Double Elimination - Online Tutor, Practice Problems & Exam Prep

Alkynes can be synthesized through double dehydrohalogenation of dihalides using two equivalents of base. Dihalides can be classified as vicinal (1,2 relationship) or geminal (1,1 relationship). Vicinal dihalides have halogens on adjacent carbons, while geminal dihalides have both halogens on the same carbon. This method effectively forms triple bonds, expanding the toolkit for organic synthesis involving alkynes.

General features of double dehydrohalogenation.

Now I want to talk about a way that we can use elimination to make alkynes instead of alkenes. So actually making triple bonds out of elimination. And it turns out that all we have to do in this case is do a dehydrohalogenation twice. So what this is going to be called is double dehydrohalogenation. So let's go ahead and figure out how this works. Basically, when we react either when we react to some kind of dihalide with 2 equivalents of base, it's going to be able to react twice making an alkyne instead of an alkene. Okay?

Now, there are actually 2 different ways that we can make dihalides and we haven't talked a whole lot about dihalides yet, so I'm going to introduce them here. There are actually 2 very common types of dihalides. There are vicinal dihalides and there are geminal dihalides. Now these words vicinal and geminal aren't unique to dihalides. They are actually just position words that we use in organic chemistry just like we use the term terminal and internal, stuff like that. These are words that just identify the locations of substituents. So vicinal, you may have already heard it, but vicinal just means that they're next to each other. So a vicinal is basically a 1, 2 relationship. Okay? And the way that I like to remember it, if you are struggling to remember what vicinal might mean, there's this word called "Oh, you're in the vicinity." Okay? Vicinity means that you're close by. And basically that's what vicinal is: that they're close to each other, they're not exactly in the same place, but they're in the vicinity of each other. They're in a one-two relationship with each other. Alright? Then we have another word, geminal. Geminal is another position word. In this case, it means that you have a one-one relationship. Okay? So that means that you're literally on the same carbon. So, Geminal, the way I like to think of it is that Geminal sounds like Gemini and Gemini means twins. You can think that these both things, whether it's geminal diols, geminal halogens, are both coming off the same carbon. Okay? Regardless, both of these different types of dihalides, whether you're talking about that they're next to each other or whether you're talking about that they're on the same carbon, both of them are going to be open to this type of attack: a type of attack called double dehydrohalogenation.

The double dehydrohalogenation mechanism.

Go ahead and see how that works. Basically, what we would do is I'm going to do the first one as a worked example and then you guys will do the second one on your own. So, first of all, let's go ahead and identify if we have a dihalide. Yes. What type is it? This would be a vicinal dihalide. Cool. So I have a vicinal dihalide and now I have 2 equivalents of base. Okay? Now that I have 2 equivalents of base, what that means is that this is going to favor the formation of an alkyne, not just to do whatever other types of reactions. This is actually going to favor doing a double dehydrohalogenation. So what would the mechanism look like? Well, it would just basically be an E2 mechanism twice.

So let's go ahead and identify our β-hydrogens. If this is my chlorine, then the β-hydrogen I would use for this one would be the H down here. Because I need the hydrogen that's β to the α carbon, and the α would be right there. This would be my β. For the red chlorine, if I want to eliminate this one, I'm going to have to use a hydrogen that's also β to it. So this would be the α for that chlorine. This would be the β. So then I would use a hydrogen over here. Okay? And that's basically the way it would work. So, you would start off with your first equivalent. Okay? And we would go ahead and eliminate this blue one first, make a double bond and kick out the Cl. Okay?

So now what that's going to make as a middle step is that's going to make a molecule that looks like this. Okay? So now I have a double bond and I have a chlorine present. Now, just so you know, this is the product that you get after one equivalent of base. Okay? And if you were asked to provide the products for just one equivalent, this could be one of the products. Okay? So just so you know, this would be like halfway. But we know that we can perform another elimination now. Why? Because remember that this red hydrogen is still over here and this chlorine is still here. So we still have a β-hydrogen and a chlorine that can be eliminated.

