Core ConceptsIn this article, you will learn about the reagents, mechanisms, stereochemistry, and applications of Catalytic Hydrogenation. We will explore the different products of this reaction depending on the different starting materials. We will discuss various examples of how different unsaturated compounds, such as alkenes, alkynes, and aromatic rings, react with hydrogen in the presence of specific catalysts to yield a wide array of saturated products.IntroductionOrganic compounds can be classified as saturated or unsaturated. Saturated compounds have only single bonds between carbon atoms. Unsaturated compounds contain one or more double or triple bonds, resulting in fewer total hydrogen atoms in the molecule. The degree of saturation refers to that compound’s hydrogen content compared with the absolute number of hydrogen atoms the carbon atoms could carry. As an example, ethane (C2H6 or CH3-CH3) is more saturated than ethene (C2H4 or CH2=CH2). Catalytic hydrogenation uses a metal catalyst like palladium, platinum, or nickel to add hydrogen to unsaturated organic compounds, such as alkenes or alkynes. Specifically, the catalyst facilitates the breaking of hydrogen molecules into atomic hydrogen, which then reacts with the unsaturated bonds, converting them into saturated compounds. Let’s look at some of the most important catalytic hydrogenations of different compounds!Why These Metals?Metals like palladium, platinum, and nickel are used for catalytic hydrogenation because they possess unique properties that make them highly effective for this type of reaction. These metals have the ability to adsorb hydrogen gas and dissociate it into individual hydrogen atoms, a crucial step for hydrogenation. Additionally, they have the right balance of reactivity, being reactive enough to facilitate the hydrogenation process, but not so reactive that they bind the hydrogen too strongly, which would prevent its transfer to the substrate.Other metals may not have these optimal properties, leading to slower reactions, lower yields, or a lack of selectivity. Palladium, platinum, and nickel are thus ideal due to their combination of catalytic activity, durability, and adaptability to various hydrogenation reactions.Catalytic Hydrogenation of AlkenesHydrogenation of an alkene is formally a reduction, with H2 adding across the double bond to give an alkane. This process typically requires a metal catalyst, such as palladium (Pd), platinum (Pt), or nickel (Ni).The stereochemistry of catalytic hydrogenation of alkenes involves syn addition, with both hydrogen atoms adding to the same side of the double bond plane.Example:MechanismHydrogenation actually takes place at the surface of the metal, where the liquid solution of the alkene comes into contact with hydrogen and the catalyst.The surface of these metal catalysts adsorbs hydrogen gas, and the catalyst weakens the H-H bond. The catalyst then dissociates the hydrogen molecules, forming metal-hydrogen bonds. The metal catalyst also absorbs the alkene onto its surface. A hydrogen atom transfers to the alkene, forming a new C-H bond. A second hydrogen atom transfers forming another C-H bond. Finally, the newly formed alkane desorbs from the catalyst surface, allowing the catalyst to be reused.Catalytic Hydrogenation of AlkynesCatalytic hydrogenation of an alkyne takes place in two stages, with an alkene intermediate. With platinum, palladium, or nickel catalysts, stopping the reaction at the alkene stage is typically impossible.Partial Catalytic Hydrogenation to cis AlkenesHydrogenation of an alkyne can be stopped at the alkene stage by using a “poisoned” (partially deactivated) catalyst. This is made by treating a good catalyst with a compound that makes the catalyst less effective. Lindlar’s catalyst is a heterogeneous catalyst with palladium on calcium carbonate, deactivated with lead acetate or quinoline. Nickel boride (Ni2B) offers a newer alternative to Lindlar’s catalyst, and it is easier to make and often gives better yields.The partial catalytic hydrogenation of alkynes is similar to the hydrogenation of alkenes, and both proceed with syn stereochemistry.Lindlar’s catalyst does not affect alkenes or aromatic rings like benzene. Examples:Catalytic Hydrogenation of Ketones and AldehydesCatalytic hydrogenation of ketones and aldehydes involves the addition of hydrogen (H₂) to the carbonyl group (C=O). This reaction reduces the carbonyl compounds to their corresponding alcohols. Ketones are reduced to secondary alcohols, and aldehydes are reduced to primary alcohols. This addition commonly uses Raney nickel as the catalyst.Raney Nickel is a highly porous, finely divided form of nickel. Markedly, it is made by alloying nickel with aluminum and then leaching out the aluminum with a strong base like sodium hydroxide. This process creates a spongy structure with a high surface area, making it very effective for hydrogenation. However, carbon-carbon double bonds are also reduced under these conditions, so any alkene double bonds in the starting material will also be reduced. In most cases, sodium borohydride is more convenient for reducing simple ketones and aldehydes. Let’s look at some examples:Raney nickel is more specific to aldehydes and ketones because of its highly porous structure, which provides a large surface area that enhances its ability to activate and reduce the polar carbonyl group (C=O) present in these compounds. The C=O bond is more stable and requires a more powerful catalyst to break it, which Raney nickel can efficiently do due to its high reactivity and affinity for polar functional groups.Catalytic Hydrogenation of Aromatic RingsHydrogenation of benzene into cyclohexane typically requires high temperatures and pressures, with ruthenium or rhodium-based catalysts facilitating the process. When benzene rings with substituents undergo this reaction, they produce substituted cyclohexanes, and disubstituted benzenes tend to form a mix of cis and trans isomers.Aromatics require harsher conditions for hydrogenation because their ring structures are highly stable due to aromaticity. This involves a delocalized π electron system that resists addition reactions. To overcome this stability, more energy, in the form of heat and pressure, is needed. Ruthenium or rhodium-based catalysts are effective because they are highly active and can provide the necessary energy and surface environment to weaken and break the aromatic bonds.
