Hey guys. So now we're going to talk about a named reaction called dehydrohalogenation. I know the name sounds tricky, but actually, it turns out that you already know all the parts of this mechanism already. So it's actually pretty easy. Let's go ahead and check it out. As you can see, the name is pretty long, but all this really is is an E2 mechanism because if you think about the name, it's saying dehydro, we're taking away 1 hydrogen and we're taking away 1 halogen. Well, that's exactly what happens with a typical E2 mechanism. Remember that we always break those 2 sigma bonds and make a pi bond at the end. And that's exactly what we're going to do. So let's go ahead and check it out. Basically, you would have an alkyl halide and in this case, do we prefer that alkyl halide to be like primary or tertiary? What do you think is better? We just said this is an E2 reaction, so that means that we're going to prefer the more substituted alkyl halide that's going to favor elimination more. Okay? So that means that hopefully, we have like a secondary or tertiary alkyl halide and we're reacting that with some kind of base. Now notice here I'm just using the word base in general, but remember the type of base could lead to a different type of product. Okay? Because we had Zaitsev and we had Hoffman and the type of base that you use could prefer one product over another. Let's just go ahead and just draw the general E2 elimination product right now. I would take my base and where would those arrows go to? Do you remember? Remember that you'd always take off a Beta hydrogen. This is actually called Beta hydrogen elimination. So I'll take my minus, grab a Beta hydrogen. Now notice that the geometry of that beta hydrogen is in a special position and it's in the anticoplanar position. Remember that that's important because if you were to make a Newman projection out of this guy, you would want to make sure that your groups are facing opposite directions or in the anti position so that they can be in the most favorable orientation to eliminate. Okay? So I would take that, but remember that elimination always has 3 arrows. So I would take the electrons from here and make a double bond and finally I would kick out my X and what I'm going to get at the end is just a new double bond where basically these 2 methyl groups here are now located here and these 2 methyl groups here are now located there. Plus I would get, obviously, my base with the new hydrogen on it, so that would be the conjugate acid And I would also get the leaving group X-. Okay? So that was really easy. But now you guys just understand that that's the name associated with this type of reaction. Whenever we're using a strong base to eliminate an alkyl halide through an E2 mechanism, that's called dehydrohalogenation. And you have to think of all those things in terms of anticoplanar, in terms of Zaitsev and Hoffman, all of that is fair game. Okay? So now I have a practice problem for you guys. I want you guys to take your time trying to draw the products based on exactly what reagents you see and then I'll give you the answer. So anyway, go for it.
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- 1. A Review of General Chemistry5h 5m
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9. Alkenes and Alkynes
Dehydrohalogenation
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