Triacylglycerol Reactions: Hydrogenation: Study with Video Lessons, Practice Problems & Examples

Hydrogenation of triacylglycerol molecules involves adding hydrogen to carbon double bonds, converting them to single bonds, which decreases unsaturation and increases melting points. Complete hydrogenation reduces all pi bonds, requiring three moles of hydrogen gas with a nickel catalyst, resulting in fully saturated fats. Partial hydrogenation, using two moles of hydrogen, leaves one pi bond, potentially isomerizing from cis to trans configurations, producing trans fats, which can be harmful. This process is crucial in creating margarine and adjusting the hardness of lipid products.

Triacylglycerol Reactions: Hydrogenation Concept 1

Hey, everyone. So in this video, we're going to take a look at the hydrogenation of Triacylglycerol Molecules. Now recall that under this type of reaction, 2 hydrogens are added to 1 pi bond. We're going to say the conversion from double bonds to single bonds is going to cause a decrease in our levels of unsaturation, which will cause an increase in our melting point. Now, when we talk about hydrogenation, we can look at it in terms of complete hydrogenation or partial hydrogenation. We're going to say here, we're starting out with the same type of a triacylglycerol molecule. Here, we have the same types of fatty acid chains attached. They each have 1 pi bond apiece. And we're going to say here with complete hydrogenation, we're going to say this is when all carbon double bonded carbon bonds are reduced to single bonds. Because we have 3 pi bonds, this will require 3 moles of hydrogen gas. Now remember with hydrogenation when we talked about it in earlier chapters with alkenes and alkynes, we'd have to use some type of metal catalyst. Here, we're using Nickel as the metal catalyst of choice. Now, because it's a complete hydrogenation, all the pi bonds here get reduced. So now they're only single bonds. We've gone from an unsaturated fatty acid group of chains to completely saturated. With partial hydrogenation, we're going to say this is when some, but not all carbon carbon double bonds are reduced to single bonds. Here we have 3 pi bonds And at the end, we only have one left. Because there's only one left, we're going to say, we only used 2 moles of hydrogen gas. So, we were able to reduce 2 of them, leaving one behind. We're still using our Nickel catalyst. Now, here, a couple of observations. First of all, we're going to say, we noticed that the configuration went from cis here to trans here. And we're also going to see that this is commercially manufactured. You might see terms or hear about this or heard about this of partially hydrogenated oils. Now, here we're going to say here that partial hydrogenation converts oils to margarines whose ultimate consistency is based on the number of piebots. So this is a way of making margarines and different types of fatty acid or lipid products that you buy in the store. You can actually adjust the number of pi bonds you have at the end. This actually adjusts the hardness of different types of margarines that you might buy. Now, here we're going to say during hydrogenation, some of the double bonds can isomerize to produce, and as we see, we went from cis to trans here, so it can produce trans pi bonds. Another way we can look at trans is e configuration. You might hear the term trans fats. This is a way of producing trans fats. We know that trans fat fats can be deleterious or harmful to our bodies. Again, this can happen. We sometimes can't control the production of these trans fats. Back decades ago, a lot of things had trans fats, but the process has become more streamlined and more technically based, better based that we're able to eliminate a lot of trans fats from different types of foods. So again, something you might have heard about or read about, we can see its application here under this idea of partial hydrogenation. Right? So here, we don't reduce all the pi bonds, some that remain. There's a chance they may go from one configuration to another. In this case, we're going from the cis or z configuration to the trans or e configuration.

Determine a possible triacylglycerol molecule formed when linoleic acid undergoes partial hydrogenation and consumes 1 mole of hydrogen gas.

• A. Palmeitoleic acid
• B. Stearic acid
• C. Linolenic acid
• D. Oleic acid

A triacylglycerol molecule in the form of linoleic acid consumes 2 moles of hydrogen gas. Which of the following fatty acid represents the product formed?

• A. Myristic acid
• B. Stearic acid
• C. Palmitic acid
• D. Oleic acid

Assuming a complete reaction with hydrogen gas, which of the following molecules would have the greatest increase in melting point?

• A.
• B.
• C.
• D.

Here’s what students ask on this topic:

What is the hydrogenation of triacylglycerol molecules?

Hydrogenation of triacylglycerol molecules involves adding hydrogen (H₂) to the carbon-carbon double bonds (C=C) in the fatty acid chains, converting them to single bonds (C-C). This process decreases the level of unsaturation and increases the melting point of the fat. Hydrogenation can be complete, where all double bonds are reduced, or partial, where only some double bonds are reduced. A metal catalyst, typically nickel (Ni), is used to facilitate the reaction.

What is the difference between complete and partial hydrogenation of triacylglycerols?

Complete hydrogenation of triacylglycerols reduces all carbon-carbon double bonds (C=C) to single bonds (C-C), requiring a stoichiometric amount of hydrogen gas (H₂) and a metal catalyst like nickel (Ni). This results in fully saturated fats. Partial hydrogenation, on the other hand, reduces only some of the double bonds, leaving others intact. This process can lead to the formation of trans fats due to the isomerization of remaining double bonds from cis to trans configurations.

Why is nickel used as a catalyst in the hydrogenation of triacylglycerols?

Nickel (Ni) is used as a catalyst in the hydrogenation of triacylglycerols because it effectively facilitates the addition of hydrogen (H₂) to carbon-carbon double bonds (C=C). Nickel provides a surface for the hydrogen molecules to dissociate into atoms, which then react with the double bonds in the fatty acid chains, converting them to single bonds (C-C). This process increases the efficiency and rate of the hydrogenation reaction.

What are trans fats, and how are they formed during partial hydrogenation?

Trans fats are a type of unsaturated fat with at least one double bond in the trans configuration, where hydrogen atoms are on opposite sides of the double bond. During partial hydrogenation, some of the remaining double bonds in the fatty acid chains can isomerize from the cis configuration (hydrogens on the same side) to the trans configuration. This isomerization can occur due to the conditions of the hydrogenation process, such as the presence of a nickel catalyst and the specific reaction environment.

How does hydrogenation affect the melting point of triacylglycerols?

Hydrogenation increases the melting point of triacylglycerols by converting carbon-carbon double bonds (C=C) to single bonds (C-C), thereby reducing the level of unsaturation. Unsaturated fats, which contain double bonds, have lower melting points and are typically liquid at room temperature. By hydrogenating these double bonds, the resulting saturated fats have higher melting points and are more likely to be solid at room temperature.

What is the role of hydrogen gas in the hydrogenation of triacylglycerols?

Hydrogen gas (H₂) plays a crucial role in the hydrogenation of triacylglycerols by providing the hydrogen atoms needed to convert carbon-carbon double bonds (C=C) to single bonds (C-C). During the reaction, hydrogen molecules dissociate into atoms on the surface of a metal catalyst, such as nickel (Ni). These hydrogen atoms then react with the double bonds in the fatty acid chains, resulting in the formation of single bonds and a decrease in unsaturation.