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Naming conventions for alkane chains that have an ethyl group branch. Created by Sal Khan.
Video transcript
I think we're ready now to tackle some more, or even more complicated examples. So let's draw something crazy here. Let me draw a chain. Let me draw it like that. And so like we've done in all of the examples, you'll want to find the longest chain. We can count from here one, two, three, four, five, six, seven, eight. Or maybe it's one, two, three, four, five, six, seven. No. Or maybe it's 1, 2, 3, 4, 5, 6, 7, 8, 9, 10. That is our longest chain. Let me just make that in green. So our longest chain here is in green. So this backbone has 10 carbons in it. The prefix for 10 is dec. It is a alkane since it has all single bonds. So we can write decane for the backbone. And then it has a group right here. And this group consists of one, two carbons attached to the backbone. The prefix for two carbons is eth, so this is an ethyl group. The y-l is because it's a group attached to the main alkane chain. So we call this ethyl decane. But we have to specify where the ethyl group is attached, and we want to give it as low of a number as possible so we start counting on the side closest to it. So it's one, two, three, four, five. So this is 5-ethyldecane. Now let's complicate this a little bit more. So let me just copy and paste this. So I have pasted it there, and let me complicate this molecule a little bit more. Let me add another ethyl group to it. So let's say we have another ethyl group over there. Now what is this going to be? Well the longest chain is still going to be that thing in green. So it's still going to be a decane. But now we have two ethyl groups. One on the five carbon, one, two, three, four, five, and then one on the six carbon. You might be tempted to write 5-ethyl 6-ethyldecane, which really wouldn't be wrong, but it would just be maybe more letters than you want to write. Instead you write 5 comma 6-diethyldecane. The 5, 6, tells us the two carbons on the main backbone that the ethyl groups are attached to, and the di says that we have two ethanes, or two ethyl groups I should say, not ethane groups, two ethyl groups, one over here, and one right over here.