Halogenation - Online Tutor, Practice Problems & Exam Prep

Halogenation is a key addition reaction involving the addition of diatomic halogens to double bonds, resulting in anti vicinal dihalides. The mechanism features a bridged ion, specifically a halonium ion, which leads to anti addition products due to the ring's strain. The reaction does not involve rearrangements or Markovnikov's rule since two identical halogens are added. Solvents like CCl₄ are inert, aiding the reaction without participating. The final products include enantiomers, produced in equal amounts, indicating a racemic mixture.

General properties of halogenation.

Now we're going to talk about one of the most common addition reactions in this entire section, and that's called halogenation. So let's just get right into it. Halogenation is the process of taking a double bond and adding a diatomic halogen, and at the end, what we're going to get is anti dihalides, specifically vicinal dihalides. Alright? So let's go ahead and just talk about the general features of this reaction.

First of all, the mechanism is going to have an intermediate, and that intermediate is going to be similar to others we've learned. It's going to be a bridged ion. Okay? So maybe you can already start to visualize what that might be. Sorry about that. The stereochemistry is going to be anti. As I just showed you, you're going to get anti dihalides, so it would be anti. My product is going to be, like I said, vicinal dihalides.

Now, just to remind you, the word vicinal is used to mean that there are two things next to each other. So this relationship here would be vicinal because they are right next to each other. Vicinal is also the same as saying 1, 2 — basically, something on the one position and something at the two position, and that would be vicinal.

So, will there be rearrangements in this mechanism? No, there won't be because there's no carbocation. And then finally, since I'm adding two of the same thing, I'm not going to worry about Markovnikov's rule because I'm adding two of the same things, so it doesn't matter. So let's just scratch that out.

As you can see, the reaction looks pretty simple. We've got a double bond, so we know this is an addition reaction. We've got an electrophile that we're adding. Now notice that I have CCl₄ down here. If you remember, this is actually going to be an apolar solvent. And apolar solvents are inert; they're not going to do anything, so don't even worry about it. That's just something that it happens to help the reaction, but it's not going to do anything. Another common solvent that you might see is like CH₂Cl₂. That just means it's the same thing, just instead of having four chlorines, you have two chlorines and two H's. Regardless, these are just solvents that don't do anything.

At the end, what we get is those vicinal dihalides. Let's go ahead and look at the reaction.

Halogenation Mechanism

So the mechanism works like this. Basically, you've got a double bond and that double bond is nucleophilic. It's looking for something to give its electrons to. And I know that halogens seem to have a lot of electrons, but they're also very what's called polarizable. So what that means is that electrons wind up moving a lot and wind up causing imbalances in the electron cloud. So at some point, there's going to be a part of the halogen that's not going to have too many electrons, and the double bond is going to pounce on that. So it's going to be a part of the halogen that's not to have too many electrons and the double bond is going to pounce on that. So what's going to happen is that the double bond is going to grab one of the halogen atoms. Now the halogen doesn't like to have that many bonds, so it's going to go ahead and break a bond. If you make a bond, you have to break a bond. But, similar to other bridged ions that we've looked at before, if we make that bond, then we have to make another bond back to the double bond. What that's going to show is it's going to make a bridge instead of just a single bond to one side. Okay. So those are our 3 arrows. What we're going to wind up getting is a bridged ion. In this case, this is called a halonium ion because instead of having a carbocation, I have a 3-membered ring and one of those atoms is a halogen. For example, if this was a bromine, I would call it a bromonium ion. Alright? Now this positive charge is going to be distributed throughout all three of those atoms. So I'm going to have a partial positive here, I'm going to have a partial positive there. Okay? But one of these atoms is going to have the most positive density. Can you guys guess which one? It's going to be the atom that is the best at stabilizing positive charges. And that one's going to be the most substituted side. So if they both had the same substitution, then this halogen negative could real

Here’s what students ask on this topic:

What is halogenation in organic chemistry?

Halogenation is an addition reaction where a diatomic halogen (such as Br₂ or Cl₂) is added to a double bond in an alkene. This process results in the formation of anti vicinal dihalides, meaning the two halogen atoms are added to adjacent carbon atoms in an anti configuration. The reaction mechanism involves the formation of a halonium ion intermediate, which leads to the anti addition of halogens due to the strain in the three-membered ring structure.

What is the mechanism of halogenation of alkenes?

The mechanism of halogenation of alkenes involves several steps. First, the nucleophilic double bond attacks a diatomic halogen, forming a halonium ion intermediate. This intermediate is a three-membered ring with a positive charge distributed over the halogen and the two carbon atoms. The halide ion then performs a backside attack (SN2 mechanism) on the more substituted carbon, leading to the anti addition of halogens. The final product is a vicinal dihalide with the halogens in an anti configuration.

What are vicinal dihalides?

Vicinal dihalides are organic compounds where two halogen atoms are attached to adjacent carbon atoms in a carbon chain. The term 'vicinal' means 'neighboring,' indicating that the halogens are on carbons next to each other. In the context of halogenation, the addition of diatomic halogens to a double bond results in the formation of vicinal dihalides with the halogens in an anti configuration.

Why does halogenation result in anti addition?

Halogenation results in anti addition due to the formation of a halonium ion intermediate. This intermediate is a three-membered ring with a positive charge, creating significant ring strain. When the halide ion attacks the intermediate, it does so from the opposite side (backside attack) to relieve this strain. This leads to the halogens being added to opposite sides of the double bond, resulting in anti addition.

What role do solvents like CCl₄ play in halogenation reactions?

Solvents like CCl₄ (carbon tetrachloride) are used in halogenation reactions as inert solvents. They do not participate in the reaction but provide a medium for the reactants to interact. CCl₄ is apolar and does not interfere with the reaction mechanism, ensuring that the halogenation proceeds smoothly to form the desired vicinal dihalides.