes. Show what the products would be from hydration of each compound.
c. hex-1-yne