8.6 Reduction of Alkenes: Hydrogenation
3 min read•may 7, 2024
transforms alkenes into alkanes using a metal catalyst and gas. This process adds two hydrogen atoms to the carbon-carbon double bond, resulting in a single bond. It's a key reaction for creating saturated compounds from unsaturated ones.
The reaction is , with both hydrogens adding to the same side of the alkene. This leads to different stereochemical outcomes depending on the starting alkene geometry. is widely used in organic synthesis and industrial applications.
Reduction of Alkenes: Hydrogenation
Process of catalytic hydrogenation
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- Adds H2 to an alkene forming an
- Uses a (Pt, Pd, Ni, Rh)
- Catalysts often adsorbed onto solid support (carbon, )
- Performed under elevated pressure of H2 gas
- Mild reaction conditions (room temperature to 100°C)
- Mechanism involves on metal surface
- Alkene and H2 adsorb onto catalyst surface
- H2 dissociates into atomic hydrogen
- Alkene double bond coordinates to metal weakening
- Hydrogen atoms add to alkene forming new C-H
- Saturated alkane product desorbs from catalyst surface
Stereochemistry in alkene hydrogenation
- Stereospecific occurring via of H2
- Both hydrogen atoms add to same face of alkene
- Product stereochemistry depends on starting alkene structure
- Hydrogenation of gives anti products
- Hydrogenation of gives syn products
- Catalyst approach can influence stereochemical outcome in certain cases
- Bulky substituents near double bond may hinder catalyst approach from one face
- Leads to preferential hydrogen addition from less hindered face
- Chiral catalysts can induce
- Enables synthesis of optically active compounds from
Alkene reactivity vs other groups
- Alkenes highly reactive towards hydrogenation compared to most other functional groups
- Isolated alkenes can be reduced selectively in presence of other functional groups
- Alkynes undergo hydrogenation to give alkenes and alkanes
- Partial hydrogenation of alkynes achieved using (Pd on CaCO3 poisoned with lead acetate and quinoline)
- Aromatic rings resistant to hydrogenation under typical conditions
- Require harsher conditions (high pressure and temperature) and specialized catalysts
- (aldehydes, ketones) can be reduced to alcohols by hydrogenation
- Generally requires higher temperatures and pressures than alkene hydrogenation
Catalytic Hydrogenation Mechanism and Catalyst Properties
- : Solid catalyst in a different phase from reactants
- : Process by which reactants bind to catalyst surface
- : Strong chemical bonding between adsorbate and catalyst surface
- : Detailed stepwise process for alkene hydrogenation
- Hydrogen dissociation on catalyst surface
- Alkene adsorption and formation of half-hydrogenated intermediate
- Addition of second hydrogen to form alkane
- : Cleavage of carbon-carbon or carbon-heteroatom bonds by hydrogen
Applications in laboratory and industry
- Synthesis of saturated hydrocarbons and other reduced compounds
- Removal of in fats and oils to improve stability and shelf life
- Preparation of active pharmaceutical ingredients and other fine chemicals
- Hydrogenation of biomass-derived compounds in biofuel production
Key Terms to Review (35)
(E)-Alkenes: (E)-Alkenes, also known as trans-alkenes, are a class of organic compounds characterized by the presence of a carbon-carbon double bond with the substituents on opposite sides. This structural feature is particularly relevant in the context of the reduction of alkenes through hydrogenation, a key topic in organic chemistry.
(Z)-Alkenes: (Z)-Alkenes, also known as cis-alkenes, are a type of alkene isomer where the two largest substituents are on the same side of the carbon-carbon double bond. This structural arrangement has important implications in the context of the reduction of alkenes through hydrogenation reactions.
Addition Reaction: An addition reaction is a type of chemical reaction where two or more reactants combine to form a single product. In the context of organic chemistry, addition reactions typically involve the addition of atoms or molecules to an alkene or alkyne, resulting in the formation of a new compound with a different structure and properties.
Adsorption: Adsorption is the process by which atoms, ions, or molecules from a substance become attached to the surface of a solid or liquid. It is a surface phenomenon that occurs when a gas or liquid solute accumulates on the surface of a solid or a liquid, forming a molecular or atomic film on the adsorbent's surface.
