Hydrogenation of Alkynes

Professor Dave Explains
13 Jun 201808:42
EducationalLearning
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TLDRIn this educational video, Professor Dave explores the hydrogenation of alkynes, detailing three distinct methods to achieve different products. He explains the full hydrogenation to alkanes using palladium, the selective hydrogenation to Z-alkenes with a Lindlar catalyst, and the conversion to E-alkenes using lithium or sodium in ammonia. The summary also clarifies the geometric considerations for alkynes and demonstrates the outcomes of each method on a 5-carbon alkyne example, emphasizing the importance of stereochemistry in synthetic pathways.

Takeaways
  • 🧪 Hydrogenation of alkynes can lead to different products based on the conditions used.
  • 🔍 Simple hydrogenation over palladium metal converts alkynes to alkanes by fully saturating the molecule.
  • 🌟 Lindlar catalyst is a 'poisoned' palladium catalyst that selectively hydrogenates alkynes to Z alkenes without further hydrogenation to alkanes.
  • 📚 The Z alkene configuration is characterized by having both substituents on the same side of the double bond, following the 'zee zame zide' rule.
  • 🔄 Alternative methods to achieve E alkene involve using lithium and ethyl amine or sodium and ammonia, which stop the reaction at the alkene stage.
  • 🔄 E alkenes are formed with substituents on opposite sides of the double bond, in contrast to the Z configuration.
  • 📝 Understanding the geometry of alkynes is crucial; they exhibit linear geometry due to sp hybridization of carbons in triple bonds.
  • 📐 Proper depiction of alkynes is essential for accurate representation, avoiding zigzag depictions that misrepresent the linear geometry.
  • 🔬 The choice of hydrogenation method is critical in synthetic pathways, allowing chemists to control the product's structure.
  • 🛠 Recognizing the conditions for each type of hydrogenation is key to predicting the outcome of the reaction.
  • 🧠 Memorizing the effects of different catalysts on alkynes helps in quickly identifying the expected products in chemical reactions.
Q & A

  • What is the general process of hydrogenation of alkynes?

    -Hydrogenation of alkynes involves the addition of hydrogen atoms to the pi bonds of the alkyne, converting it to an alkene or further to an alkane, depending on the conditions used.

  • Why can't hydrogenation of alkynes stop at the alkene level with simple hydrogen over palladium?

    -Hydrogenation with simple hydrogen over palladium cannot stop at the alkene level because hydrogen will continue to interact with the alkene, leading to full saturation and the formation of an alkane.

  • What is a Lindlar catalyst and how does it affect the hydrogenation of alkynes?

    -A Lindlar catalyst is a poisoned palladium catalyst that limits its ability to hydrogenate. It can hydrogenate alkynes to alkenes but not further to alkanes, specifically producing the Z alkene due to its stereoselectivity.

  • What is the significance of the Z alkene produced by the Lindlar catalyst?

    -The Z alkene produced by the Lindlar catalyst has a specific stereochemistry where the substituents are on the same side of the double bond, which is important for certain synthetic pathways.

  • How can the E alkene be selectively produced from an alkyne?

    -The E alkene can be selectively produced by using a reaction involving lithium and ethyl amine or sodium and ammonia, which stops the hydrogenation at the alkene level and specifically forms the E alkene.

  • What is the difference between the E and Z alkene in terms of stereochemistry?

    -The E alkene has substituents on the double bond that project in opposite directions, while the Z alkene has substituents on the same side of the double bond.

  • Why is it important to control the stereochemistry during the hydrogenation of alkynes?

    -Controlling the stereochemistry is important for synthetic pathways where specific isomers are required for further reactions or to achieve desired biological activity.

  • How does the geometry of carbons in a triple bond affect the depiction of alkynes?

    -Carbons in a triple bond are sp hybridized and exhibit linear geometry. This must be accurately depicted in the structure of alkynes to reflect their true geometry.

  • What is the IUPAC name for a 5-carbon alkyne?

    -The IUPAC name for a 5-carbon alkyne is 2-pentyne.

  • Can you provide an example of a scenario where different hydrogenation conditions would lead to different products?

    -Yes, if you start with a 5-carbon alkyne and use hydrogen over palladium, you get pentane. With Lindlar catalyst, you get a Z alkene, and with lithium and ethyl amine or sodium and ammonia, you get an E alkene.

  • What is the mnemonic 'Zee same zide' used for in the context of this script?

    -The mnemonic 'Zee same zide' is used to remember that in a Z alkene, the larger groups are on the same side of the double bond.

Outlines
00:00
🧪 Hydrogenation of Alkynes: From Alkynes to Alkanes

Professor Dave introduces the concept of hydrogenation of alkynes, explaining the process of converting alkynes to alkenes or alkanes through addition reactions. He discusses the use of hydrogen over palladium, which leads to the formation of alkanes by fully saturating the pi bonds. This method does not stop at the alkene stage, as hydrogen also reacts with alkenes, thus proceeding to alkanes. The paragraph emphasizes the direct conversion from alkynes to fully saturated alkanes under these conditions.

05:04
🔬 Controlled Hydrogenation: Producing Alkenes from Alkynes

The script explains alternative methods for the partial hydrogenation of alkynes, focusing on the production of alkenes rather than alkanes. It introduces the Lindlar catalyst, a 'poisoned' palladium catalyst that selectively hydrogenates alkynes to Z-alkenes without further reacting with the alkene product. The paragraph also mentions other methods involving lithium and ethyl amine or sodium and ammonia, which selectively produce E-alkenes. The summary includes the stereochemical outcomes of these reactions, highlighting the Z and E configurations determined by the addition of hydrogen atoms to the alkyne. The paragraph concludes with an example of how these hydrogenation techniques can be applied to a specific alkyne molecule, resulting in different products based on the reaction conditions used.

