Alkane Nomenclature

In chemistry, every single molecule needs a unique name. We can’t have two molecules with the same name or that would get super confusing! For this, we use IUPAC nomenclature. 

Alkane Nomenclature Overview

Concept: The different parts of an IUPAC name 

4m
Video Transcript

So what I want to do is I want to introduce the concept of alkane nomenclature. If we learn this, then we're going to be able to use this systematic method in order to name a bunch of other molecules later.
A little bit of a history lesson really quick. Before 1919, chemists had no uniform way to name molecules so they would basically just make up common names on the spot. That's what it was called. It was called a common name. Literally, some chemists would name molecules after their dog, after their ex-wife, after something they saw in the movies. I don't know. It got really confusing because they were a ton of names that people started having to memorize as people started discovering more molecules.
So 1919, the IUPAC convention gets together and they decide, hey this is getting out of hand. This is getting crazy. We need to figure out a systematic method to name all these molecules and that's when IUPAC nomenclature was born.
The way that it basically works is that there's four different things that we look at for IUPAC nomenclature. Here you'll see that what I'm giving you is an alcohol. This is actually a little bit beyond the scope of what we're going to talk about for this first set. We're just going to talk about alkanes, but I'm just going to show you that what we do is we break up a big molecule into more manageable pieces.
The first and most important thing that we always look at is the root. This is also called, let me write it in black, this is also called the parent chain. It can be called the root chain or the parent chain. That's usually just going to be – I'm going to go through these rules in a second – that's usually just going to be your longest chain.
All right, as I said, I'm not going over the rules just yet. I'm just helping you guys see the differences between these different parts of the name.
Then what they said was okay, well, not everything was on the root chain, through. There's other things coming off of it. So what they said is that anything that's coming off of it, they're going to call that a substituent. So a substituent is just a branch. If you ever see that word, it just means there's a branch coming off the chain. In this case, I have a carbon that is not on the black line and it's just sticking out, so that means that would be a branch.
But then, if we have branches we need to know where is that branch because I can't just say there's one branch. Where is it? Is it at the end? Is it in the middle? So we're going to need locations for those branches. So another part of the name is the fact that I need to have number locations. I'm just going to put numbers in order to know where those branches are.
Does that make sense so far? Basically, I have a main chain. I have things coming off of it and then I have to say where those things are. Easy so far. It's kind of like giving someone directions. You have to say what's the street, what's the zip code, all that different stuff.
Then you have this one last thing called the modifier. The modifier is just basically the functional group. I'm just going to put FG for functional group. The functional group determines the actual, basically a suffix of the root. So you're going to add a suffix at the end depending on what the functional group is and that's going to tell you what kind of molecule your molecule is actually going to react as. Like I said, this suffix has to do with the function of the molecule. Does that make sense?
Like I said, this is beyond the scope of – naming this molecule is beyond the scope of what we're doing right now, but I just want to show you an example of everything – of all the different components.

Basically there are 4 different parts to naming most molecules:

  • The Root or Parent Chain (this tells us about overall chain length)
  • Modifiers (tells us what functional groups are attached to the chain)
  • Substituents (tells us about what is branching off that chain)
  • Numerical Locations (tells us where those branches are)

Alkane Prefixes

Concept: Learning Alkane Prefixes up to 12 Carbons in Length 

2m
Video Transcript

I just want to look at this chart here. This chart is going to be your cheat sheet for the different root names or parent names that you need to know. Now some professors don't require you to know all the way up to 12. Some professors will have you end at 8. Some professors have you end at 10. Just it's up to your professor. I'm just doing 12 just to be thorough, just in case your professor wants you to know all 12.
Let's go ahead and do these one at a time.
What would be the prefix for one carbon? And it would be meth-. You have meth-. Then the second one, so two carbons, would be eth-. You guys can maybe start saying these with me because maybe you know some of them from gen chem or you learned them in class.
Then we have prop-. Then we have but-. Then we have pent-.Then we have hex-.
Now this one gets people sometimes when we start at seven. It's not sept-, it's hept-. Hept-, oct-, non-, dec-. Those aren't so bad.
Now 11 and 12 are a little bit weird. 11 is going to be undec-. That one actually makes sense. It's like one and ten. Then 12 is dodec-. I didn't know if you can just somehow remember that that's one and two plus ten then that will help.
Those are the prefixes that you need to know. These are going to be the ones that tell you basically how long that root or parent chain is. 

Memorizing sucks, I get it. But unfortunately this is something you’re just gonna have to remember. Not all professors will make you memorize all 12- check with your professor to figure out how many you need to know!

IUPAC Alkane Naming Rule #1

Concept: Naming the root chain 

2m
Video Transcript

There's another part to this rule, though, and let's go back up to that. What if you have two different pathways, two different chains, that are of equal length? How about if you have a tie between longest chains?
Then what we're going to do is we're going to choose the chain that gives the most substituents. What that means is if you have the choice between two, let's say ten-carbon chains, one of them gives you three substituents if you follow it, the other one gives you four. You're going to pick the chain that gives you four substituents.
Why would that be? That actually sounds more complicated. The reason is because if you can break down your substituents into more, the chances are more likely that those substituents will be smaller and easier to name. So it's actually a good thing as an organic chemist, it's good for me to have a bunch of small substituents that are easy to name instead of having a few really big substituents that are terrible to name.
Does that make sense so far? That where we're starting. That's rule number one. Let's go ahead and move on to the next rules.

