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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9. Alkenes and Alkynes
Hydrogenation of Alkynes
4:13 minutesProblem 38a
Textbook Question
What reagents would you use for the following syntheses?
a. (Z)-3-hexene from 3-hexyne
Verified step by step guidance1
Step 1: Recognize that the problem involves converting 3-hexyne (an alkyne) into (Z)-3-hexene (a cis-alkene). This requires selective reduction of the triple bond to a double bond while ensuring the cis configuration.
Step 2: Understand that a common method for achieving cis-selective reduction of alkynes is using Lindlar's catalyst. Lindlar's catalyst is a palladium catalyst modified with lead and quinoline, which selectively reduces alkynes to cis-alkenes without further reducing them to alkanes.
Step 3: Write the reaction mechanism conceptually: 3-hexyne reacts with hydrogen gas (H₂) in the presence of Lindlar's catalyst. The catalyst facilitates the addition of hydrogen atoms to the triple bond, forming a double bond with the cis configuration.
Step 4: Confirm that the product is (Z)-3-hexene. The 'Z' designation indicates that the two substituents on the double bond are on the same side, which is consistent with the cis configuration achieved using Lindlar's catalyst.
Step 5: Summarize the reagents needed for this synthesis: 3-hexyne, hydrogen gas (H₂), and Lindlar's catalyst. These will selectively reduce the alkyne to the desired cis-alkene.
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Video duration:
4mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Alkyne to Alkene Conversion
The conversion of alkynes to alkenes typically involves the use of reagents that can selectively reduce the triple bond to a double bond. Common methods include catalytic hydrogenation or the use of Lindlar's catalyst, which allows for the formation of cis-alkenes from alkynes. Understanding this process is crucial for synthesizing (Z)-3-hexene from 3-hexyne.
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Stereochemistry of Alkenes
Stereochemistry refers to the spatial arrangement of atoms in molecules and is essential in determining the properties of alkenes. In the case of (Z)-3-hexene, the 'Z' configuration indicates that the highest priority substituents on the double bond are on the same side. Recognizing how to manipulate stereochemistry during synthesis is vital for achieving the desired product.
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Reagents for Selective Reduction
Choosing the right reagents for selective reduction is key in organic synthesis. For converting 3-hexyne to (Z)-3-hexene, one might use hydrogen gas with a catalyst like Lindlar's or sodium in liquid ammonia, which selectively reduces the alkyne to the cis-alkene. Familiarity with these reagents and their mechanisms is essential for successful synthesis.
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