Textbook Question
Answer Problem 39, parts a–h, using 2-butyne as the starting material instead of propyne.
h. H2/Lindlar catalyst
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9. Alkenes and Alkynes
Hydrogenation of Alkynes
4:31 minutesProblem 55b
Textbook Question
What stereoisomers are obtained from the following reactions?
b. 
Verified step by step guidance1
Step 1: Analyze the starting material, which is a terminal alkyne (CH3CH2C≡CCH2CH3). The triple bond is the reactive site in this molecule.
Step 2: The first reaction involves H2 with a Lindlar catalyst. This selectively reduces the alkyne to a cis-alkene. The Lindlar catalyst prevents further reduction to an alkane, ensuring the formation of a cis-alkene stereoisomer.
Step 3: The second reaction involves D2 with Pd/C. This is a catalytic hydrogenation reaction, but instead of H2, D2 is used. The alkene formed in the previous step undergoes syn-addition of deuterium atoms across the double bond, resulting in a fully saturated alkane with deuterium atoms added.
Step 4: Consider stereochemistry. The Lindlar catalyst ensures the cis configuration of the alkene intermediate. The syn-addition of D2 maintains the stereochemical integrity during the second step.
Step 5: The final product is a saturated alkane with deuterium atoms added. The stereoisomers obtained depend on the spatial arrangement of substituents around the original triple bond and the cis-alkene intermediate.
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Video duration:
4mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Stereoisomerism
Stereoisomerism refers to the phenomenon where compounds have the same molecular formula and connectivity of atoms but differ in the spatial arrangement of their atoms. This can lead to different physical and chemical properties. In organic chemistry, stereoisomers can be classified into two main types: geometric isomers (cis/trans) and optical isomers (enantiomers), which are crucial for understanding the outcomes of reactions involving chiral centers.
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Determining when molecules are stereoisomers.
Hydrogenation of Alkynes
Hydrogenation is a chemical reaction that involves the addition of hydrogen (H2) to an unsaturated compound, such as an alkyne. The use of a Lindlar catalyst allows for the selective partial hydrogenation of alkynes to cis-alkenes, while the use of Pd/C can lead to complete hydrogenation to alkanes. Understanding the conditions and catalysts used in hydrogenation is essential for predicting the stereochemical outcomes of the reaction.
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Lindlar Catalyst
The Lindlar catalyst is a palladium-based catalyst that is poisoned to selectively hydrogenate alkynes to cis-alkenes without further reducing them to alkanes. It is crucial in organic synthesis for controlling the degree of hydrogenation and achieving specific stereochemical configurations. Recognizing the role of the Lindlar catalyst helps in predicting the stereoisomers formed during the reaction process.
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