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Alkane with isopropyl group

Naming conventions for alkane chains that have an isopropyl group branch. Created by Sal Khan.

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Video transcript

- [Instructor] Let's try to name this molecule right over here. And so the first thing we wanna do is identify the longest chain of carbons. So let's see, it could be one, two, three, four, five, six, seven, eight. Or, let's see, maybe it's one, two, three, four, five, six, seven, eight, nine, 10. Yes, indeed, that's the longest chain. And if you go one, two, three, four, five, six, seven, or one, two, three, four, five, six, seven. So those aren't the longest. So the longest one is this one. One, two, three, four, five, six, seven, eight, nine, 10, 10 carbons. And this is going to be an alkane, 'cause it's all single bonds. And an alkane that's a chain of 10 carbons, we would use a prefix deck for 10. So this is a decane. Let me write that right over here. This is a decane, 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, we're not talking about the backbone, this is an ethyl group. 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 one, two, three carbons. So you could think about, well, this has three carbons. Our prefix for there carbons is prop. So you could say it maybe this is a propyl group. This right over here. This right over here. You could say maybe this is a 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 to only one carbon, it would be a primary carbon. So you could call this, since we're attached to the secondary carbon right over here. This is sometimes called a sec, a sec-propyl group. A sec-propyl group. And it's also sometimes called iso-propyl, an isopropyl group. Isopropyl, and you'll actually see isopropyl a little bit more frequently. And these would both be referred to as common names, common names for this group. Now, if you wanted to name this systematically, then you would do it very similar to the way 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, is a chain of two carbons. And so you could call, so that the backbone right over here is ethyl, is, let me write this right over here, is ethyl, 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, we would start counting right where we're 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 an methyl group, or a methyl group, this 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 one methyl, one, one methyl, one methyl-ethyl, one methyl-ethyl is 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 will see this thing right over here just called isopropyl. And sometimes you would see it called sec-propyl or 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, your main, kind of this decane backbone. And you would count from the end that bumps into the most groups faster. So, for example, if you count from this end, this would be the one carbon, two carbon, three carbon, four carbon, five carbon. On the five carbon, we bump into two groups. If we started over here, this would be the one carbon, two carbon, three carbon, four carbon, five carbon. On the five carbon, we do bump into a group, but only one group. And we'd have to wait until the sixth carbon to bump into two groups. So we get to the two groups faster, we would start counting on this end. So this is the one carbon, two carbon, three carbon, four carbon, five carbon, six carbon, seven carbon, eight, nine 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. 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 methyl-ethyl. 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 'em it is decane. And so let's talk about the ethyls first. And since we had two ethyls we can say that this is diethyl. So let me write that down. So we can write diethyl, diethyl. And, of course, we need to specify where those two ethyls are. One is at the five carbon, one is at the sixth carbon. So we could call this five, five, six, diethyl. The five refers to this ethyl group. Six refers to this ethyl group. And now we could talk about the isopropyl group. The isopropyl group is also on the five carbon. So we could say five, six diethyl, five, five isopropyl. Isopropyl, isopropyl, decane. Or, if wanted to use sec-propyl instead of isopropyl, we could write sec-propyl here. Or if want to do the systematic naming we could call this five, comma, six, di, diethyl. Five, instead of writing isopropyl here we could write all of this, one methyl-ethyl here. So actually let me just copy and paste that. Let me just copy this one methyl-ethyl right over here. Copy and paste. Let me stick that there. One methyl ethyl. And, of course, the main backbone is decane. Decane, decane. So either one, 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 it group one methyl-ethyl.