When two equivalents of halide are present, bases can eliminate twice, creating alkynes as products
Concept: General features of double dehydrohalogenation.3m
Now I'm going to talk about a way that we can use elimination to make alkynes instead of alkenes. So we're 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 some kind of dihalide with two equivalents of base, it's going to be able to react twice, making an alkyne instead of an alkene. Now there's actually two 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's actually two different very common types of dihalides. There's vicinal dihalides and there's geminal dihalides. Now these words vicinal and geminal aren't unique to dihalides, they're 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 vicinal is basically a one, two relationship. And the way that I like to remember it if you are struggling to remember what vicinal might mean, there's this word called – there's like, oh, you're in the vicinity. The vicinity means that you're close by and basically that's what vicinal is. They're close to each other. They're not exactly the same place, but they're in the vicinity of each other. They're in a one, two relationship with each other.
Then we have another word, geminal. Geminal is another position word. In this case, it means that you have a one, one relationship. 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. So you could think these both things whatever they are, whether it's geminal diols, geminal halogens, that they're both coming off of the same carbon.
Regardless, both of these different types of dihalides, whether you're talking abut 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 called double dehydrohalogenation.
There are two types of dihalides that can create alkynes. These are described by the position words vicinal and geminal.
Example: The double dehydrohalogenation mechanism.5m
Example: Supply the mechanism and product for the following reaction.3m