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The removal of a hydrogen atom from an alkane compound creates an alkyl group.
Concept #1: Classification of Alkyl Groups
Hey guys! In this new video, we're going to take a look at the concept of alkyl groups. We're going to say the removal of a hydrogen from an alkane compound creates what we call an alkyl group. Remember, these alkyl groups are the first step in learning how to name organic compounds. If we take a look at this first compound, CH4. Ch4 is an alkane. Remember, alkanes have the formula of CnH2n+2, where n is the number of carbons. It has one carbon so n is one. Then that'll be H 2 times 1 plus 2. That's how we got the formula CH4. This is our saturated hydrocarbon. Saturated meaning the carbon has the maximum amount of hydrogens possible.
What do I decide to do? I decided to remove a hydrogen from the methane. Here, I’ve removed it so now it's just a CH3. What does this line here mean? This line here means that this CH3 is connected to some other structure I don't know. Maybe it could be an OH. Maybe it could be a larger carbon chain. I have no idea, and it doesn't matter. What we need to realize here is I'm connecting this carbon to something different. Carbon can’t make five bonds. I couldn't have the carbon connect to that thing beforehand because carbon can only go up to four bonds that's why I needed to remove the H from it so that it could connect to some new thing.
Notice the difference between the two structures. Obviously, we go from CH4 which is methane to CH3 which is methyl. But what other thing you should notice here is at the end of the name changes when we become an alkyl group, if you take up a hydrogen from an end carbon, the ending changes to -yl. Here we’re going from -ane to -yl. When we make an alkyl group, the ending changes from -ane to -yl. Here methane is CH4, methyl is Ch3. Here we can say that CH2 just means we have a carbon connected to two hydrogens and then it's connected to two other things I don't know what they are. I don't really care.
Here I'm showing you these guys now because when you get to Organic, professors tend to gloss over them and never talk about what their names are. Here they always focus on what a methane is and what a methyl is, but they never explain what a CH2 is called or what a CH is called. CH is connected to three things we don't know what they are because remember carbon must make four bonds. We're going to say that a CH2 is called a methylene group. That's methylene. A CH is called methyne. Here, it's not too important in terms of Gen Chem but just realize later on when you get to Organic 1, you will have to know what the names of all four of these are. Professors tend to gloss over methylene and methyne a lot. You basically have to figure out on your own, that's what they're called. But I'm telling you that now. Make a mental note. Write a little note somewhere so that you can remember when you get Organic 1 that, “Hey, Jules told me that CH2 is methylene and CH2 is methyne.” If I'm reading and I see those names, I know what the heck I'm looking at.
If we take an H off of methane, it gives us methyl. Then what else could we take an H off, name as an alkyl group. We're going to say when you have a two-carbon chain, then the alkane name will be, let's see. Two carbons saturated. The prefix is eth- and it is an alkane, so the ending is -ane. Remember, we're taking off an H from one of the end carbons. When we do that, we create an alkyl group and the ending changes from -ane to -yl. It changes from -ane to -yl. The prefix name stays the same, so it's eth- and the ending changes to -yl so it becomes ethyl. This is ethyl. Again, we don't know what it's connected to. It can be connected to something. I don't know what it is but it's connected to it somehow. That's what an ethyl would look like. Later on, you'll take a look at it in terms of skeletal formulas. For right now we won't worry too much about that. You’ll tackle that more so when you get to Organic 1.
If we could take it off of a two-carbon chain, what could we do when we’re taking off a three-carbon chain? Here, three carbons is prop. It's an alkane so it's propane. Here, this is an end carbon, this is an end carbon. I could take an H from either end and remember when you're taking off an H from either end, it just changes the ending from -ane to -yl. This becomes propyl. But now we finally have a new situation where we don't only have end carbons, we also have a middle carbon here which means that I can remove the H from the middle as well. The ending still is -yl but now it's not going to be propyl because we're taking it from the middle, this structure is called isopropyl. Iso just means same in the carbon that is having its H removed so it can be connected to something else. It’s connected to two of the same groups, which basically means these two CH3s. Iso means same. I’ll making propyl or isopropyl.
