Only one alkyne forms an aldehyde when it undergoes the mercuric-ion-catalyzed addition of water. Identify the alkyne.

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Understand the reaction: The mercuric-ion-catalyzed addition of water to an alkyne is a hydration reaction. In this reaction, the alkyne reacts with water in the presence of Hg²⁺ and an acid (commonly H₂SO₄) to form a ketone or an aldehyde via the enol intermediate, which tautomerizes to the more stable carbonyl compound.

Recall Markovnikov's rule: In the hydration of alkynes, the addition of the hydroxyl group (-OH) follows Markovnikov's rule. This means the -OH group will attach to the more substituted carbon of the triple bond, leading to the formation of a ketone in most cases.

Identify the exception: The only alkyne that forms an aldehyde under these conditions is **ethyne (acetylene, CH≡CH)**. This is because ethyne is a terminal alkyne with no substituents on either carbon of the triple bond. When water adds to ethyne, the enol intermediate tautomerizes to form an aldehyde (ethanal, CH₃CHO).

Write the reaction mechanism: The reaction proceeds as follows: (1) The triple bond of ethyne is activated by the Hg²⁺ catalyst, making it more electrophilic. (2) Water adds to the triple bond, forming an enol intermediate. (3) The enol undergoes keto-enol tautomerization to form the aldehyde.

Conclude: Ethyne is the only alkyne that forms an aldehyde under mercuric-ion-catalyzed hydration because it is the simplest terminal alkyne, and the addition of water leads to the formation of an aldehyde rather than a ketone.

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Key Concepts

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

Alkynes

Alkynes are hydrocarbons that contain at least one carbon-carbon triple bond. They are unsaturated compounds and are generally more reactive than alkenes and alkanes due to the presence of the triple bond. The simplest alkyne is ethyne (acetylene), and they can undergo various reactions, including addition reactions, which are crucial for understanding their reactivity.

Mercuric-Ion-Catalyzed Hydration

The mercuric-ion-catalyzed hydration of alkynes involves the addition of water across the triple bond in the presence of mercuric ions (Hg2+). This reaction typically leads to the formation of a ketone or an aldehyde, depending on the structure of the alkyne. The mechanism involves the formation of a mercuric alkynyl intermediate, which is then hydrolyzed to yield the carbonyl compound.

Aldehyde Formation

Aldehydes are organic compounds characterized by the presence of a carbonyl group (C=O) at the end of a carbon chain. In the context of alkyne hydration, an aldehyde is formed when a terminal alkyne undergoes hydration, resulting in the addition of a water molecule to the carbon-carbon triple bond. This process is significant because it highlights the unique reactivity of terminal alkynes compared to internal alkynes.

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Textbook Question

For each of the following alkynes, draw the products obtained from (1) the acid-catalyzed addition of water (mercuric ion is added for part a) and from (2) hydroboration–oxidation:

a. 1-butyne

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Textbook Question

Describe the alkyne you should start with and the reagents you should use if you want to synthesize

b. cis-2-butene.

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Textbook Question

Describe the alkyne you should start with and the reagents you should use if you want to synthesize

a. pentane.

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