Hydrohalogenation - Online Tutor, Practice Problems & Exam Prep

Hydrohalogenation involves adding a hydrogen and a halogen to a double bond, resulting in alkyl halides. The reaction features a carbocation intermediate, which is trigonal planar, leading to unknown stereochemistry. Markovnikov's rule dictates that the halogen attaches to the more substituted carbon. Rearrangements may occur due to the carbocation's stability. The general reaction can be represented as a double bond reacting with HX, yielding products with the halogen on the more substituted side and hydrogen on the less substituted side, while the stereochemistry remains ambiguous.

General properties of hydrohalogenation.

All right. So this brings us to our first named reaction of this section, and it turns out that it's a really easy reaction because it's just the one that we've already been practicing with. It's called hydrohalogenation. Just like the name says, all we're going to be doing is we're going to be adding a hydrogen and a halogen as sigma bonds. That's it. So, we just want to fill out a few facts before we begin. Let's talk about the intermediate. Because all these addition reactions are going to have intermediates. So either intermediates or transition states. So let's go ahead and talk about what this is. The intermediate is going to be a carbocation. So, we know that we're going to have a carbocation intermediate and the stereochemistry is going to be unknown. The reason is that whenever you have a carbocation in place and you're attacking it, remember that carbocations are trigonal planar. So what that means is that they have a front side and a back side that are both easy to attack. So what that means is that when something attacks it, I'm not going to know exactly where it's coming from. Is it coming from the front, back? We don't really know, so you're actually just going to get a mixture. So, the product here is going to be alkyl halides. So what that means is I'm going to go from double bonds to alkyl halides using hydrohalogenation.

Now let's talk about rearrangements. This is basically a checkbox. Could we expect carbocation rearrangements or not? The answer is that any time you have a carbocation intermediate in your mechanism, you always need to be thinking about rearrangements. So this is going to get a huge check mark because of the fact that, yeah, we have carbocation intermediate, so we would expect that you have to be able to shift if it wants to. Then finally, we've got what we call the regiochemistry. I'm just going to go ahead and define this word really quick. Regiochemistry. What regiochemical reaction has to do with is actually where the reactive site is. Where does the electrophile add? So, I'm just going to put where does electrophile add? That's the regiochemistry. The stereochemistry says what's the shape of the end product? Is it trans? Stuff like that. Regiochemistry says where does the electrophile add? Does it add to a specific atom? The answervs yes. The regiochemistry is going to be Markovnikov because Markovnikov's rule says that your carbocation is going to form in the most stable place and that your electrophile attacks your carbocation. So that's the regiochemistry. Let's go ahead and look at the general reaction. The general reaction is simply going to be this, a double bond reacts with HX, hydrogen halide. And what we're going to wind up getting is if it's asymmetrical, your halogen is going to attach to the more substituted side or the Markovnikov side. So that would be this right here. And then your hydrogen is going to attach to the less substituted side. So that would be this hydrogen right there.

Now you might be wondering what is the squiggly line if you've never seen that before. Maybe you have, maybe you haven't. The squiggly line just has to do with the unknown stereochemistry. Basically, all that's saying is that I don't know if the hydrogen is facing towards the front. I don't know if it's facing towards the back because when it attacked, it was trigonal planar. So I don't really know exactly where it is. Now notice that if that H is there, there's actually no chiral center on this molecule at all. So what that means is that this x that I just drew here, I could have drawn it on the front, I could have drawn it in the back, it doesn't really matter. All I'm trying to show you is that honestly, we're not going to know the stereochemistry of the end product. So there you go. Pretty easy. Let's go ahead and do an example of this. I want you guys to try it yourself and then I'll go ahead and give you guys the answer.