Alkane: Alkanes are a class of saturated hydrocarbons composed entirely of single-bonded carbon and hydrogen atoms. They are the simplest organic compounds and form the basis for many other organic molecules and reactions.
Alumina: Alumina, also known as aluminum oxide (Al2O3), is a hard, white, and crystalline compound that is the primary constituent of the mineral corundum. It is a key material in various industrial and chemical applications, particularly in the context of the reduction of alkenes through hydrogenation reactions.
Anti Addition: Anti addition refers to the stereochemical outcome of an electrophilic addition reaction, where the incoming electrophilic species adds to the opposite face of the alkene or alkyne relative to the existing substituents. This results in the formation of the anti-addition product, where the new substituents are arranged in an anti-configuration.
Carbonyl Compounds: Carbonyl compounds are a class of organic compounds that contain a carbon-oxygen double bond (C=O), known as the carbonyl group. This functional group is found in a variety of important molecules, including aldehydes, ketones, carboxylic acids, esters, and amides, which are all integral to many organic chemistry topics and reactions.
Catalytic Hydrogenation: Catalytic hydrogenation is a chemical process where hydrogen gas is used to reduce unsaturated organic compounds, such as alkenes, aromatic rings, and carbonyl groups, in the presence of a metal catalyst. This reaction allows for the selective and controlled addition of hydrogen to these functional groups, leading to the formation of new, more saturated compounds.
Chemisorption: Chemisorption is the process by which a gas or liquid adheres to the surface of a solid material through the formation of a chemical bond. It is a fundamental concept in the field of heterogeneous catalysis, where the interaction between reactants and the catalyst surface plays a crucial role in the overall reaction mechanism.
Enantioselectivity: Enantioselectivity refers to the preference of a chemical reaction to produce one enantiomer (mirror-image molecule) over another. It is a crucial concept in organic chemistry, particularly in the context of the reduction of alkenes through hydrogenation reactions.
Heat of hydrogenation: The heat of hydrogenation is the amount of energy released when a double bond in an alkene reacts with hydrogen gas to form a single bond, turning it into an alkane. This process is exothermic, indicating that energy is given off during the conversion.
Heterogeneous Catalysis: Heterogeneous catalysis is a catalytic process where the catalyst is in a different physical state than the reactants. This is typically seen when the catalyst is a solid and the reactants are in the liquid or gaseous phase.
Heterogeneous Catalyst: A heterogeneous catalyst is a catalyst that exists in a different physical phase from the reactants in a chemical reaction. It is typically a solid material that facilitates the reaction between gaseous or liquid reactants, providing an alternative pathway with lower activation energy.
Horiuti-Polanyi Mechanism: The Horiuti-Polanyi mechanism is a model that describes the step-by-step process of the catalytic hydrogenation of alkenes. It provides a detailed explanation of how hydrogen gas interacts with an alkene substrate in the presence of a metal catalyst to reduce the carbon-carbon double bond.
Hydrogen: Hydrogen is the lightest and most abundant chemical element in the universe. It is a highly reactive, colorless, odorless gas that forms polar covalent bonds and plays a crucial role in various chemical reactions, including the reduction of alkenes through hydrogenation.
Hydrogen bond: A hydrogen bond is a weak type of chemical bond that is formed when a hydrogen atom covalently bonded to a highly electronegative atom, such as nitrogen, oxygen, or fluorine, experiences an attraction to another electronegative atom in a nearby molecule or within the same molecule. It plays a crucial role in determining the structure and properties of water, proteins, and nucleic acids.
Hydrogenation: Hydrogenation is a chemical reaction in which hydrogen gas (H2) is added to an organic compound, typically an alkene or alkyne, to produce a more saturated compound. This process is commonly used in the food industry to convert unsaturated fats into more stable, saturated fats.
Hydrogenolysis: Hydrogenolysis is a chemical reaction where a carbon-heteroatom bond, such as a carbon-oxygen or carbon-nitrogen bond, is cleaved by the addition of hydrogen. This process is commonly used in organic chemistry for the selective removal of protecting groups and the reduction of various functional groups.
