Hydrogenation Reaction - Online Tutor, Practice Problems & Exam Prep

Hydrogenation reactions involve the addition of hydrogen (H₂) to unsaturated hydrocarbons, specifically alkenes and alkynes. A catalyst, typically a metal, is necessary to break the H-H bond. For alkenes, one mole of H₂ adds two hydrogens, converting the pi bond to a single bond, resulting in an alkane. In alkynes, two moles of H₂ are required to saturate both pi bonds, also yielding an alkane. The overall goal of hydrogenation is to achieve a stable alkane product from less stable alkenes or alkynes.

Hydrogenation Reactions Concept 1

This video, we're going to take a look at hydrogenation reactions. Under this reaction, 2 hydrogens are added to 1 pi bond. Now, due to the stability of H2, a catalyst is required to break the hydrogen-hydrogen bond first. So if we take a look here at this alkene, we have here our alkene, we only have 1 pi bond within our alkene, so we only require 1 mole of H2. Our catalyst being used here is usually some type of metal catalyst, we don't need to go too in-depth in terms of it, we'll save that for organic 1 and organic 2. But in essence what happens is we break the Pi bond in order to add the 2 hydrogens, one to each formally double-bonded carbon. At the end, we make an alkene product.

Now, with an alkyne we have 2 pi bonds now, and for every pi bond we require 1 mole of our reagent. Since there's 2 pi bonds, we need 2 moles of H2. We'd still use our catalyst, so here we add 4 hydrogen atoms, and look, whether we start with an alkene or an alkyne, the end product for a hydrogenation reaction is an alkane. So you're always trying to get to an alkane where you have only single-bonded carbon and hydrogen bonds. Right? So here we'd have 2 alkane products at the end.

Hydrogenation Reactions Example 1

Identify the major product of the following hydrogenation reaction. So here we're starting out with an alkene, and we're reacting it with 1 mole of H₂. We use a catalyst to jumpstart the process. Remember, in essence what's occurring here is we're transforming an alkene into an alkane. We're adding these 2 hydrogens to these 2 double-bonded carbons. They each already have a hydrogen on them. They're still there. We're just adding an additional H to each, which comes from this H₂. So here, we're going to make this alkane product as our final answer. Its name, we have an ethyl group connected to this ring, so this is Ethylcyclohexane. We're not asked to name the final product, but here I'm just giving it to you, so you can see that we make an alkane at the end.

 Write a hydrogenation reaction of the following alkyne and name the organic product formed.

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Here’s what students ask on this topic:

What is a hydrogenation reaction in organic chemistry?

A hydrogenation reaction in organic chemistry involves the addition of hydrogen (H₂) to unsaturated hydrocarbons, such as alkenes and alkynes. This process requires a catalyst, typically a metal like palladium, platinum, or nickel, to break the H-H bond. For alkenes, one mole of H₂ adds two hydrogen atoms, converting the pi bond to a single bond, resulting in an alkane. For alkynes, two moles of H₂ are needed to saturate both pi bonds, also yielding an alkane. The goal of hydrogenation is to transform less stable unsaturated hydrocarbons into more stable alkanes.

Why is a catalyst necessary in hydrogenation reactions?

A catalyst is necessary in hydrogenation reactions because hydrogen (H₂) is a very stable molecule, and breaking the H-H bond requires significant energy. Metal catalysts like palladium, platinum, or nickel provide a surface that facilitates the dissociation of H₂ into individual hydrogen atoms. These atoms can then interact with the pi bonds of alkenes or alkynes, allowing the addition of hydrogen to the unsaturated hydrocarbon. Without a catalyst, the reaction would proceed very slowly or not at all under normal conditions.

What is the difference between hydrogenation of alkenes and alkynes?

The main difference between the hydrogenation of alkenes and alkynes lies in the number of pi bonds and the amount of hydrogen (H₂) required. Alkenes have one pi bond, so they need one mole of H₂ to add two hydrogen atoms, converting the pi bond to a single bond and forming an alkane. Alkynes have two pi bonds, requiring two moles of H₂ to add four hydrogen atoms, saturating both pi bonds and also resulting in an alkane. Thus, hydrogenation of alkenes and alkynes both yield alkanes, but the latter requires more hydrogen.

What are the common catalysts used in hydrogenation reactions?

Common catalysts used in hydrogenation reactions include metals such as palladium (Pd), platinum (Pt), and nickel (Ni). These metals are effective because they provide a surface that facilitates the dissociation of hydrogen (H₂) into individual hydrogen atoms. This dissociation is crucial for the hydrogen atoms to interact with the pi bonds of unsaturated hydrocarbons like alkenes and alkynes, allowing the addition of hydrogen and the formation of alkanes. Each of these catalysts has its own specific applications and advantages depending on the reaction conditions and desired outcomes.

What is the end product of a hydrogenation reaction involving alkenes or alkynes?

The end product of a hydrogenation reaction involving alkenes or alkynes is an alkane. In the case of alkenes, one mole of hydrogen (H₂) is added to the pi bond, converting it to a single bond and forming an alkane. For alkynes, two moles of H₂ are required to saturate both pi bonds, also resulting in an alkane. The overall goal of hydrogenation is to transform the less stable unsaturated hydrocarbons (alkenes or alkynes) into more stable saturated hydrocarbons (alkanes).