Hydrogenation - Online Tutor, Practice Problems & Exam Prep

Catalytic hydrogenation is a reaction that adds hydrogen to double or triple bonds, converting them into alkanes through syn addition, resulting in cis products. This process utilizes metal catalysts like nickel, platinum, or palladium, or the Wilkinson's catalyst, which employs rhodium. The reaction involves the transformation of pi bonds into sigma bonds, with the addition of two hydrogens for each double bond. The stereochemistry is consistent due to the coordination of hydrogens on the same side of the double bond, ensuring no rearrangements occur.

General properties of catalytic hydrogenation.

Alright, guys. So now we're going to talk about an addition mechanism that some of you might already be familiar with. But regardless, we're going to talk about it in this section because it has to do with addition. And that's called catalytic hydrogenation and also Wilkinson's catalyst. So let's go ahead and check it out. Basically, catalytic hydrogenation is a way of adding hydrogen to double bonds. So remember that I said that any addition reaction is a reaction where you take 1 pi bond and you make 2 sigma bonds. Well, actually, one of the easiest ways that you could do that is just to say, let's take that 1 pi bond and make 2 hydrogen bonds out of it. Okay? That would still be an addition reaction because I'm taking 1 pi and then making 2 sigma bonds. Alright? And that's what happens with both of these reagents. Okay?

So, how does this work in general? Basically, we're going to have a double bond or triple bond and whenever you have one of those, we're just going to add as many H's to it as possible. So what that means is that the product of these reactions is simply going to be alkanes. Okay? We haven't learned a whole lot of reactions that make alkanes. This is one of them because you can take double bonds and triple bonds, add hydrogens to them until they're fully saturated and become alkanes. Alright? The stereochemistry for this reaction, both of these, is going to be syn addition and what that means is that your end products would be cis products. Okay? What that means is that your hydrogens are going to add from the same side of the double bond at all times. And why is that? Because it turns out that our intermediate or the thing that's going to catalyze this is going to be a metal catalyst. Okay? And that metal catalyst is going to come in and it's going to coordinate the H's to be on one side of that double bond. So that's why you're always going to get syn addition. Alright?

So now for these two things, I'm actually going to kind of ignore them. There are no carbocations, so I'm not going to expect rearrangements. And then I'm adding 2 of the same thing. I'm always adding 2 H's. So I don't really have a choice between Markovnikov and anti-Markovnikov. In fact, that's not really going to apply since I'm adding 2 of the same thing. Alright? So let's go ahead and see the general reaction. The general reaction basically looks like this. I have 2 different mechanisms and 2 different reagents that do the same thing. This combination of reagents here is called catalytic hydrogenation. Okay? How do you know that it's catalytic hydrogenation? Because we've got H₂ with a metal catalyst such as nickel, platinum, or palladium. Whenever you see H₂ with one of those 3, that means it's going to be catalytic hydrogenation. Okay? But there's also another reagent that does a very similar reaction and that's if I have H₂ over what's called rhodium. This is the Wilkinson's catalyst. Okay? I'm just going to write that here. Wilkinson's catalyst. Okay? And the Wilkinson's catalyst is really going to do the same exact thing just using a different metal catalyst. It's going to have a rhodium and then 3 triphenylphosphines and chlorine. You don't need to be able to draw this. What's important is that you just recognize that this is going to do the same thing. It's going to add 2 H's for every double bond that it encounters.

But it's not going to do anything else to anything else. So that means that my general reaction, what I would expect, is that I get the same carbon backbone as I had in my beginning. I would get a single bond there. Even my alcohol doesn't change. Nothing changes about that. The only difference is that now instead of having that double bond on this side of the molecule over here, now I'm just going to have 2 H's. So in fact, I would just draw it like that. I don't need to draw the H's because H's are implied. Okay? And then over here, what I would expect is that I would just have a single bond. Why is that? Because basically, there were 2 pi bonds here. What that means is that I add 2 hydrogens for every pi bond. So I would've added hydrogen here, hydrogen here, and then I would've also added hydrogen here, hydrogen here. And that's what I get at the end. What I get is 2 Hydrogens here and, well, I'm sorry, one of each color, one there and one there. And Then I would get a hydrogen there and a hydrogen there. Those are the 4 hydrogens that were added through my 2 hydrogenation or Wilkinson's catalyst reactions.

Maybe you're wondering, hey, but doesn't this carbon need to have 3 H's? Yeah, it does. It has that last H that came from the original compound. Remember there was always one H at the end to begin with, that's that H right there. But now what I did was I added 2 blue H's from the first equivalent and then I added 2 red H's from the second equivalent, basically giving me an alkane as my end product. Does that make sense? Cool. So what I want you guys to do now is I want to just provide the products for the following reaction. That means you're going to have to recognize what it is and then think about the stereochemistry if it's required and try to draw the end product. Now notice that instead of using H₂, I'm using D₂. But that's really the same thing because remember that deuterium is just heavy hydrogen. So go ahead and try to draw the end product for this and then I'm going to go ahead and explain it to you guys. So go for it.

Provide the product of the following reaction.     

• A. No Reaction
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• C.
• D.

Here’s what students ask on this topic:

What is catalytic hydrogenation in organic chemistry?

Catalytic hydrogenation is a chemical reaction where hydrogen (H₂) is added to double or triple bonds in organic compounds, converting them into alkanes. This reaction uses metal catalysts such as nickel, platinum, or palladium. The process involves the transformation of π bonds into σ bonds through syn addition, resulting in cis products. The metal catalyst coordinates the hydrogens to add on the same side of the double bond, ensuring consistent stereochemistry.

What is Wilkinson's catalyst and how is it used in hydrogenation?

Wilkinson's catalyst is a specific type of catalyst used in hydrogenation reactions. It consists of rhodium coordinated with three triphenylphosphines and a chlorine atom. This catalyst facilitates the addition of hydrogen (H₂) to double or triple bonds, converting them into alkanes. Like other metal catalysts, Wilkinson's catalyst ensures syn addition, resulting in cis products. It is particularly useful for selective hydrogenation reactions in organic synthesis.

What is the stereochemistry of catalytic hydrogenation?

The stereochemistry of catalytic hydrogenation is syn addition, meaning that the hydrogen atoms add to the same side of the double bond. This results in cis products. The metal catalyst used in the reaction coordinates the hydrogens to ensure they add from the same side, maintaining consistent stereochemistry throughout the process.

What are the common metal catalysts used in catalytic hydrogenation?

Common metal catalysts used in catalytic hydrogenation include nickel (Ni), platinum (Pt), and palladium (Pd). These metals facilitate the addition of hydrogen (H₂) to double or triple bonds, converting them into alkanes. Another specific catalyst used is Wilkinson's catalyst, which contains rhodium (Rh) coordinated with triphenylphosphines and chlorine.

How does catalytic hydrogenation convert alkenes to alkanes?

Catalytic hydrogenation converts alkenes to alkanes by adding hydrogen (H₂) to the double bonds of alkenes. The reaction uses a metal catalyst such as nickel, platinum, or palladium, which facilitates the addition of hydrogen atoms to the same side of the double bond (syn addition). This process transforms the π bonds in the alkene into σ bonds, resulting in a fully saturated alkane.