Current time:0:00Total duration:8:19
0 energy points
Naming conventions for alkane chains that have an isopropyl group branch. Created by Sal Khan.
Video transcript
Let's try to name this molecule right over here. And so the first thing we want to do is identify the longest chain of carbons. Let's see, it could be 1, 2, 3, 4, 5, 6, 7, 8. Or let's see, maybe it's 1, 2, 3, 4, 5, 6, 7, 8, 9, 10. Yes, indeed, that's the longest chain. And if you go 1, 2, 3, 4, 5, 6, 7, or 1, 2, 3, 4, 5, 6, 7, so those aren't the longest. So the longest one is this one-- 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10 carbons. And this is going to be an alkane, because it's all single bonds. And an alkane that's a chain of 10 carbons, we would use a prefix dec for 10, so this is a decane. Let me write that right over here. This is a decane. Now let's think about the groups that are attached to this decane. So we have this group right over here. This has two carbons in it, one carbon, two carbons. And so, because it has two carbons, we would use the prefix eth. Remember, meth is one carbon, eth is two carbons. And since it's a group and we're not talking about the backbone, this is an ethyl group. And we have another ethyl group right over here. Two carbons attached right over here. This is also an ethyl group. And now, this group right over here is interesting. We can count the carbons in it. So it has 1, 2, 3 carbons. So you could think about, well, this has three carbons. Our prefix for three carbons is prop. So you could say, hey, maybe this is a propyl group. This right over here, you could say maybe this is propyl group. And you wouldn't be completely off base by saying that. But we have to be a little bit more careful when we name it. Because a propyl group, you would assume that you're attaching to one end of the propyl group. But we're not attaching to one end of the propyl group. We're attaching essentially to the second carbon, to the middle carbon. And this is a secondary carbon. The reason why it's called a secondary carbon is because it's attached to two other carbons. If it was attached to three other carbons, it would be a tertiary carbon. If it was attached only one carbon, it would be a primary carbon. Since we're attached to the secondary carbon right over here, this is sometimes called a sec-propyl group. And it's also sometimes called isopropyl, an isopropyl group. And you'll actually see isopropyl a little bit more frequently. And these would both be referred to as common names for this group. Now, if you wanted to name this systematically, then you would do it very similar to the way that you would name the entire molecule. You would look for the longest chain here. And the longest chain in this molecule, starting with where you are attached, is a chain of two carbons. And so the backbone right over here is ethyl. Let me write this right over here. It's an ethyl backbone here. And then you could view this carbon as a group attached to that ethyl backbone. And we would start counting right where we are attached to the main chain. So this is the one carbon, this is the two carbon. So this right over here, this is just one carbon group. This right over here is a methyl group. So you have a methyl group attached to the one carbon of an ethyl group. So the systematic name for this, and this is a little bit less typical for a group as small as a propyl group, but you could call this 1-methylethyl. 1-methylethyl the systematic name for this. Now, the systematic name, you might say, hey, why go through the pain of doing this for something so simple that we could just call isopropyl? This is useful if this was a much larger or a much more complex group that was attached to this main chain. But more typically, and this is why it's called the common name, you'll see this thing right over here just called isopropyl. And sometimes you would see it called sec-propyl even s-propyl. Now that we've named all of the groups, let's think about what carbons they are attached to and where we can start counting from. And the way that this is done is that you would start counting from the end of your carbon chain, this decane backbone, and you'd count from the end that bumps into the most groups faster. So, for example, if you count from this end, this would be the 1 carbon, 2 carbon, 3 carbon, 4 carbon, 5 carbon. On the 4 carbon, we bump into two groups. If we started over here, this would be the 1 carbon, 2 carbon, 3 carbon, 4 carbon, 5 carbon. On the 5 carbon, we do bump into a group, but only one group. And we'd have to wait until the 6 carbon to bump into two groups. So we'd get to the two groups faster, we would start counting on this end. So this is the 1 carbon, 2 carbon, 3 carbon, 4 carbon, 5 carbon, 6 carbon, 7 carbon, 8, 9, and 10 carbon. And so when we think about which groups are we going to refer to first, do we refer to the ethyl groups first or do we refer to this isopropyl group first? We just think about what letter they start with in alphabetical order. So, for example, these ethyl groups, they clearly start with an e. E comes before in the alphabet than the i in isopropyl, before the p in sec-propyl-- you would normally ignore the sec or the tert when you're thinking about alphabetical order-- and e also comes before the m in methylethyl. So we will talk about the ethyl groups first. So remember, our backbone, 10 carbons, all single bonds. It's an alkane. Since there's 10 of them, it is decane. And so let's talk about the ethyls first. And since we have two ethyls, we can say that this is diethyl. So let me write that down. So we can write diethyl. And of course, we need to specify where those two ethyls are. One is at the 5 carbon, one is at the 6 carbon. So we could call this 5,6-diethyl. The 5 refers to this ethyl group, 6 refers to this ethyl group. And now we could talk about the isopropyl group. The isopropyl group is also on the 5 carbon. So we could say 5,6-diethyl-5-isopropyldecane. Or if we wanted to use sec-propyl instead of isopropyl, we could write sec-propyl here. Or if we wanted to do the systematic naming, we could call this 5,6-diethyl. Instead of writing isopropyl here, we could write all of this. 1-methylethyl here. So actually, let me just copy and paste that. Let me just copy this 1-methylethyl right over here. Copy and paste, let me stick that there. 1-methylethyl. And of course, the main backbone is decane. So all of these are reasonable ways to name it. This would be the common way, where we use isopropyl. Here we're doing it more systematically by calling the group 1-methylethyl.