Hydrogenation Reactions - Online Tutor, Practice Problems & Exam Prep

Hydrogenation reactions involve the addition of hydrogen atoms to unsaturated hydrocarbons, such as alkenes and alkynes, to produce alkanes. This process requires a metal catalyst to facilitate the breaking of the hydrogen-hydrogen bond and the subsequent addition of hydrogen atoms to the carbon atoms that were originally part of a double or triple bond. For each π bond present in the starting material, one mole of hydrogen (H₂) is necessary. The result of hydrogenation is the conversion of alkenes, with one π bond, or alkynes, with two π bonds, into alkanes, which contain only single bonds between carbon and hydrogen atoms. This reaction is essential in organic chemistry for modifying the saturation level of hydrocarbon chains.

Hydrogenation Reactions

In this video we're going to take a look at hydrogenation reactions. Under this reaction, 2 hydrogens are added to one Pi bond. Now due to the stability of H₂A, 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 one Pi bond within our alkene, so we only require one 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 one in organic 2.

But in essence, what happens is we break the Pi bond in order to add the two hydrogens, one to each formally double bonded carbon. At the end we make an alkene product. Now with an alkene we have 2 Pi bonds now, and for every Pi bond we require one 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'd add 4 hydrogen atoms. And look, whether we start with an alkene or an alkyne, the end product for 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. All right. So here we have two alkane products at the end.

Hydrogenation Reactions Example

Identify the major product of the following hydrogenation reaction. So here we're starting out with an alkene and we're reacting it with one mole of H₂. We use a callus to jumpstart the process. Remember, in essence what's occurring here is we're transforming an alkene into an alkane.

We're adding these two hydrogens to these two 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 if this is ethyl cyclohexane, we're not asked to name the final product, but here I'm just giving 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.

• A.
• B.
• C.
• D.

Here’s what students ask on this topic:

Which metal catalyst is often used in the hydrogenation reactions that convert them into alkanes?

In hydrogenation reactions that convert unsaturated compounds into alkanes, the metal catalyst most commonly used is platinum (Pt). However, other metals like palladium (Pd) and nickel (Ni) are also frequently used for this purpose. These catalysts help to facilitate the addition of hydrogen (H₂) across the double or triple bonds of unsaturated hydrocarbons, such as alkenes and alkynes, to yield saturated alkanes. The catalyst provides a surface where the reactants can adsorb, and the hydrogen molecules dissociate into atoms and become more reactive. This process allows the hydrogen atoms to add to the carbon atoms with multiple bonds more efficiently, resulting in the formation of single-bonded carbon atoms, or alkanes, with a higher saturation of hydrogen.

What are hydrogenation reactions?

Hydrogenation reactions are chemical processes where hydrogen molecules are added to other compounds, typically unsaturated organic molecules like alkenes and alkynes. This addition of hydrogen generally occurs in the presence of a catalyst, such as palladium, platinum, or nickel, and often requires the application of heat and pressure.

During hydrogenation, the double or triple bonds between carbon atoms in the unsaturated compounds are converted into single bonds as hydrogen atoms are added. This process results in the saturation of the molecule, turning alkenes into alkanes for instance. A common example of a hydrogenation reaction is the conversion of vegetable oils, which contain unsaturated fats, into semi-solid fats like margarine, a process that increases the melting point of the oil.

Hydrogenation is widely used in the food industry, as well as in the production of chemicals and pharmaceuticals, where controlling the saturation level of molecules is crucial for the properties and functionality of the final products.

Draw the major organic product for each of the following hydrogenation reactions and indicate which stereoisomer is formed if more than one is possible?

To answer your question on drawing the major organic product for hydrogenation reactions, I'll provide a general explanation since I don't have the specific reactions you're referring to.

In hydrogenation reactions, an alkene or alkyne is typically converted into an alkane through the addition of hydrogen (H₂) in the presence of a catalyst, usually palladium (Pd), platinum (Pt), or nickel (Ni). The reaction typically proceeds through a syn addition, where both hydrogen atoms add to the same side of the double or triple bond.

For alkenes:

• The hydrogenation of a simple alkene will yield a single product, an alkane, with no stereoisomers since alkanes do not have chiral centers or geometric isomerism.
• If the alkene is disubstituted and has cis/trans (E/Z) isomerism, the product will still be a single alkane without stereoisomers.

For alkynes:

• The hydrogenation of an alkyne can proceed in two stages. The first stage adds one molecule of hydrogen to give a cis-alkene. The second stage adds another molecule of hydrogen to give an alkane.
• If the reaction is stopped at the alkene stage, you may have cis/trans (E/Z) isomerism depending on the substituents attached to the carbon-carbon double bond.