Hydrohalogenation

All right, so this brings us to our first named reaction of the 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, you're 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 because whenever you have a carbocation in place and you're attacking it, remember that carbocations are trigonal planar so what that means is 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 actually you're 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 checkmark because of the fact that we have carbocation intermediate so we'd 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 regiochem 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 cis? Is it trans? Stuff like that.
Regiochem says where does the electrophile add? Does it add to a specific atom? And the answer is 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 an HX, hydrogen halide. And what we're going to wind up getting is that 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 attack to the less substituted side. So that would be this hydrogen right there.
Now you might be wondering what is the squiggly line. We've never seen that before. Maybe you have, maybe you haven't. Squiggly line just has to do with the unknown stereochem. 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.