8.5 Catalytic Hydrogenation of Alkenes and Heats of Hydrogenation | Organic Chemistry

Chad's Prep

20 Nov 202008:15

EducationalLearning

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TLDRThe video script delves into the concept of catalytic hydrogenation, also known as catalytic reduction, where an alkene is converted into an alkane by adding two hydrogens from a molecule of molecular hydrogen. The process is characterized by a syn addition, meaning both hydrogens are added to the same face of the alkene, and does not involve a carbocation intermediate. The stereoselectivity of the reaction is highlighted, along with the fact that it doesn't follow Markovnikov's rule due to the addition of identical hydrogen atoms. The lesson also touches on the heats of hydrogenation, explaining that these reactions are exothermic and that the degree of exothermicity can be predicted based on the substitution of the alkene. The more substituted the alkene, the less exothermic the reaction, with the least substituted alkenes releasing the most heat upon hydrogenation. The video is part of a series on organic chemistry, aimed at providing educational content throughout the 2020-21 school year.

Takeaways

🌟 **Catalytic Hydrogenation Overview**: The process involves adding two hydrogens from a molecule of hydrogen (H2) to an alkene, converting it into an alkane.
🔍 **Regioselectivity Absence**: Unlike some other addition reactions, there is no regioselectivity in catalytic hydrogenation, meaning there is no preference for Markovnikov or anti-Markovnikov addition.
🕴️ **Stereoselectivity**: The addition is stereoselective and proceeds via a syn addition, meaning both hydrogens are added to the same face of the alkene.
⛓ **Mechanism Uncertainty**: The exact mechanism of catalytic hydrogenation is not fully understood, which means students do not need to memorize the mechanism for exams.
🔬 **Catalyst Role**: Common metal catalysts used in this reaction include palladium, platinum, or nickel, often in the form of palladium on carbon.
🔄 **Adsorption and Weakening**: Hydrogen adsorbs onto the metal catalyst surface, which weakens the H-H bond, allowing the alkene to add both hydrogens simultaneously.
🧬 **Chiral Center Formation**: When two hydrogens are added to an alkene with specific substituents, it can lead to the formation of chiral centers and potentially multiple products.
🔄 **Meso Compounds**: If a reaction leads to the formation of a meso compound, which is achiral despite having two chiral centers, only one product is formed, not four.
🌡️ **Heats of Hydrogenation**: Catalytic hydrogenation reactions are exothermic, and the heat released can vary depending on the substitution of the alkene.
⬇️ **Exothermic Ranking**: More substituted alkenes, which are more stable and start at lower energy, will have less exothermic reactions compared to less substituted alkenes.
📚 **Study Resources**: For further study and practice, premium courses and study guides related to alkene addition reactions are available on chatsprep.com.

Q & A

What is catalytic hydrogenation also known as?
-Catalytic hydrogenation is also referred to as catalytic reduction.
What happens to an alkene during catalytic hydrogenation?
-During catalytic hydrogenation, an alkene is converted into an alkane by the addition of two hydrogens from a molecule of molecular hydrogen.
Is there any regioselectivity in the addition of hydrogen to an alkene during catalytic hydrogenation?
-No, there is no regioselectivity when adding hydrogen to an alkene during catalytic hydrogenation because two identical hydrogens are added.
What is the stereoselectivity of the addition in catalytic hydrogenation?
-The stereoselectivity of the addition in catalytic hydrogenation is syn addition, meaning both hydrogens are added to the same face of the alkene.
Do we know the exact mechanism of catalytic hydrogenation?
-The exact mechanism of catalytic hydrogenation is not fully understood, but it is known that hydrogen adsorbs to the surface of a metal catalyst, which weakens the hydrogen-hydrogen bond, allowing the alkene to add both hydrogens simultaneously.
Which metals are typically used as catalysts in catalytic hydrogenation?
-Common metal catalysts used in catalytic hydrogenation include palladium, platinum, or nickel, with palladium on carbon being a frequently used form.
What is the term for the addition of hydrogen to both sides of an alkene without forming any chiral centers?
-When hydrogen is added to both sides of an alkene without forming any chiral centers, the process is known as a reduction and specifically, catalytic reduction in the context of catalytic hydrogenation.
What is a meso compound in the context of stereochemistry?
-A meso compound is a molecule with two or more chiral centers that is superposable with its mirror image, making it achiral despite having chiral centers.
Why does forming a meso compound result in fewer products than expected when two chiral centers are formed?
-Forming a meso compound results in fewer products because the molecule has a plane of symmetry, making the two chiral centers identical and thus not leading to distinct diastereomers.
What is the heat of hydrogenation and how does it vary with the substitution of the alkene?
-The heat of hydrogenation is the amount of heat released during the hydrogenation of an alkene to form an alkane. It varies with the substitution of the alkene, with more substituted (and thus more stable) alkenes leading to less exothermic reactions compared to less substituted alkenes.
How can the heat of hydrogenation be used to characterize alkenes?
-The heat of hydrogenation can be used to characterize alkenes by comparing the amount of heat released during their hydrogenation to form alkanes. Alkenes that release more heat are generally less stable and have a higher energy content.
What is the relationship between the stability of an alkene and the exothermicity of its hydrogenation reaction?
-The stability of an alkene is inversely related to the exothermicity of its hydrogenation reaction. Less stable alkenes, which are higher in energy, will have more exothermic hydrogenation reactions compared to more stable alkenes.