Mindmap
Keywords
💡Hydrogenation
Hydrogenation refers to the chemical reaction where hydrogen atoms are added to a molecule, often an unsaturated organic compound like an alkene or alkyne. In the video, hydrogenation is a central theme, as it discusses the addition of hydrogen to pi bonds in alkynes, leading to the formation of alkenes or alkanes. The script provides examples of different hydrogenation processes, such as using palladium as a catalyst.
💡Alkynes
Alkynes are a class of hydrocarbons with at least one carbon-carbon triple bond. In the context of the video, alkynes are the starting compounds for the hydrogenation reactions. The script discusses how alkynes can be converted to alkenes or alkanes through various hydrogenation methods, emphasizing the importance of alkynes in organic synthesis.
💡Palladium
Palladium is a chemical element used as a catalyst in many organic reactions, including hydrogenation. In the script, palladium is mentioned as a catalyst for the hydrogenation of alkynes to alkanes, highlighting its role in facilitating the addition of hydrogen atoms to pi bonds.
💡Lindlar Catalyst
The Lindlar Catalyst is a specific type of catalyst used in the selective hydrogenation of alkynes to form Z-alkenes. The script explains that this catalyst, which is palladium 'poisoned' with substances like quinoline, limits the hydrogenation process to stop at the alkene stage and specifically produces the Z isomer, as opposed to the E isomer or the fully saturated alkane.
💡Z Alkene
A Z alkene is an alkene with a specific stereochemistry where the substituents on the double bond are on the same side of the plane containing the double bond. The script uses the term 'Z' to describe the product of the hydrogenation of alkynes using the Lindlar Catalyst, emphasizing the stereoselectivity of the reaction.
💡E Alkene
An E alkene is an alkene with a stereochemistry where the substituents on the double bond are on opposite sides of the plane containing the double bond. The video script contrasts the E alkene with the Z alkene, describing a method using lithium and ethyl amine or sodium and ammonia to selectively hydrogenate alkynes to E alkenes.
💡Stereochemistry
Stereochemistry is the aspect of chemistry concerned with the three-dimensional arrangement of atoms in molecules. The script discusses stereochemistry in the context of alkenes, specifically how the addition of hydrogen can lead to either the E or Z isomer, and how different catalysts can be used to control this outcome.
💡Cahn-Ingold-Prelog Convention
The Cahn-Ingold-Prelog (CIP) convention is a set of rules used to determine the absolute configuration of asymmetric centers in molecules and the stereochemistry of double bonds. The script refers to the CIP convention when explaining how to determine whether an alkene is E or Z, which is crucial for understanding the selectivity of the hydrogenation reactions discussed.
💡Alkanes
Alkanes are saturated hydrocarbons with only single bonds between carbon atoms. In the script, alkanes are the end products of the complete hydrogenation of alkynes, where all pi bonds are converted to single bonds, resulting in a fully saturated carbon chain.
💡Synthetic Pathway
A synthetic pathway refers to a series of chemical reactions designed to produce a specific compound. The script mentions synthetic pathways in the context of selectively hydrogenating alkynes to either Z or E alkenes for further chemical reactions, highlighting the importance of controlling the products in organic synthesis.
💡Alkali Metals
Alkali metals are a group of elements in the periodic table known for their reactivity, including lithium and sodium. The script discusses the use of alkali metals in conjunction with ammonia or ethyl amine to selectively hydrogenate alkynes to E alkenes, demonstrating their role in facilitating specific types of hydrogenation reactions.
Highlights

Hydrogenation of alkynes can be achieved through different methods to obtain alkanes, Z alkenes, or E alkenes.

Simple hydrogenation over palladium metal converts alkynes to alkanes by reacting with both pi bonds.

Lindlar catalyst is used to selectively hydrogenate alkynes to Z alkenes without further reduction to alkanes.

Lindlar catalyst is a poisoned palladium that limits hydrogenation to alkynes only, producing Z alkenes.

Z alkenes have the same stereochemistry as the groups attached to the double bond, following the zee zame zide rule.

Alternative methods to produce E alkenes from alkynes involve using lithium and ethyl amine or sodium and ammonia.

These alkali metal-based methods selectively hydrogenate alkynes to E alkenes without reducing to alkanes.

E alkenes have opposite stereochemistry with the groups attached to the double bond projecting in opposite directions.

Understanding the different hydrogenation conditions is crucial for synthetic pathways in organic chemistry.

The choice of catalyst determines the final product: alkane, Z alkene, or E alkene from an alkyne.

Hydrogenation over palladium metal is a one-step process converting alkynes directly to alkanes.

Lindlar catalyst provides a controlled way to stop at the Z alkene stage in the hydrogenation of alkynes.

Alkali metal methods with lithium or sodium offer a selective route to E alkenes from alkynes.

Stereochemistry is an essential consideration in the selective hydrogenation of alkynes to alkenes.

The cahn-ingold-prelog Convention helps assess whether an alkene is E or Z.

Depicting alkynes accurately with sp hybridization and linear geometry is important for understanding reactions.

Different hydrogenation conditions lead to distinct products, illustrating the versatility of alkyne chemistry.

Practical applications of selective hydrogenation include synthesizing specific compounds in organic chemistry.

Transcripts

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Related Tags
HydrogenationAlkynesAlkanesAlkenesZ-AlkeneE-AlkeneLindlar CatalystChemistry TutorialSynthetic PathwayStereochemistryPalladium Reaction