Textbook Question
Suggest alkynes that might be used to make the following trans-alkenes.
(a)
9. Alkenes and Alkynes
Hydrogenation of Alkynes
2:25 minutesProblem 23a
Textbook Question
Predict the product of the following hydrogenation reactions run with a poisoned catalyst.
(a) 
Verified step by step guidance1
Step 1: Understand the concept of hydrogenation with a poisoned catalyst. A poisoned catalyst, such as Lindlar's catalyst, is used to selectively hydrogenate alkynes to alkenes without further reducing the alkene to an alkane.
Step 2: Identify the starting material in the reaction. Determine whether the compound is an alkyne, alkene, or another functional group that can undergo hydrogenation.
Step 3: Recognize the stereochemistry of the reaction. Hydrogenation with a poisoned catalyst typically results in a cis-alkene (Z-configuration) due to the syn addition of hydrogen atoms.
Step 4: Write the reaction mechanism. Show how the alkyne interacts with the poisoned catalyst and hydrogen gas to form the cis-alkene. Use MathML to represent the chemical structures and transformations.
Step 5: Predict the product based on the reaction conditions. Ensure the product is a cis-alkene and verify that no further reduction occurs due to the presence of the poisoned catalyst.
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Video duration:
2mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Hydrogenation
Hydrogenation is a chemical reaction that involves the addition of hydrogen (H2) to unsaturated organic compounds, typically alkenes or alkynes, converting them into saturated hydrocarbons. This process is commonly facilitated by catalysts, which speed up the reaction without being consumed. The reaction can lead to the formation of alkanes, and the conditions under which it occurs can significantly influence the product distribution.
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00:48
The definition of hydrogenation.
Poisoned Catalyst
A poisoned catalyst is a catalyst that has been intentionally modified or contaminated to reduce its activity. In hydrogenation reactions, a poisoned catalyst selectively inhibits the reaction, allowing for the formation of specific products rather than complete saturation. This is particularly useful in organic synthesis, where controlling the degree of hydrogenation is essential for obtaining desired compounds.
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Selectivity in Reactions
Selectivity refers to the preference of a chemical reaction to produce one product over others. In the context of hydrogenation with a poisoned catalyst, selectivity is crucial as it determines which unsaturated bonds are hydrogenated and to what extent. Understanding selectivity helps chemists predict the outcome of reactions and design pathways to synthesize specific molecules efficiently.
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