Naming alkanes with ethyl groups | Organic chemistry | Khan Academy

I think we're ready now to tackle some more or even more complicated examples. So let's draw something crazy here. So let's see, let me draw a chain. Let me draw it like that, and so like we've done in all of the examples, you want to find the longest chain.

We could 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 one, two, three, four, five, six, seven, eight, nine, ten. That is our longest chain. Let me just let me make that in green. So our longest chain here is in green.

So this decade, this, sorry, this backbone has 10 carbons in it. The prefix 410 is dec. It is an alkane since it has all single bonds, so we can write decane for the backbone. And then it has a group right here. It has a group right here, and this group consists of one, two carbons attached to the backbone. The prefix for two carbons is f, so this is an ethyl group. The yl is because it's a group attached to the main alkane chain, so we call this ethyl decane.

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 five ethyl decane. Now let's complicate this a little bit more.

So let me just copy and paste this, copy, and then paste, so I have pasted it there. And let me complicate this molecule a little bit more. Let me add another ethyl group to it. 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, still, the longest chain is still going to be that thing in green, so it's still going to be a decane. 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.

So what we write here is you might be tempted to write five ethyl six ethyl decane, which really wouldn't be wrong, but it would just be maybe more letters than you want to write. Instead, you write five, comma, six diethyl decane. 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.