Markovnikov - Online Tutor, Practice Problems & Exam Prep

Markovnikov's Rule is essential for predicting the outcome of electrophilic additions to asymmetric alkenes. It states that the more stable carbocation intermediate will form during the reaction, leading to the major product. Tertiary carbocations, stabilized by more alkyl groups, are preferred over primary ones. This results in a single product, known as the Markovnikov product, rather than a mixture. Understanding this rule is crucial for mastering regioselectivity in organic reactions involving alkenes.

How to add to asymmetrical double bonds.

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? And 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 2 different products because now you could 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 2. 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 there are other ways to describe it. For example, some professors say that Markovnikov's rule is that the goes where the other are. I don't know why professors keep saying that because that's not the real definition of where a carbocation is ruled. 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 2 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 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 2 is going to be the more stable carbocation? And if you guys remember, carbocations are stabilized by R groups. So the more R groups that I can pile around that carbocation, the better. So one of these is actually 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. Hs 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.

Provide the mechanism

So let's go ahead and do an example. And for this one, once again, I want you guys to pause the video, try to draw the entire mechanism on your own, and then predict the correct product. So, go ahead and pause the video now. All right. I really hope you guys gave this a try because this is a very important mechanism that you need to know. So my double bond would grab the H. I would kick out the X. What I would wind up getting is well, where's the H going to attach to? Is it going to attach to the secondary spot, which is right here, or the tertiary spot, which is right there? I hope you said secondary. When I ask it like that, it's confusing. But the H would attach here, so that my carbocation can form there. The biggest difference is that I want my carbocation tertiary as much as possible, so I want it to go right there. Now that I have it there, my X- is free to attack, and my final product would just look like this. It would be a methyl group and an X, both coming off of that tertiary position. So I hope that made sense to you guys. This isn't really hard. It's just something that you need to always apply. Markovnikov's rule is very prevalent in this whole section, so it's very important that you guys understand what it is and has to do with the carbocation. So if you have any questions, let me know. But if not, let's move on to the next topic.

Here’s what students ask on this topic:

What is Markovnikov's Rule in organic chemistry?

Markovnikov's Rule is a principle used to predict the outcome of electrophilic addition reactions to asymmetric alkenes. It states that the more stable carbocation intermediate will form during the reaction, leading to the major product. This means that the hydrogen atom (H) will add to the carbon with the most hydrogen atoms already attached, while the electrophile (e.g., halogen) will add to the carbon with fewer hydrogen atoms. This results in the formation of a single product, known as the Markovnikov product, rather than a mixture of products.

Why is the stability of carbocations important in Markovnikov's Rule?

The stability of carbocations is crucial in Markovnikov's Rule because the rule predicts that the more stable carbocation intermediate will form during the reaction. Carbocations are stabilized by alkyl groups through hyperconjugation and inductive effects. Tertiary carbocations, which have three alkyl groups attached, are more stable than secondary or primary carbocations. This increased stability lowers the energy barrier for the reaction, making the formation of the more stable carbocation more favorable. As a result, the major product of the reaction is determined by the formation of the most stable carbocation intermediate.

How does Markovnikov's Rule apply to asymmetric alkenes?

Markovnikov's Rule is particularly useful for predicting the outcome of electrophilic addition reactions to asymmetric alkenes, which have different substituents on either side of the double bond. When an electrophile adds to an asymmetric alkene, there are two possible products depending on which carbon the electrophile attaches to. According to Markovnikov's Rule, the electrophile will add to the carbon that leads to the formation of the more stable carbocation intermediate. This ensures that the major product is the one where the carbocation is most stabilized by alkyl groups.

Can you provide an example of a reaction that follows Markovnikov's Rule?

Sure! Consider the addition of HBr to propene (CH₃-CH=CH₂). Propene is an asymmetric alkene. According to Markovnikov's Rule, the hydrogen (H) from HBr will add to the carbon with more hydrogen atoms (the terminal carbon), and the bromine (Br) will add to the carbon with fewer hydrogen atoms (the middle carbon). This results in the formation of 2-bromopropane (CH₃-CHBr-CH₃) as the major product. The reaction proceeds through the formation of a more stable secondary carbocation intermediate.

What is the difference between Markovnikov and anti-Markovnikov addition?

Markovnikov addition refers to the addition of an electrophile to an alkene where the hydrogen atom attaches to the carbon with more hydrogen atoms, leading to the formation of the more stable carbocation intermediate. Anti-Markovnikov addition, on the other hand, occurs when the hydrogen atom attaches to the carbon with fewer hydrogen atoms, often facilitated by radical mechanisms or specific reagents like peroxides. This results in the formation of the less stable carbocation intermediate. Anti-Markovnikov addition is less common and typically requires specific conditions to occur.