Alkyne Hydrohalogenation - Online Tutor, Practice Problems & Exam Prep

Alkynes undergo hydrohalogenation, reacting with halogens to form geminal dihalides through double addition. Initially, a Markovnikov halogen adds to the alkyne, creating a vinyl carbocation that cannot rearrange. A second equivalent of HX leads to further addition, resulting in a product with two halogens and two hydrogens on the same carbon. This process emphasizes the importance of understanding carbocation stability and regioselectivity in reactions involving alkynes, highlighting the unique behavior of triple bonds compared to double bonds.

General properties of double addition reactions to alkynes.

It turns out that alkynes can react with halogens in much the same way that double bonds can, except for the fact that double bonds have 1 pi bond and triple bonds have 2. So, what that means is that some reagents that we expose to an alkyne are actually going to react twice, and that's what we're going to study right here. There are 2 different reactions that alkynes will react twice with, and these are going to produce double addition products. That just means that anything I was expecting to get from my double bond, just double that, and that's going to be my expected product for a triple bond. Now keep in mind, if we have carbocations in any of these mechanisms, which we will, that includes vinyl carbocations. Now remember what the word vinyl means. It means something directly on a double bond. That means a carbocation that looks like this. Vinyl carbocations cannot easily rearrange. So, what that means is that you shouldn't be thinking about shifts in these mechanisms because they're really just not going to happen.

Double hydrohalogenation of alkynes.

So let's go ahead and look into the first one. And the first one is a hydrohalogenation of an alkyne. Now a hydrohalogenation, remember, is just an HX with a triple bond. Well, if I were to react that one time, I would expect to get a Markovnikov halogen added. But it turns out that if you react this twice with alkynes, what you're going to end up getting is actually geminal dihalides. Now remember that the word "gem" stands for geminal, and that means that they're both on the same carbon. So let's go ahead and see how this works.

If I were to react this with one equivalent, I would expect it to look like this, with a positive charge here and an H here. Okay? And then that positive charge would grab, or I'm sorry, the X would grab the positive charge and I would get my intermediate, my first product. Okay? But if I expose it to more than one equivalent, for example, let's say that I exposed it to 2 equivalents total, then it could react again. So then I would get this double bond attacking the H, kicking out the X. What I would end up getting is a carbocation that now looks like this, where my X is there, my H is there. And once again, I would get my X. And now, by the way, there are 2 H's there. And now my X would attack there again. So what I would end up getting is a product that looks like this where I have a 5-membered ring with 2 X's in the Markovnikov position. Okay. So it's the geminal dihalide part and it's the Markovnikov part.

And then I would have 2 H's that came from my addition reaction. Okay? I know you're wondering, well, what about that carbon? Shouldn't it have 3 H's? Shouldn't it be CH₃? Yeah. Well, that H was always there though. That kind of looks messy. I'm just going to redraw that. That H was always there because that's the H that was originally on that triple bond anyway. I'm just saying that through this reaction, we end up adding H twice and X twice. Does that make sense? I hope so. I hope that it flows. It's really not a complicated reaction as long as you know what hydrohalogenation is. Okay?

Here’s what students ask on this topic:

What is the mechanism of alkyne hydrohalogenation?

Alkyne hydrohalogenation involves the addition of hydrogen halides (HX) to a triple bond. Initially, the alkyne reacts with one equivalent of HX, leading to the formation of a vinyl carbocation. This carbocation is stabilized by the addition of the halogen (X) in a Markovnikov fashion, resulting in a haloalkene. If a second equivalent of HX is added, the reaction proceeds further, forming a geminal dihalide. The final product has two halogens and two hydrogens on the same carbon. The key steps are the formation of the vinyl carbocation and the regioselective addition of the halogen.

What are geminal dihalides in the context of alkyne hydrohalogenation?

In alkyne hydrohalogenation, geminal dihalides are the final products formed when two equivalents of a hydrogen halide (HX) react with an alkyne. The term 'geminal' means that both halogen atoms are attached to the same carbon atom. Initially, the alkyne reacts with one equivalent of HX to form a haloalkene. When a second equivalent of HX is added, the reaction proceeds to form a geminal dihalide, where the carbon that was part of the triple bond now has two halogens and two hydrogens attached to it.

Why can't vinyl carbocations rearrange during alkyne hydrohalogenation?

Vinyl carbocations, which are intermediates in alkyne hydrohalogenation, cannot rearrange because they are directly attached to a double bond. The structure of a vinyl carbocation is such that the positive charge is on a sp² hybridized carbon, making it less stable and less likely to undergo rearrangement compared to other carbocations. This lack of rearrangement is crucial for predicting the products of alkyne hydrohalogenation accurately.

How does Markovnikov's rule apply to alkyne hydrohalogenation?

Markovnikov's rule states that in the addition of HX to an alkyne, the hydrogen atom (H) will attach to the carbon with the greater number of hydrogen atoms, while the halogen (X) will attach to the carbon with fewer hydrogen atoms. This rule helps predict the regioselectivity of the reaction. In alkyne hydrohalogenation, the first equivalent of HX adds according to Markovnikov's rule, forming a haloalkene. The second equivalent of HX also follows this rule, leading to the formation of a geminal dihalide.

What is the difference between alkyne hydrohalogenation and alkene hydrohalogenation?

Both alkyne and alkene hydrohalogenation involve the addition of hydrogen halides (HX) to unsaturated carbon-carbon bonds. However, alkynes have a triple bond, while alkenes have a double bond. In alkyne hydrohalogenation, the reaction can proceed twice, leading to the formation of geminal dihalides, whereas in alkene hydrohalogenation, the reaction typically stops after the addition of one equivalent of HX, forming a haloalkane. Additionally, the intermediates in alkyne hydrohalogenation are vinyl carbocations, which do not rearrange, unlike the carbocations in alkene hydrohalogenation.