Remember, if there is a TIE between two chains of equal length, go with the chain that gives the MOST substituents!

IUPAC Alkane Naming Rule #2

Now we know the length of the root chain, but nothing else! We need to determine which carbon gets the “1” location. 

Concept: How to determine the direction of the root chain 

2m
Video Transcript

So rule number two, once you've already figured out the longest chain, is that we have to decide the direction of that chain. The way we do that is to start from the closest substituent. Remember that substituent just means branch.
What that means is that I want my branch numbers to be as small as possible. Remember that each branch needs a location number. So in order to get those numbers as low as possible, I want to start from the side that's going to have the closest branch, so then it's going to have the lowest location possible.
Unlike, if you're reading, in the English language you always read from left to right. It's not the same in organic chemistry. In organic chemistry, you can read from any direction as long as that's the direction that gives you the smallest numbers in terms of location.
Then there's a few additional rules because this happens a lot. How about if there's a tie between substituents, meaning that on one side you get to the first substituent in three carbons, on the other side you also get to the first substituent in three carbons? Well, then we're going to compare the next closest substituents. What that means is that I would say okay, both of these are tied. Let's move on to the next set and maybe there's going to be a difference there.
Then there's an, even more, nightmare situation, which is what if there's still a tie. What if all throughout the substituents are in exactly symmetrical locations? Then what we would do is we would determine the direction, meaning which side gets the number one position, is it the left, is it the right, or is it the bottom or the top? Using alphabetical order. 

  • Start numbering the root chain starting from the closest substituent
    • If there is a tie between locations, compare the locations of the next substituents
    • If there is STILL a tie between ALL locations, decide using alphabetical order.

Example: Name the longest carbon chain and determine the direction of the root chain 

5m
Video Transcript

So what I want to do is just do this first one as a worked example between us. So it says here name the longest carbon chain and determine the direction of the root chain. I know that can be difficult to relate to.
First of all, let's figure out what the longest carbon chain is. Go ahead and think about it for a second. Then I'll – and get back to me.
All right, so there are actually a few different longest chains. For example, I could have had this be one of the chains. That would be how many carbons? That looks like it's going to be seven carbons. Are you guys cool with that? But I could have also had this be one of my longest chains. Do you agree? That one would also be seven carbons. Oops.
What I'm doing right here is I'm actually still going through rule number one. I haven't even gotten to two yet. Two is direction. One is just which chain is going to be the longest one or is going to be my root chain.
What do you guys think? The green area means that that's going to be the chain no matter what, but should I go to the yellow? Should I go up or should I go down to the blue? There's actually a rule to determine this. I said that if there's a tie between two different ways that you could make a chain, a longest chain, how do you determine which one is the winner? The way you determine is by the one that gives you the most substituents.
What I want to do is I want to erase both of these. I want to erase one and we're going to see how many substituents each one gives.
If I have the yellow chain, how many substituents do I have total? I have just one substituent. Remember that a substituent just means a branch, so I just have one thing that's not on the yellow. Does that make sense? One thing branching off, so I'd call that one.
Now what we're going to do is we're going to erase the yellow and I'm going to draw the blue. Now if I have the blue chain, how many substituents do I get? Well, I'm going to erase the red just in case you guys are confused. So for the blue chain, how many things do I have sticking off of the blue chain? Actually two. I have something here and I have something there.
What that means is that one of these is going to give me more substituents and it's going to be the blue. So this is actually my longest carbon chain right here. So my root is going to be heptane and it's going to be that particular heptane. Cool?
So now all we have to do is determine the direction. Direction means simply is this going to be the first carbon or is this going to be the first carbon? Let me use different colors. Is the blue going to be my first carbon or is the red going to be my first carbon? Now we're on to rule number two.
Do you guys remember how to decide that? What we would do is we would go start from the closest substituent. The closest substituent, if I go to blue, I would have to go to two, three, four, five. My first substituent starts at five if I start numbering from the blue direction. So five is the number to beat. Now if I start numbering from the red direction, I get to two and I already have a substituent. Do you see that? So I get to the second carbon, I already have a branch coming off.
So which of these is going to give me the lower number? Which of them is going to be the one that gives me the direction? And it's going to be red. So I'm going to erase the blue. The blue is wrong.
What that means is that this is going to be one, two, three, four, five, six, seven. Later on when I have to name this molecule, which we're not going to do yet, we're not there yet, but later on when we have to name this, this is going to help me because now I'm going to have substituents at the two and at the three and that is way better than having substituents at the five and at the six, which would have been the blue direction. Does that make sense?
So the direction was this way and that is the end of the question. So you guys did an awesome job. Now what I want to do is I want you guys to do this one on your own. So go ahead and try to do the entire thing and then go to the next video when you're done.