So now this is where my second equivalent comes in. I would use my second equivalent of base and I would do this again. Make a triple bond and kick out the Cl. What this is now going to create is simply a cyclopentane with a triple bond coming off of it and that's it. And then plus, we would have my 2 equivalents of ethanol and we would have my 2 leaving groups. Okay? So that would be what we would get there.

Now, some of you guys might be wondering, does this triple bond have a hydrogen on it? Yes, it does. There's a hydrogen right there. I'm writing it in black. That's my black hydrogen. Where did that hydrogen come from? Well, it came from the fact that there was always one more hydrogen here; I just didn't draw it because it wasn't part of my reaction and it was here too. So, the entire time that hydrogen was there, it just w

Supply the mechanism and product for the following reaction.

In this case, we started off with what we would call a gem dihalide or geminal dihalide. Because as you'll notice both of my Bromines are coming off the same carbon. Now in order to conclude that we could do a double dehydrohalogenation on this, we just have to make sure that one of the beta carbons has at least 2 hydrogens on it that can react twice. And it turns out that this one is my beta carbon to both of them and it has 2 H's. Okay? Well, what would have only what would have happened if I only had 1 H instead of 2? Well, if you only had 1 H, then you could only eliminate once. Okay? But in this case, since I have 2, I will be able to do this double dehydrohalogenation. So let's go ahead and get started. LDA, as we know, is a very strong bulky base. It's going to pull off these hydrogens. What LDA actually looks like in case you want to actually draw it out is like that with a lithium. Okay? So we could just go ahead and use that version of LDA to pull off these hydrogens. I wo

Here’s what students ask on this topic:

What is double dehydrohalogenation in organic chemistry?

Double dehydrohalogenation is a chemical reaction used to synthesize alkynes from dihalides. This process involves the removal of two equivalents of hydrogen halide (HX) from a dihalide compound using a strong base. The dihalides can be either vicinal (halogens on adjacent carbons) or geminal (halogens on the same carbon). The reaction proceeds in two steps, each involving the elimination of one HX molecule, ultimately forming a triple bond (alkyne). This method is particularly useful for creating alkynes, expanding the synthetic toolkit in organic chemistry.

How do vicinal and geminal dihalides differ in double dehydrohalogenation?

Vicinal and geminal dihalides differ in the positions of their halogen atoms. In vicinal dihalides, the halogens are on adjacent carbons (1,2-relationship), while in geminal dihalides, both halogens are on the same carbon (1,1-relationship). During double dehydrohalogenation, both types of dihalides can undergo elimination reactions to form alkynes. The process involves two steps of dehydrohalogenation, where two equivalents of a base remove two hydrogen halide (HX) molecules, resulting in the formation of a triple bond (alkyne).

What bases are commonly used in double dehydrohalogenation reactions?

Common bases used in double dehydrohalogenation reactions include strong bases such as sodium amide (NaNH₂), potassium tert-butoxide (KOtBu), and sodium hydride (NaH). These bases are effective in abstracting protons and facilitating the elimination of hydrogen halide (HX) from dihalides. The choice of base can depend on the specific dihalide and reaction conditions, but these strong bases are generally preferred due to their ability to drive the elimination reactions to completion, forming alkynes.

Can you explain the mechanism of double dehydrohalogenation?

The mechanism of double dehydrohalogenation involves two consecutive elimination (E2) reactions. In the first step, a strong base abstracts a proton adjacent to a halogen atom, leading to the formation of a double bond and the elimination of a hydrogen halide (HX). This results in an alkene intermediate. In the second step, another equivalent of the base abstracts a proton adjacent to the remaining halogen, leading to the formation of a triple bond (alkyne) and the elimination of another HX molecule. This two-step process effectively converts dihalides into alkynes.

What are some applications of alkynes synthesized through double dehydrohalogenation?

Alkynes synthesized through double dehydrohalogenation have various applications in organic synthesis and industrial chemistry. They serve as key intermediates in the synthesis of pharmaceuticals, agrochemicals, and natural products. Alkynes are also used in the production of polymers, such as polyacetylene, and in the development of materials with unique electronic properties. Additionally, alkynes can undergo various chemical transformations, such as hydrogenation, halogenation, and cycloaddition reactions, making them versatile building blocks in synthetic chemistry.