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Naming alkyl halides. Created by Sal Khan.
Video transcript
There are other things, other than carbons, that might branch off of the main chain. And in this video, I want to introduce you to the alkyl halides. And this is a situation where one of the halogen elements, and the halogen elements are the second to last, or the second to the right, or to the right most column in the periodic table. Fluorine, chlorine, bromine, iodine. It's when one of these replace one of the hydrogens attached to a carbon. So I've drawn some alkyl halides here. And let's think a little bit of how to name them. So like we name all of our organic molecules so far, you look for the longest chain first. And the longest chain that contains a double bond if there is one. In this case, there is no double bond, so the longest chain looks like this. We start here and we go over there. Or we could go either way, but this is the longest chain. So it's one, two, three, four, five, six, seven, eight carbons. Let me make sure of that. One, two, three, four, five, six, seven, eight carbons. And we want to start at the end where we're going to run into either some type of halide first or some other type of branch off of it. Just something attached to the carbon change other than a hydrogen. So we're going to start at this end. So it's one, two, three, four, five, six, seven, eight. So we're dealing with an octane. Our main chain has eight carbons, so we're dealing with an octane. And now we have to think about these different halogens that we have here. So first we have a chlorine at carbon number two. So that's going to be 2-chloro, is what we're going to call that. And we're going to put them in alphabetical order and then put them in front of the octane. And then this right here is a-- it's on the fourth carbon-- so it is a 4-bromo. The actual element is called bromine. We change the ine to an oro, so this is bromo. The ine to an o, I should say. So this is bromo. And B comes before C, but we're not done yet, because we don't have just halides here. We don't just have these halogens over here. We also have a methyl group. We have one carbon sitting over here. So we don't want to forget that guy. So that right there, that is a 5-methyl. So when we write it all out, we do it in alphabetical order. B takes precedence. So this will be 4-bromo-- I'll do it over here-- 4-bromo. Then the next thing we have is C. So 4-bromo-2-chloro -5-methyloctane. Now if we take look at this structure over here, our main carbon branch, we only have two carbons in it right here. So we're dealing with eth or ethane. It's a single bond. So this tells us that we're dealing with an ethane. And let's see what we have attached to it. We have a bunch of these halogens attached to it. We replaced a bunch of our hydrogens with halogens. So we have these fluorines here. And so we can start at the end that has more things attached to it. So in this case, we have one, two, three fluorines attached. So this is going to start off with-- and then fluorines, well I'll just leave it like that for now. I was going to write it, but then I have to remember, you're going to have to alphabetize it. So let me just write out. So at some point in our name, we're going to have attached-- we're going to start here-- that's going to be the one carbon. That is the one carbon. That is the two carbon. So at one point in our chain-- let me switch colors actually-- we're going to have one, two, three flourines. So it's going to be a trifluoro. And they're all at the one carbon. So it's going to be a 1,1,1-trifluoro. That'll take care of those three fluorines there. And then we're going to have, at some point in our name, this bromo is attached to the two carbon, so 2-bromo. And then you also have this chloro attached to the two carbon. So you're also going to have 2,2-chloro. So then you go in alphabetical order. B takes precedence. And then you're going to go to C, and then the trifluoro. You ignore the tri. But F still comes after B and C. So this is going to be 2-bromo 2-chloro-- I want to do that same shade of green-- 2-chloro, same idea. And then we have 1,1,1-trifluoroethane. Now I want to make one thing clear, when you do this. Double bonds, we saw on the last video-- we learned a little bit about alkenes-- still take precedence over everything. So if we go to this original compound-- this original molecule up here-- if I were to throw a double bond here. So let's say I threw a double bond. Then all of a sudden that'll change the naming mechanism. First of all, we're not doing with an octane anymore. We're dealing with an octene. So in this situation, the chain-- we'll start at the end closest to the double bond-- there's one, two, three, four, five, six, seven, eight. So now this becomes 7-chloro. This will be 5-bromo. This is 4-methyl. And this compound over here, it will no longer be this thing over here, because I've added this double bond. It will now be going in alphabetical order. It would be 5-bromo. Let me have some space here. It will be 5-bromo-7-chloro-4-methyl. And then we have oct. But it's an alkene, so it's oct. We have to specify where we start our double bond, oct-2-ene. Now if we had two double bonds. Let's say we had a situation where it was, maybe we had, let's see we don't want to put too many bonds here. Carbon will only have four bonds. This carbon has three. This hydrogen, we could have double bond over here. In that situation, we're going to have-- and I actually didn't specify it correctly in the last video, so this is a bit of a correction-- instead of being 2-ene, this will now be-- well let me make it instead of being 2-ene when I add that second carbon there, or that second double bond, it will now be-- scroll over to the right a little bit-- it will now be oct dash. We have a double bond at the two. Starting at the two and at the four. 2 comma 4. And it's no longer just an ene. That was a mistake I did in the last video. I said hept, and I just put an ene. Now that I have two double bonds, it's a diene. So you don't want to forget that. That tells us that we have two double bonds here. Although it's kind of obvious, because I'm listing a 2 and a 4. So it's a little bit redundant. 2 and 4 diene. So if you have it with these two double bonds, we have 5-bromo-7-chloro -4-methyl-oct-2,4-diene. And the whole reason why I introduced this is really to make a bit of a correction. Well to show you two things. One, that the double bond takes precedence when you are deciding which direction to number in. And then also to correct a little bit of a mistake in the last video where I forgot to put the di when I had two double bonds.