Provide the mechanism

All right, guys. So what kind of alkyl halide did you get at the end? What degree? Hopefully, your answer is tertiary. If you wrote a secondary alkyl halide, that means you forgot to do one very critical thing. Let's go ahead and draw the mechanism and then maybe it will become more clear how this is a tertiary alkyl halide at the end. My arrow comes from my double bond to my H. Then I dump my electrons onto the carbocation that looks like this. And my carbocation is going to form in the secondary position. And then just hanging out. So my question is what's the next step? The Br has to attack the carbocation, right? Wrong. It turns out that because there's a carbocation, we have to be aware of carbocation shifts. Remember that carbocations are going to shift anytime that they can move to a more stable position just by moving one atom over. So I've got 2 different directions. My carbocation could either shift to the right or to the left, but it's definitely going to want to shift to the left to go to the tertiary position. Now I do have a hydrogen attached to that carbon. This is going to mean a 1,2-hydride shift. So I'm going to go ahead and draw here a 1,2-H shift. This is going to give me a new carbocation that looks like this. Now that I have the more stable carbocation, now it's time to go ahead and attack it with my nucleophile. So I've got my Br⁻ attacking the positive, and my final product, like I said, should be a tertiary alkyl bromide. So that's what you should have gotten for your final product. If the carbocation shift confused you, that just means you have to brush up on shifts. You guys might be wondering why didn't I draw the H at the end product, the H that moved? As once again, H's are implied. I didn't draw any of the H's at all. I never drew the first H, which was the H that would have attached here. That's the one that came from the H there. I never drew the H that shifted because I can imply those. The only important thing is to draw the Br in the correct place. Hopefully, that makes sense to you guys. Let me know if you have questions. Let's move on to the next topic.

Here’s what students ask on this topic:

What is hydrohalogenation in organic chemistry?

Hydrohalogenation is a reaction in organic chemistry where a hydrogen halide (HX) is added to an alkene (a molecule with a carbon-carbon double bond). The result is the formation of an alkyl halide. The reaction proceeds through a carbocation intermediate, which is trigonal planar, leading to unknown stereochemistry. According to Markovnikov's rule, the halogen attaches to the more substituted carbon, while the hydrogen attaches to the less substituted carbon. Rearrangements may occur to form a more stable carbocation.

What is the role of Markovnikov's rule in hydrohalogenation?

Markovnikov's rule plays a crucial role in hydrohalogenation by dictating the regiochemistry of the reaction. According to this rule, during the addition of a hydrogen halide (HX) to an alkene, the hydrogen atom attaches to the carbon with the greater number of hydrogen atoms (the less substituted carbon), and the halogen attaches to the carbon with fewer hydrogen atoms (the more substituted carbon). This occurs because the carbocation intermediate forms at the more stable, more substituted carbon.

What is a carbocation intermediate, and why is it important in hydrohalogenation?

A carbocation intermediate is a positively charged carbon atom that forms during the hydrohalogenation reaction. It is important because it determines the reaction's pathway and the final product's regiochemistry. The carbocation is trigonal planar, allowing for attack from either the front or back, leading to unknown stereochemistry. The stability of the carbocation can also lead to rearrangements, where the structure shifts to form a more stable carbocation, affecting the final product.

Can rearrangements occur during hydrohalogenation, and why?

Yes, rearrangements can occur during hydrohalogenation due to the formation of a carbocation intermediate. Carbocations are unstable and will rearrange to form a more stable carbocation if possible. This can involve shifts of hydrogen atoms (hydride shifts) or alkyl groups (alkyl shifts) to adjacent carbons, resulting in a more stable intermediate and ultimately affecting the final product's structure.

What is the general reaction equation for hydrohalogenation?

The general reaction equation for hydrohalogenation is:

$C_2=C_2+H_X\to C_2{H}_5{X}_1$

In this reaction, an alkene (C₂=C₂) reacts with a hydrogen halide (HX) to form an alkyl halide (C₂H₅X). The halogen (X) attaches to the more substituted carbon, and the hydrogen (H) attaches to the less substituted carbon, following Markovnikov's rule.