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Naming Alkanes - Concept
M.Ed., Columbia Teachers College
Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.
Naming alkanes can be difficult because each alkane consists of a parent chain and one or more branches. First, it is necessary to count the number of atoms in the longest chain. Then we name the branches based upon how many atoms they contain and number the branches by the atom in the parent chain they connect to, keeping the numbers as low as possible. For multiple identical branches, prefixes are necessary to indicate quantity.
Alright. So we're going to talk about naming alkanes. And alkanes are in hydrocarbon, where all the carbons are singly bonded to each other. We're going to also call them saturated hydrocarbons, meaning that they have the maximum amount of hydrogen attached to all these carbons as possible.
So, let's look at C4H10. It's a hy- it's a alkane. Notice all the bonds are si- all the carbons are singly bonded to each other and we have the maximum number of hydrogens that we can have. We can structurally put it, make it this way with carbons attached to it in a chain, in a straight chain or we can have it attached this way where three carbons in a row with one carbon coming up the center. They're both the same, C4H10 but they're structurally very different. They actually have very different chemical properties, and they're used in very different ways. So how are we going to be able to distinguish between this guy and this guy if our structure is the sa- if the chemical formula is exactly the same?
We're actually going to have to name them differently and there's actually like rules that we're going to abide by to name these different compounds so we can communicate with amongst chemists in a better more efficient way. So we can communicate well.
So one of the lingos that we're going to actually discuss is a thing called parent chain. And a parent chain is the longest carbon chain. So we, that's what we call the parent backbone or the backbone of the carbon chain, also known as the parent chain. We also have with- among the, within the parent chain, we might have side shoot offs called substituent groups. And those are those side branches with- within those carbon chains.
Alright. So we to name those side branches and they're going to be used, the naming of them is actually very similar to how we name alkanes to begin with, and we know we talk about about one carbon, we're going to use the prefix, meth. So if you have one carbon side chain, r represents our parent chain, we're going to say that that's a methyl group. If we have two carbon, we're going to say that that's an ethyl group. Don't forget eth is the prefix for two. If we were talking about a propyl group we'd say three carbon attached to the parent chain. But we can actually rearrange this differently. We can have it attached this way or we can have it attached to the central carbon. And we're going to call this, it's different. We're going to call this an isopropyl group. And then there's a butyl group which is attached which is four carbons attached to the parent chain and that can be also rearranged. We'll talk about that. And then we get back to our normal prefixes like pentyl group, hexyl group, all the prefixes we use in dealing with covalent bonding to begin with.
Alright. So let's actually, there are more rules when we're talking aobut this. Let's actually discuss them and do an example while we're doing that. So let's say we have this massive carbon chain with all these side branches and this really long carbon chain that might be used, you might see this when dealing with polymers and dealing with plastics or things like that. This is very co-, something like this is very common. So you have to be able to learn how to communicate that would, you know what kind of compounds you're dealing with.
So the first thing we're going to do is count the number of carbons in the longest carbon chain. Okay. So we have to think about, what is the longest carbon chain. So let's, let's count this. So let's look at this straight chain that we have at the bottom and count how many carbons are there. We can say one, two, three, four, five, six, seven, eight carbons but can, is there a possibility that we can have more than eight carbons in a row? Well, just because it's bent doesn't mean that it's not part of a chain. As long as we go in a line or in the same, we don't break from that chain, then we can still call that a parent chain. So let's actually include these. So let's say one, two, three, four, five, six, seven, eight, nine. Well, nine's larger than eight. So I'm going to say that this is my parent chain. It's my longest carbon chain. Okay. So now that I know my carbon, my longest carbon chain or my parent chain, I can now go in and talk about the substituent groups.
Alright. We're going to number each carbon giving all the substituents the lowest numbers. So let's look at our, make sure that we know where our substituents are. We have one here, we have one here and we have one here. Okay. So we have to make sure taht these are the lowest number of carbons. If we're numbering the, we have to number the carbons in a way that these are in their lowest places. So we could either name it, this being carbon number one and go one, two, three, making this carbon methyl, a methyl group of carbon three. Or we can number it this way. Going one, two, lower. And this carbon, and this methyl group's at carbon two. Two is lower than three so we're going to go this way. So then we name this [IB] carbon one, two, three, four, five, six, seven, eight, nine. Okay. So now we're going to name the substituents. Well, we have three of them. If we have more than one of the same substituents we're going to use prefixes. So the prefix for three, we're going to go back to our normal prefixes, we know it's tri. We're going to call this trimethyl. We have trimethyl group. Trimethyl, okay? Because it's three methyl groups. Methyl meaning one carbon each.
If we have more than one side chain name it alphabetical order. We don't have more than one side chain in this example. So we don't, we can skip this idea. Use hyphens to separate the numbers and wor- from words and the commas to separate the numbers. Okay. We have to figure out where these three methyl groups are. One's a group two, one's a carbon two, one's a carbon four and one's a carbon seven. So we're going to say two, four, seven hyphen trimethyl. And what is this? This is nonane. so we're going to call it nonane because don't forget the prefix non means nine. We have nine carbons. And then we are going to end it with ane. Because everything is singly bonded. This is an alkane so we're going to end it in ane. So the name of this guy is two, four, seven trimethyl nonane. Okay? We are actually going to go the opposite way.
Let's look at something like this. Don't let this intimidate you. This is basically just directions on how to draw this carbon chain. So we're going to say two, three dimethyl five propyldecane. Well, first I know, it ended in decane and firstly I want say that it's an ane. So it's a single bonded carbon chain. Dec, the prefix dec means ten. So I'm going to draw ten carbons. Okay? One, two, three, four, five, six, seven, eight, nine, ten. Okay. At the fifth carbon, there is a propyl group. Don't forget propyl means three carbons. So at the fifth one, one, two, three, four, five, there is a propyl group. One, and I'm just going to go this way just for a [IB] three. So we have three carbons coming off of carbon five. We're going to call this, we're going to label it just for [IB] Nine, ten. Okay. Then we have two three dimethyl. So we know we have two methyl groups. One at carbon two and one at carbon three. So we have a carb CH3 here and we have a CH3 here. This is essentially what we just did. See how easy this was. These are basic directions to get to this actual formula or this structure. Now I can just fill in all my hydrogens with where they go because I know that carbons have to have four bonds and I'm going to just fill in hydrogens where they, to make it all correct. And so on and so forth I don't need the whole thing but you know, you see where I'm getting at. We'd fill all the carbons in and then we can say this we just drew is two three dimethyl five propyldecane. Which sounds intimidating at first but actually it's not that bad. So these actu-, don't forget this instructions because these instructions will help naming, when you're naming alkenes and alkynes. Double bonds and triple bonds. these are the basic instructions on how to name hydrocarbons.
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