Lindlar's Catalyst: Lindlar's catalyst is a heterogeneous catalyst used in the selective hydrogenation of alkynes to alkenes. It is a versatile tool in organic synthesis for the controlled reduction of carbon-carbon triple bonds while preserving other functional groups.
NMR Spectroscopy: NMR (Nuclear Magnetic Resonance) spectroscopy is an analytical technique that uses the magnetic properties of atomic nuclei to provide detailed information about the structure and composition of organic compounds. It is a powerful tool for identifying and characterizing chemical compounds, particularly in the context of organic chemistry.
Palladium: Palladium is a rare and valuable transition metal that has unique catalytic properties, making it an important element in various organic chemistry reactions. It is commonly used as a catalyst to facilitate chemical transformations and is particularly relevant in the context of biological reactions, the reduction of alkenes, oxidation and reduction processes, and the Wolff-Kishner reaction.
Platinum: Platinum is a rare, dense, and highly valuable precious metal that is widely used in various scientific and industrial applications. It is known for its exceptional catalytic properties, corrosion resistance, and high melting point, making it a crucial element in organic chemistry and related fields.
Prochiral Alkenes: Prochiral alkenes are alkenes that have two enantiotopic hydrogen atoms or substituents attached to a carbon-carbon double bond. This means that the two hydrogen atoms or substituents can be replaced with different groups to generate two distinct stereoisomers.
Raney Nickel: Raney nickel is a highly active heterogeneous catalyst composed of nickel that is commonly used in organic chemistry for the selective hydrogenation of various functional groups. It is named after its inventor, Murray Raney, and is known for its ability to facilitate the reduction of alkenes, nitro compounds, and other reducible groups while maintaining selectivity.
Reduction: Reduction is a chemical process that involves the gain of electrons by a molecule or atom, resulting in a decrease in its oxidation state. This term is particularly important in the context of various organic chemistry reactions and transformations.
Rhodium: Rhodium is a rare, silvery-white, hard, and corrosion-resistant transition metal. It is primarily used as a catalyst in various chemical processes, including the reduction of alkenes through hydrogenation and the synthesis of amino acids.
Sabatier: The Sabatier process, also known as the Sabatier reaction, is a catalytic process used in the reduction of alkenes through hydrogenation. It involves the addition of hydrogen gas to an alkene in the presence of a metal catalyst, typically nickel, to produce an alkane.
Saturation: Saturation refers to the extent to which a chemical compound or functional group has the maximum number of hydrogen atoms that it can accommodate. It is a fundamental concept in organic chemistry that is closely tied to the reduction of alkenes through hydrogenation.
Stereospecific: Stereospecificity refers to the ability of a chemical reaction to produce a specific stereoisomer as the sole or predominant product. This term is particularly relevant in the context of organic chemistry reactions involving alkenes, where the stereochemistry of the reactants and products is of great importance.
Syn Addition: Syn addition is a type of organic reaction where two substituents are added to the same side of a carbon-carbon double bond, resulting in the formation of a new stereocenter with a specific stereochemical configuration. This term is particularly relevant in the context of various organic chemistry topics, including electrophilic addition reactions of alkenes, hydration of alkenes, reduction of alkenes, and oxidation of alkenes.
Transition Metal Catalyst: A transition metal catalyst is a type of catalyst that contains a transition metal element, which can facilitate chemical reactions by providing an alternative pathway with lower activation energy. These catalysts are widely used in various organic reactions, including the reduction of alkenes and the oxidation of alkenes.
Unsaturation: Unsaturation refers to the presence of carbon-carbon double or triple bonds in organic molecules. This structural feature has important implications for the reactivity and physical properties of these compounds, particularly in the context of reduction reactions like hydrogenation.
π Bond: A π bond is a type of covalent bond that forms between atoms when they share a pair of electrons in a side-to-side arrangement, rather than the head-to-head arrangement of a σ bond. π bonds are crucial in understanding the structure and reactivity of organic compounds.
σ Bonds: A σ bond is a type of covalent bond formed by the head-on overlap of atomic orbitals, resulting in a high electron density between the bonded atoms. These bonds are fundamental to the structure and stability of organic molecules.