Outlines

00:00

🌟 Catalytic Hydrogenation: Understanding Syn Addition and Heats of Hydrogenation

This paragraph introduces the concept of catalytic hydrogenation, also known as catalytic reduction, where hydrogens from a molecule of molecular hydrogen are added across an alkene to form an alkane. The process lacks regioselectivity but exhibits syn stereoselectivity. The exact mechanism is not well understood, which is beneficial for students as they don't need to memorize it. The lesson also touches on the exothermic nature of these reactions and how the degree of substitution in the alkene can predict the heat of hydrogenation. The video is part of a series on organic chemistry that will be released weekly throughout the 2020-21 school year.

05:02

🔍 Stereoselectivity and Meso Compounds in Catalytic Hydrogenation

The second paragraph delves deeper into the stereoselectivity of catalytic hydrogenation, explaining that it is a syn addition and does not proceed through a carbocation intermediate. It clarifies that when two chiral centers are formed, the reaction typically yields only two products due to the syn addition, not four, as one might initially assume. An important note is made regarding meso compounds, which are achiral despite having two chiral centers, leading to a single product rather than two. The paragraph also discusses the use of hydrogenation as a method for characterizing alkenes and how the heat of hydrogenation can vary depending on the stability of the alkene, with less substituted alkenes releasing more heat upon hydrogenation.

Mindmap

Keywords

💡Catalytic Hydrogenation

Catalytic hydrogenation, also known as catalytic reduction, is a chemical process where hydrogen is added to a molecule, specifically an alkene, to convert it into an alkane. This process is central to the video's theme as it is the primary reaction being discussed. The video explains that this reaction does not involve regioselectivity but is a syn addition, meaning the hydrogens are added to the same face of the alkene.

💡Regioselectivity

Regioselectivity refers to the selectivity of a chemical reaction for different possible reaction sites. In the context of the video, it is mentioned that catalytic hydrogenation does not exhibit regioselectivity because two identical hydrogen atoms are added to the alkene, resulting in the same product regardless of which side the hydrogens are added to.

💡Stereoselectivity

Stereoselectivity is the ability of a reaction to produce different spatial arrangements of atoms in a molecule. The video emphasizes that while there is no regioselectivity in catalytic hydrogenation, stereoselectivity is observed as the reaction is a syn addition. This means that the two hydrogens from the hydrogen molecule (H2) are added to the same face of the alkene, which is significant for understanding the stereochemistry of the resulting alkane.

💡Syn Addition

Syn addition is a type of chemical reaction where two groups are added to a molecule from the same side. The video uses the term to describe the stereochemistry of the hydrogenation reaction, where the hydrogens from the H2 molecule are added to the same face of the alkene, resulting in a specific spatial arrangement of the new hydrogen atoms in the product alkane.

💡Metal Catalyst

A metal catalyst is a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change. In the video, metal catalysts such as palladium, platinum, or nickel are mentioned as being commonly used in catalytic hydrogenation to facilitate the addition of hydrogen to the alkene.

💡Palladium on Carbon

Palladium on carbon (or palladium on charcoal) is a form of palladium used as a catalyst in various chemical reactions, including catalytic hydrogenation. The video mentions it as a common catalyst used in the hydrogenation process, where it helps in the adsorption of hydrogen and facilitates the reaction without being consumed.

