Naming Alkyl Halides - Online Tutor, Practice Problems & Exam Prep

Alkyl halides are named by adding halogen prefixes—fluoro, chloro, bromo, and iodo—before the root chain, indicating their location as substituents. They hold no priority in chain numbering; the closest substituent dictates the starting point. Understanding these naming conventions is crucial for identifying and working with functional groups in organic chemistry, as they play a significant role in reactions involving nucleophiles and electrophiles, influencing regioselectivity and stereochemistry.

How to name alkyl halides

So I've got some good news. We're all done with alkanes. The bad news is that now we have to add some functional groups to the mix. Alright? So the first and easiest one we're going to start off with is alkyl halides. Alright? So alkyl halides are simply named by adding them as a substituent before the root chain indicating their location. Okay? So what that means is that remember that at the beginning of this lesson we talked about substituents and modifiers? Alkyl halides don't have modifiers. They're just called substituents just like an alkane would be, like an alkyl or whatever. Okay?

So one thing we should know is our prefixes for these different halogens. So fluorine would get the prefix fluoro. Okay. Chlorine gets the prefix chloro. Bromine gets the prefix bromo. And then iodine gets the prefix iodo. Okay. And these are going to be the prefixes that we use to assign the name to the alkyl halide. Okay.

The next thing that you should know is that alkyl halides have no priority. Zero priority when it comes to numbering the direction of the chain. So what that means is a lot of people think oh, I've got this long chain and this side has a chlorine on it, so that must be the side that I start from. No. Not at all. Use the same rules that we did before. Just you look at the closest substituent. It doesn't matter what it is. It could be a chlorine, it could be a methyl, it doesn't matter. You still give your lowest your you still make the chain start from the side that has the closest substituent.

So what I want to do is apply this to this alkyl halide. Go ahead and try to solve it out on your own first, draw it out and then when you're done, you have your answer figured out, then go ahead and go to the next video.

Name the following alkyl halide

Alright guys, so this one was actually similar to one that we did earlier where it kind of looked like a tert-butyl, but it wasn't. My longest chain would look like that. So that would be, it looks like 6 carbons. 1, 2, 3, 4, 5, 6. So my root is going to be hexane. Alright, now I have to decide which one gets the number 1. Is it red or is it blue? And for this, I need to go ahead and see which one has the closest substituent. Red has a substituent on 2. Blue has a substituent on 2. So far, there's a tie. So then, I go to the next substituent. Red has its next substituent on 5. Blue keeps its substituent on 2 because there were 2 methyls in that location. It turns out that even though the iodine is all the way over here, my number 1 is still going to be the blue. Okay. So what that means is that now I look at substituents. So for substituents, what do I got? I've got 2,2-dimethyl and it looks like I also have a 5-iodo. Now all I have to do is piece that together. In terms of alphabetical order, which one would come first? Remember, di doesn't count, so I look at m versus I and I comes in the alphabet first, so it would be 5-iodo-2,2-dimethylhexane. Not so bad, right? Now, hopefully, if you have questions, let me know. But if not, move to the next question. Now I'm just going to let you guys know, the hardest part about the next question is actually just figuring out which chain you're going to use. So I want you guys to pay extra careful attention to that because this one's a little tricky. So go ahead and get started.

Name the following alkyl halide

Like I said, the hardest part of this one was just figuring out which chain we're going to use as our root chain. So let's go ahead and look at the different possibilities. I could have used this blue chain right here. That would have been one option. If I used that blue one, how many substituents would I get? Two. So I'm just going to say blue equals 2. I could have also used this red one down here. Keep in mind that the red one down there is the same thing as if I would have made it go down this way, like here, because both roads lead to the same place. How many substitutions would that have had? That would have also had two. Lastly, we could have also done this. How many substituents would that have had if I made that my longest chain? That would actually have three. So, it looks like green is going to win, and that is going to be our longest chain. Alright. If you didn't get that as your longest chain, you already got the question wrong. That's why I told you guys to be really careful about which one you pick as your longest chain.