Let's say you have a four-carbon chain. We're going to say here that this four-carbon chain here is called butane. You could take it from either one of the end carbons. Again when you take it from the end carbon, it just changes the ending to -yl, so this becomes butyl. But we also have a new situation where we can take it off from one of the middle guys, which are the same. We're going to stay here when you take it off from one of the middle guys, it becomes sec-butyl.
What the heck does sec mean here? Think of sec, meaning secondary. But what does that mean? What is a secondary carbon? A secondary carbon is a carbon connected to two other carbons. The carbon that has the bond, this carbon, it is connected to two other carbons. Namely, it's connected to this CH3 and this CH2. It’s a secondary carbon or sec-carbon and because it’s a secondary carbon, that's why we call it sec-butyl. It’s important that you know what a butyl looks like and what a sec-butyl group looks like. We've figured out half of the names.
But where did the other two come? The other two come from the constitutional isomer of butane. What the heck does constitutional isomer mean or structural isomer? We’re going to say constitutional or structural isomer. Constitutional isomer or structural isomer means you have the same exact molecular formula but different connections. Because you have different connections, you have different names. On the previous page, we saw that butane was this right here. This was butane. If you count the number of carbons and hydrogens, it was C4H10. This one here is called isobutane. It is a structural or constitutional isomer of butane, because if you look at it, how many carbons does it have? It has four as well. How many hydrogens does it have? It has 10 as well. Isobutane is a constitutional isomer of butane, same formula different connections.
Here, this is an end carbon, this is an end carbon and technically up here is an end carbon. If we remove from any one of those, it changes the ending to just -yl. The beginning state is isobut and then the ending changes to -yl. This would be called isobutyl. Here, we don’t only have those options but we can remove it here from this middle carbon. It's not one of the end carbon so we can't just simply say isobutyl anymore. Here we're going to say that it's called tert-butyl or t-butyl. What does tert mean? Tert equals tertiary. What does tertiary mean? Tertiary means that you are a carbon connected to three other carbons. Tertiary means you are carbon connected to three other carbons. If we look at this carbon here, it is connected to one, two, three other carbons. It is a tertiary carbon. The name is tertiary butyl or tert-butyl for short or t-butyl even shorter.
A carbon connected to two carbons is secondary. A carbon connected to three carbons is tertiary. A carbon connected to one carbon is called a primary carbon. Primary is usually on denoted by a one with a degree sign. Tertiary is usually three with the degree sign. Secondary is a two with a degree sign. A carbon connected to one carbon is primary. A carbon connected to two carbons is secondary. A carbon connected to three carbons is tertiary. What do you think a carbon connected to four carbons is called? A carbon connected to four carbons would be called quaternary. We tend not to talk too much about quaternary carbons. There are a few places here and there in Organic that they ever talk about them. But just realize that a quaternary carbon is a carbon connected to four other carbons. The whole sec-butyl and tert-butyl kind of go off of this idea of that carbon that's making a bond, how many carbons are connected to it. That's where their names come from.
Knowing this is important because knowing this will help us answer this following question. We’re being asked to draw the three isomers of pentane. Basically I'm asking us to draw the constitutional isomers of pentane. What are three different ways I can draw this formula and it’d be correct? We're going to take a look at this question in the following video. Make sure you come back and attempt to do it alongside me and see what strategy I use in order to draw this correctly. Here's a huge hint. What we just went over in terms of alkyl groups will help us tremendously when it comes to naming this compound or actually drawing these compounds. They provide shortcuts to get to the answer very quickly, shortcuts that are going to help you a lot in the beginning of Organic 1 because a typical question is draw the constitutional isomers from the given formula. Look at the strategies that I use to help guide you to the answer. Come back and look at the next video.
When an alkane becomes an alkyl group the ending will change from "ane" to "yl".
The alkyl form of ethane is called ethyl.
Propane has two alkyl forms.
Butane has two alkyl forms.
Isobutane is a constitutional isomer of butane and it has two alkyl forms.
Example #1: Based on your knowledge of chemical structures draw the three isomers of C5H12 (pentane).
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