💡Chiral Centers

Chiral centers are points in a molecule where the molecule's spatial arrangement can result in different stereoisomers. The video discusses the formation of chiral centers when two hydrogens are added to an alkene with different substituents, potentially leading to different stereoisomers or a racemic mixture, depending on the reaction's stereoselectivity.

💡Meso Compound

A meso compound is a molecule that appears chiral but is not optically active due to a plane of symmetry. The video explains that when two chiral centers are formed during a syn addition, if the molecule has a plane of symmetry, it results in a meso compound. This is significant because it affects the number of possible products and the optical activity of the molecule.

💡Heats of Hydrogenation

Heats of hydrogenation refer to the amount of heat released during the hydrogenation of a molecule. The video discusses how the exothermic nature of the hydrogenation reaction can vary depending on the substitution of the alkene, with more substituted alkenes releasing less heat upon hydrogenation compared to less substituted ones.

💡Exothermic Reaction

An exothermic reaction is a chemical reaction that releases energy, usually in the form of heat. In the context of the video, all hydrogenation reactions are exothermic, meaning they release heat. The video also touches on the concept that the extent of exothermicity can be used to predict and rank the relative stabilities of different alkenes.

💡Alkene Stability

Alkene stability refers to the tendency of an alkene to undergo reactions such as hydrogenation. The video explains that more substituted alkenes are more stable, which is why they are less reactive and release less heat upon hydrogenation. This concept is used to discuss the relative heats of hydrogenation for different alkenes.

Highlights

Catalytic hydrogenation, also known as catalytic reduction, involves adding two hydrogens from a molecule of molecular hydrogen across an alkene to form an alkane.

There is no regioselectivity in catalytic hydrogenation because two identical hydrogens are added to the alkene.

The addition of hydrogens to the alkene is a syn addition, meaning both hydrogens are added to the same face of the molecule.

The exact mechanism of catalytic hydrogenation is not perfectly understood, which means students do not need to memorize it for exams.

Hydrogen adsorbs to the surface of a metal catalyst, such as palladium, platinum, or nickel, which weakens the hydrogen-hydrogen bond.

An alkene aligns to add both hydrogens simultaneously from the same molecule of H2, resulting in syn addition.

Catalytic hydrogenation can result in the formation of chiral centers and potentially up to four products if there is no stereoselectivity.

Stereoselectivity in catalytic hydrogenation typically results in a racemic mixture of two products when two chiral centers are formed.

The formation of a meso compound from two chiral centers results in an achiral molecule and does not produce four distinct products.

Catalytic hydrogenation is an exothermic reaction, and the heat released, known as the heat of hydrogenation, varies depending on the substitution of the alkene.

The more substituted an alkene is, the more stable it is, and the less heat is released upon hydrogenation.

The least substituted alkene has the highest energy and will release the most heat upon hydrogenation.

Catalytic hydrogenation is used to characterize alkenes and can help determine their stability and heat of hydrogenation.

The lesson is part of a new organic chemistry playlist being released weekly throughout the 2020-21 school year.

To stay updated with new lessons, subscribers are encouraged to click the bell notification to receive updates.

The next lesson in the series will cover more alkene addition reactions, including catalytic hydrogenation with different metal catalysts.

For additional study materials and practice problems, students are directed to the instructor's premium courses on chatsprep.com.

Transcripts

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Takeaways

Q & A

What is catalytic hydrogenation also known as?

What happens to an alkene during catalytic hydrogenation?

Is there any regioselectivity in the addition of hydrogen to an alkene during catalytic hydrogenation?

What is the stereoselectivity of the addition in catalytic hydrogenation?

Do we know the exact mechanism of catalytic hydrogenation?

Which metals are typically used as catalysts in catalytic hydrogenation?

What is the term for the addition of hydrogen to both sides of an alkene without forming any chiral centers?

What is a meso compound in the context of stereochemistry?

Why does forming a meso compound result in fewer products than expected when two chiral centers are formed?

What is the heat of hydrogenation and how does it vary with the substitution of the alkene?

How can the heat of hydrogenation be used to characterize alkenes?

What is the relationship between the stability of an alkene and the exothermicity of its hydrogenation reaction?