Alkane Halogenation - Online Tutor, Practice Problems & Exam Prep

Alkane halogenation is a key method for functionalizing alkanes through radical halogenation, particularly effective when a molecule contains a single type of hydrogen. To convert an alkane to an alkyl halide, use Br₂ with heat or light. However, more complex multi-step syntheses may be required for specific placements of halogens. Additionally, understanding how to revert alkyl halides back to alkanes is crucial. Mastering these reactions enhances skills in retrosynthetic analysis and organic synthesis, emphasizing the importance of reagents and reaction mechanisms in achieving desired transformations.

Functionalizing Alkanes

So the synthetic cheat sheet focused on a few different important rules. Okay? And now what I want to do is focus on one of those in particular, and that's alkane halogenation. This was actually a pretty easy rule because we just remembered that this is the only way to functionalize alkanes through radical halogenation. That’s the only way. And I've already said in past lessons that radical chlorination is really only useful if you have a single type of what in your molecule? Do you remember? Hydrogen. Okay. So only if you have a single type of hydrogen will it be helpful. If you don’t, then it won’t be. Alright?

So I mean, I would love it if all of you guys got this as your retrosynthesis. This first question. So, how do you turn that alkane into this alkyl halide? Oh my God. It’s so easy. Like you could do this in your sleep. It’s just Br₂ overheat or light. So you’re thinking, man, that would be so awesome. But guess what? Your professor probably isn't going to give you that at least by itself. Okay? What you’re probably going to get is either this in a multistep retro synthesis or it’s just going to be harder than that. Okay? Because that’s too easy. That’s just way too straightforward. Okay?

So what you’re more likely to get is where you have to turn an alkane into an alkyl halide that you can’t immediately get in one step from halogenation. For example, we can easily put bromines here, but it’s not that easy to just stick a bromine on a random carbon like that or to stick a bromine on a random carbon like that. How do we do that? That's going to require multistep synthesis. That’s going to require us to kind of concentrate and figure out what reagents would it take to transform into that. Okay? So that’s the first thing.

The second thing is that you guys should also know how to take an alkyl halide like this and turn it back into an alkane. It’s important to know what reactions could go backward and take it back to the alkane. So we're going to be practicing all of that here.

Now what I want you guys to do in particular is focus on, I think, 2 of the easier ones. This is not the easiest exercise here. It's a little bit tricky. But I want you guys to focus on b and d and tell me what those reagents are because it's basically taking this alkane and putting an alkyl halide right there. So you're going to have to figure out in multiple steps, this is not just one step, how you can do that. The same thing or just putting it in a different place. Okay? So, I want you guys to try out both of these. I'll pause the video. I'll pause, you know. And you guys try your hardest to get those syntheses down and then obviously I’ll show you the answer. So let's go for it.

Predict the product:

• A. No Reaction
• B.
• C.
• D.

Propose a synthesis for following transformation:

• A.
• B.
• C.
• D.

Here’s what students ask on this topic:

What is alkane halogenation and how does it work?

Alkane halogenation is a chemical reaction where a halogen (like bromine or chlorine) is introduced into an alkane, resulting in the formation of an alkyl halide. This process typically involves radical halogenation, which requires the presence of heat or light to initiate the reaction. The general reaction can be represented as:

$R_H+X_2\to R_X+H_X$

where R-H is the alkane and X₂ is the halogen molecule. The reaction proceeds through a chain mechanism involving initiation, propagation, and termination steps.

Why is radical chlorination only useful if there is a single type of hydrogen in the molecule?

Radical chlorination is only useful if there is a single type of hydrogen in the molecule because chlorine radicals are not very selective. When multiple types of hydrogen atoms are present, the reaction can produce a mixture of products, making it difficult to isolate the desired alkyl halide. This lack of selectivity can lead to complications in purification and yield. Therefore, radical chlorination is most effective when the molecule has a single type of hydrogen, ensuring a more straightforward and predictable product.

How can you convert an alkane to an alkyl halide using Br₂?

To convert an alkane to an alkyl halide using Br₂, you need to perform a radical halogenation reaction. The general procedure involves exposing the alkane to bromine (Br₂) in the presence of heat or light (usually UV light). The reaction can be summarized as:

$R_H+{\mathrm{Br}}_2\to R_{\mathrm{Br}}+H_{\mathrm{Br}}$

where R-H is the alkane and Br₂ is bromine. The reaction proceeds through a radical mechanism, involving initiation, propagation, and termination steps.

What are the steps to revert an alkyl halide back to an alkane?

To revert an alkyl halide back to an alkane, you can use a reduction reaction. One common method is to use zinc (Zn) in the presence of hydrochloric acid (HCl). The general reaction can be represented as:

$R_X+{\mathrm{Zn}}_{+}$

where R-X is the alkyl halide. This reaction effectively replaces the halogen atom with a hydrogen atom, converting the alkyl halide back to the original alkane.

What are the challenges in placing a halogen on a specific carbon in an alkane?

Placing a halogen on a specific carbon in an alkane can be challenging due to the lack of selectivity in radical halogenation reactions. The reaction tends to produce a mixture of products, especially when multiple types of hydrogen atoms are present. To achieve specific placement, a multi-step synthesis may be required. This involves using different reagents and reaction conditions to selectively functionalize the desired carbon. Understanding the reactivity and stability of intermediates is crucial for planning these multi-step syntheses effectively.