Alkane with isopropyl group | Organic chemistry | Khan Academy

Let's try to name this molecule right over here. The first thing we want to 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, ten. Yes, indeed, that's the longest chain!

If you go one, two, three, four, five, six, seven, or one, two, three, four, five, six, seven, those aren't the longest. So the longest one is this one: one, two, three, four, five, six, seven, eight, nine, ten. Ten carbons! This is going to be an alkane because it's all single bonds.

An alkane that's a chain of ten carbons, we would use the prefix "dec" for ten. 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.

We have this group right over here. This has two carbons in it: one carbon, two carbons. Because it has two carbons, we would use the prefix "eth." Remember, "meth" is one carbon; "eth" is two carbons. 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. Now, this group right over here is interesting. We can count the carbons in it. So it has one, two, three carbons. You could think, 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, 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.

A propyl group 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. 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 second, 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 isopropyl.

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 similarly 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. So you could call the backbone right over here: it's ethyl. Let me write this right over here: ethyl. It's an ethyl backbone here.

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. 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, 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 propyl group as small as a propyl group, but you could call this 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.

Sometimes you would call it 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. The way that this is done is that you would start counting from the end of your carbon chain, your main decane backbone.

You would count from the end that bumps into the most groups faster. 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 till the six 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 ten carbon.

When we think about which groups we are 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 clearly start with an "e."

They clearly start with an "e," and "e" comes before the "i" in isopropyl and before the "p" in sec propyl. You would normally ignore the "sec" or the "ter" 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. Remember our backbone: ten carbons, all single bonds. It's an alkane since there's 10 of them; it is decane. So let's talk about the first, and since we have two ethyls, we can say that this is diethyl.

So let me write that down: diethyl. D-ethyl! And of course, we need to specify where those two ethyls are. One is at the five carbon; one is at the six carbon. So we could call this 5,5-diethyl. That refers to the five, refers to this ethyl group; six refers to this ethyl group.

Now, we could talk about the isopropyl group. The isopropyl group is also on the five carbon, so we could say 5,6-diethyl, 5,5-isopropyl. Isopropyl! 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-diaryl. 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 that group one-methyl-ethyl.