8.5 Catalytic Hydrogenation of Alkenes and Heats of Hydrogenation | Organic Chemistry
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
🌟 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.
🔍 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
💡Regioselectivity
💡Stereoselectivity
💡Syn Addition
💡Metal Catalyst
💡Palladium on Carbon
💡Chiral Centers
💡Meso Compound
💡Heats of Hydrogenation
💡Exothermic Reaction
💡Alkene Stability
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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