Right. Now we have to figure out which side is going to get number 1 and which side is going to get, I guess, number 5. So is it going to be number 1 here in red or number 1 here in blue? The first thing I do is look at the closest substituents. So blue has a substituent at position 2 and red also has a substituent at position 2. Does bromine beat methyl? No. I'm not even thinking about that yet. I'm just thinking close to substituents, there's a tie. What do I do in the event of a tie? I go to the next substituent. So now I'm going to go to 3, and 3 has a substituent there for blue, but check it out, 3 also has a substituent on this side for red. So we've got that rare situation where there's a perfect tie because the substituents are symmetrical.

Now what I need to do is use the rule to determine the direction, and that's the last rule, which was alphabetical order. Remember this was actually, I think, rule number 2. The last one said if you can't decide then use alphabetical order. So now what I'm going to do is I'm going to erase all this stuff that has to do with closest substituents and I'm just going to say which one is going to win in terms of alphabetical order, bromine or methyl? So now you guys realize that it's going to be bromine.

So that means that my root chain or my parent is going to be pentane. And what are my substituents? Well, my substituents are going to be I'm going to have a 2-bromo, I'm going to have a 3-ethyl, and I'm going to have 4-methyl. Notice that I couldn't use di, tri, tetra because all of these are different, so I can't use those. Now all I have to do is put these in the right order in a big name and I am done. And that's actually just going to be in alphabetical order, which it already is in. So it's going to be 2-bromo-3-ethyl-4-methylpentane. Tricky, right? So I hope that you guys got that. But if you didn't get that, just consider it a learning experience. You need to be very concerned with which one is your longest chain, which direction are you using before you can start actually putting the name together. Alright. So let's go ahead and move on.

Here’s what students ask on this topic:

What are the prefixes used for naming alkyl halides?

In naming alkyl halides, specific prefixes are used to denote the type of halogen present. The prefixes are as follows: fluorine is denoted by 'fluoro', chlorine by 'chloro', bromine by 'bromo', and iodine by 'iodo'. These prefixes are added before the root name of the hydrocarbon chain to indicate the presence and position of the halogen substituent. For example, if a chlorine atom is attached to the second carbon of a butane chain, the compound would be named 2-chlorobutane.

How do you determine the numbering of the carbon chain in alkyl halides?

When numbering the carbon chain in alkyl halides, the same rules used for alkanes apply. The chain is numbered from the end nearest to the first substituent, regardless of whether it is a halogen or another group like a methyl. Alkyl halides do not have priority in determining the direction of numbering. For example, in 2-chloropentane, the numbering starts from the end of the chain closest to the chlorine atom, ensuring the substituent gets the lowest possible number.

What is the significance of alkyl halides in organic chemistry reactions?

Alkyl halides are significant in organic chemistry because they are key intermediates in many reactions. They are involved in nucleophilic substitution and elimination reactions, which are fundamental in the synthesis of various organic compounds. The presence of a halogen atom makes the carbon-halogen bond polar, making the carbon atom electrophilic and susceptible to attack by nucleophiles. This property is crucial in determining the regioselectivity and stereochemistry of the reactions they undergo.

Can you provide an example of naming an alkyl halide?

Sure! Let's name the compound with the structure CH₃-CH₂-CH(Cl)-CH₃. First, identify the longest carbon chain, which in this case is butane. Next, identify the substituent, which is a chlorine atom. The chlorine is attached to the third carbon of the butane chain. Therefore, the compound is named 3-chlorobutane.

Why do alkyl halides not have priority in chain numbering?

Alkyl halides do not have priority in chain numbering because the numbering rules prioritize the position of the first substituent encountered, regardless of its type. This ensures consistency and simplicity in naming organic compounds. The goal is to give the substituents the lowest possible numbers, which helps in clearly identifying the structure of the compound. This rule applies to all substituents, not just halogens, ensuring a standardized approach to naming.