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Organic chemistry naming examples 3, containing branching and cycles.  Created by Sal Khan.
Video transcript
We've got a few more molecular structures to name, so let's look at this first one right here. The first thing you always want to do is identify the longest chain. If we start over here, we have one, two, three, four, five, six, seven, eight, nine, ten, eleven, twelve, thirteen carbons, looks pretty long. Now what if we start over here? This looks like it could also be a long chain: one, two, three, four, five, six, seven, eight, nine, ten, eleven, twelve, thirteen. So we have two different chains, depending on whether we want to go up here or whether we go over here, that have a length of thirteen. So you're probably asking, which chain do you choose? You should always, if we have two chains of equal lengths and they're the longest chains, you pick the one that will have more branches or more alkyl groups on it. So this group right here, if we pick this from here to here as our chain, we only have one group on it, that group up there. If we pick this chain, starting over here and then going over here, we have two groups on it. We would have this group over here and then we would have this group over here. This is the better chain to use because it has more groups on it. It has more groups, but the groups are smaller and simpler. So let's start counting. And the direction we want to count, we always want to start on the side of the chain where we're going to encounter something first, and everything is closer this end of the chain so we'll start counting here. One, two, three, four, five, six, seven, eight, nine, ten, eleven, twelve, thirteen, so we have thirteen carbons on our main chain. Let me draw our main chain. So our main chain is this thing in orange I'm drawing right here. That is our main chain. Thirteen, that's three and ten. The prefix is tridec- and it's tridecane because we have all single bonds here, so it's tridecane. Tridecane is the main chain. Then we have two groups over here. This one in green, this only has one the carbon branching off of the main chain, so it's prefix will be meth- and it'll be a methyl group, so that is methyl. That is a methyl group right there. Then this one down here, we have one, two, three carbons. the prefix is prop-, so this is a propyl group. And the methyl is sitting on the three carbon of our main chain and the propyl group is sitting on the four carbon: one, two, three, four. Now when we figure out what order to list them in when we actually write out the name, M, we just do it in alphabetical order. M comes before P, so we write 3-methyl before 4-propyl. The entire compound here is or the entire molecule is 3-methyl-4-propyltridecane And this is actually all going to be one word. You use dashes to separate when you have numbers, but if you have a word followed by a word, it just becomes propyltridecane, so 3-methyl-4-propyltridecane. And we're done. Let's do this one down here. Now this one seems a little bit more complex. The first thing to see is what is the largest chain or the largest ring that we have in our structure? The two candidates, we have this chain over here. This has one, two, three, four, five, six, seven carbons. Let's see how many carbons our ring has. Our ring has one, two, three, four, five, six, seven, eight, nine carbons. So the ring is the largest core structure in this molecule, so that will be our core structure. We have nine carbons-- let me highlight it-- so the ring is this right here. It has nine carbons. The prefix for nine is non-. It's all single bonds, so it's nonane, and it is in a cycle. It's a ring, so it's cyclononane. This part right here, that part right there, is cyclononane. We have several things that branch off of the cyclononane, so let's look at them one at a time, and then we'll think about how we're going to number them on the ring. We looked already at this at this chain that has seven carbons: one, two, three, four, five, six, seven. So that is a heptyl group. This over here, let's see what we're dealing with. We have one, two, three carbons, so that is just a standard propyl group. Then here, we have one, two, three, four carbons, so we could say this is a butyl group. But this isn't just any butyl group. If we use the common naming, the carbon we immediately touch on or that we immediately get to when we go off of our main ring, that branches off into three other carbons, so this is tert-, tert- for three. The tert- is usually written in italics. It's hard to differentiate that when you see it. I'll write it in cursive. This is tert. This is a tertbutyl group. Now, the next question is how do we specify where these different groups sit on this main ring? If you just had one group, you wouldn't have to specify it, but when you have more than one, what you actually do is you figure out which one would be alphabetically first, and that would be number one. Now, we have an H in heptyl, a P in propyl, and tert-butyl, you might say, well, do I use the T or do I use the B? And this is just the convention, you use the B. If you have sec-butyl or tert-butyl, you ignore the sec- or the tert-. If this was an isobutyl or an isopropyl, you actually would use the I, so it's a little bit-- I guess the best way to think about it is there's a dash here so you can kind of ignore it, but if this was isobutyl it would all be one word, so you would consider the I. So in this situation, we would consider the B, and B comes before P or an H, so that is where we will start numbering one. Then to figure out which direction to keep numbering in, we just go in the direction where we're likely to encounter the first or where we will encounter the first side chain, so we'll go in this direction because we get right to the propyl group. One, two, three, four, five, six, seven, eight, nine. So this compound, we're going to start with the alphabetically first side chain, so it's 1-tert-butyl. I'll write this in cursive. 1-tert-butyl. Then the next one alphabetically is the heptyl group. That's H for heptyl, so then it is 5-heptyl. And then we have the propyl, and then it is 2-propyl, and then finally cyclononane, and we're done!