The general addition mechanism looks pretty straightforward, but what if the π-bond is asymmetrical? How can we tell which side the –X will add to?
Concept: How to add to asymmetrical double bonds.4m
So in the previous video, I gave you guys two examples of alkenes that were perfectly symmetrical that would attack electrophiles, but it turns out that it's usually not that easy. Most of the time our double bonds are actually going to be asymmetrical meaning that they're going to have different things on different sides.
In that case, how do we decide where the carbocation goes and where the H goes or wherever the electrophile goes? For this, we're going to have to use the prevailing rule of this whole section and that's called Markovnikov's rule. So let's go ahead and check it out.
So Markovnikov's rule is basically going to be the rule that we use to figure out addition for asymmetric double bonds, which, like I said, is actually the majority of the double bonds that we're going to deal with.
Whenever you do have asymmetric double bonds, you're going to get the possibility of forming two different products because now you can add to one side or you could add to the other. Well, what Markovnikov's rule states is that it's going to predict that the carbocation is always going to form on the most stable carbon of the two. Now this is actually the definition of Markovnikov's rule. It states that the intermediate is always going to form in the most stable location.
You might have heard your professor say it that the – there's other ways to describe it. For example, some professors say that Markovnikov's rule is that the H goes where the other H's are. I don't know why professors keep saying that because that's not the real definition of Markovnikov's rule. Yes, that is a shortcut, but it's not always true. And I would rather you just memorize the true definition which is that the carbocation is just going to go in the most stable location possible.
So let's go ahead and check out this asymmetric double bond and see what our options are. Once again I'm going to take the electrons from my double bond. I'm going to attack the H and I'm going to kick out the X. This is going to leave me with two distinct possibilities. I have the possibility first of all that the H forms towards the top. If the H goes on the top, that means that my carbocation is at the bottom.
But there's also the possibility, since this is asymmetrical, notice that my double bond has completely different things at the top and the bottom, that I had the H at the bottom and my carbocation at the top. That's another possibility of this addition reaction.
So which of these two is going to be the more stable carbocation? If you guys remember, carbocations are stabilized by R groups. So the more R groups that I can pile on that carbocation, the better. So one of these actually is going to be much, much more stable and that's going to be the blue one.
So it turns out the red one is so unstable, that it would never even form. That's Markovnikov's rule for you. Markovnikov's rule is saying that you're only going to get the blue one or the one that has the most stable carbocation. Notice that this was tertiary while this one was primary. There's a huge difference there. Tertiary carbocations are much more stable than primary carbocations.
So now that I know which carbocation is going to form, now I just need to draw my final product. The X- is now going to attack there. What that means is that my final product is going to just be a molecule that looks like this. Now notice that I didn't draw the H because I don't have to. H's are always implied unless they're attached to heteroatoms. So what that means is that this is my final product. I'm not going to get a mixture of products. I'm only going to get 100% of the Markovnikov product. Does that make sense? Cool.
Markovnikov’s Rule predicts that the carbocation will ALWAYS form on the more stable carbon, or the carbon that can eventually become more stable.
Note: Some professors teach that the “H will go to the side with the most H’s”. While this is usually true, I believe this misses the greater concept behind Markovnikov’s Rule, so I stay away from this explanation.
Example: Provide the mechanism for the following addition reaction.2m