Introduction
Hydrogenation is a process that involves a chemical reaction between molecular hydrogen and another compound, such as an alkene or alkyne. This reaction typically occurs in the presence of a catalyst, such as nickel, palladium, or platinum. The primary purpose of hydrogenation is to reduce or saturate organic compounds. This is achieved by adding pairs of hydrogen atoms to a molecule, often an alkene. For the reaction to be feasible, catalysts are essential; without them, hydrogenation can only occur at extremely high temperatures. The process of hydrogenation serves to reduce double and triple bonds in hydrocarbons. The three key components of hydrogenation are the unsaturated substrate, the hydrogen (or hydrogen source), and a catalyst. The conditions under which the reduction reaction is carried out, including temperature and pressure, vary depending on the substrate and the catalyst’s activity. Since the chemical reaction predominantly occurs on the surface of the metal, making a large surface area crucial for achieving satisfactory reaction rates. To facilitate this, the metal is often subdivided into fine particles and subsequently adsorbed onto a solid substrate, which could be activated carbon or alumina. This arrangement optimizes the exposure of the metal’s surface, thereby enhancing the overall reaction efficiency.
Catalysts
Molecular hydrogen is generally unreactive with organic compounds unless metal catalysts are present. The unsaturated substrate is adsorbed onto the catalyst, covering most of the sites. In the case of heterogeneous catalysts, hydrogen forms surface hydrides (M-H), which can transfer hydrogens to the adsorbed compound too be reduced. Platinum, palladium, rhodium, and ruthenium are known to form highly active catalysts that operate at lower temperatures and pressures of hydrogen. Catalysts are typically divided into two main categories: homogeneous and heterogeneous. Homogeneous catalysts dissolve in the solvent containing the unsaturated compound, while heterogeneous catalysts are solids that are either suspended in the same solvent as the substrate or treated with a gaseous substrate.
Reaction
Regioselectivity: N/A
Stereospecificity: syn
Intermediate: none
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The reaction takes place on the surface of the metal
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Will reduce alkenes and alkynes but typically not aromatic rings unless the temperature and pressure are high enough
Mechanism
The initial stages of catalytic hydrogenation entail the coordination of both the alkene and molecular hydrogen to the metal surface. The alkene or alkyne is typically adsorbed onto the metal surface via its pi-bond (known as "coordination" in organometallic chemistry). Upon the introduction of hydrogen gas to the metal, it is transformed into metal hydrides that occupy the surface. The close proximity of the hydride and alkene facilitates the formation of a carbon-hydrogen (C-H) bond and the breaking of the carbon-carbon (C-C) pi bond, resulting in a metal-carbon bond. Subsequently, the formation of the second carbon-hydrogen bond results in an alkane. In the absence of any pi bond, the molecule dissociates from the metal surface. It’s important to note that the two atoms of hydrogen are not introduced simultaneously.
In instances where a molecule contains two or more alkenes, it is possible to hydrogenate one alkene without reducing the other by carefully introducing only one mole of H2. Typically, the least substituted alkene is hydrogenated first due to its greater accessibility to the metal catalyst (less steric hindrance).