The key here is to get the lowest numbers possible for ALL of your locations (at least for now). Now give it a shot by yourself!

Example: Name the longest carbon chain and determine the direction of the root chain 

3m

If you got this one wrong, don’t worry too much. You’ve got plenty more chances to nail this. 

IUPAC Alkane Naming Rule #3

So we know the length and direction of the root chain, which is great. But if there are ANY branches on this chain, we need to name those too.

P.S. The term “substituent” is just a nerdy word for a “branch”. 

Concept: How to identify and locate branches (substituents) 

4m
Video Transcript

All right, so this is the page where we really get into naming. We're going to learn how to put everything together and make a full alkane name by the end of this page.
Rule number three, this is after we've already identified the longest chain and the direction of that chain. Rule number three is to designate numerical locations of substituents. What that means is that I'm going to want to actually say, okay is this substituent on two or is it on three.
Now when there's more than one identical substituent, so let's say that I have two one-carbon groups, instead of naming them both individually, we want to cluster them together to save some space and to save some time, so what we're going to do is we're going to use prefixes. These prefixes are going to tell us how many of that type of substituent we have.
The prefix for two is di-. The prefix for three is tri-. The prefix for four is tetra-. So di-, tri- and tetra-. When we do represent these chains, these branches, they aren't true alkanes. The reason is because they're always going to be missing an H because of the fact that they are on a branch.
For example, let me just give you an example. Here I have a long chain. Then I have a two-carbon chain coming off of it. The molecular formula of this two-carbon chain is CH2CH3. The molecular formula for a normal two-carbon chain is CH3CH3. That would be if it was just by itself. It would just be a stick. Does that make sense?
The name of that alkane, the red one right here, the name of that one if it was just on its own would be what? Alkane prefixes. It would be ethane. Does that make sense so far?
But notice how the molecular formula is slightly different here. I have a three, but when it's a substituent or a branch it has a two. This is not – you can't just call it ethane. Instead, what we're going to do is we're going to represent it using a -yl suffix, so basically alkanes become alkyls.
I told you guys this already earlier when we were talking about functional groups that if you have a chain coming off, you put a -yl suffix for the alkane. That means that this would be called ethyl, not ethane, the actual branch off right there. That means that we're missing one hydrogen.
What I want you guys to do is we're going to put this all together. We're not going to name the entire thing yet, but we're pretty much going to get all the pieces. I want you guys to name the root chain, determine the direction of the root chain and then identify and locate all of the substituents.
We're going to do this as a worked practice, so we're going to do this together. But just go ahead and try to solve it on your own and don't go to the next video until you're done with it. 

NOTE: Alkane substituents require a “-yl” suffix to indicate that they are a branch! (i.e. ethane becomes ethyl). 

Example: Name the root chain, determine the direction of the root chain and then identify & locate all substituents 

3m
Video Transcript

All right guys, so hopefully you got that the longest chain was actually this one right here. I know that can be tricky to visualize because you're not used to thinking of curving around stuff. You're used to seeing things just from left to right and things in a straight line. But the longest chain just is the longest continuous chain of atoms so that would be that swirly one right there.
So how many is that? That looks like 1, 2, 3, 4, 5, 6, 7, 8, 9, so this is going to be that the root equals nonane. Cool with that so far?
Now we have to determine the direction. This one's actually pretty easy. This is my one in blue. This is my one in red. Which one is going to be the direction? Which one is going to be a better direction and obviously, it would be the blue because the blue, I'm going to get to my first substituent at the 3 position, whereas with the red, I wouldn't get to my first substituent until the 5 position. So that one's just a loser. Red sucks.
Now I have my numbering determined. Now I have to identify and locate all of the substituents. The easiest way to do this is just to number the entire chains. I'm just going to say 2, 3, 4, 5, 6, 7, 8, 9. How many substituents do I have? I have two. I have one on the 3 and one on the 5.
The way we're going to actually name these – I'm just going to put here subs. Now I'm getting hungry. The way we're going to put these is by naming the location with a hyphen and then we're going to say what the actual alkyl group is. I'm going to teach you more about this as we move down the page, but I'm just telling you this for right now.
One of the substitiuents would be 3-methyl. By the way, if you didn't add the hyphen, that's totally fine because I haven't taught you that yet. But just so you know, if you just put 3 methyl, that's fine. But it's actually 3-methyl. That would be one substituent.
Another substituent would be 5-ethyl. Does that make sense? Because I have an ethyl group on the 5 and a methyl group on the 3. Awesome. 

Sick job! We’re getting closer to fully naming these guys. 

IUPAC Alkane Naming Rule #4 and #5

Now we have all the pieces we need to name most alkanes, but we need to work on our formatting! Chemists are surprisingly analytical making sure all your commas and numbers are in the right place.

Concept: Proper name ordering and punctuation 

2m
Video Transcript

So now I pretty much have all the pieces that we need to put a name together. I just need to teach you two additional rules. Let's go ahead and work on that.
The next rule is that – now we have all the pieces we just have to figure out how to make this big name. What we're going to do is we're going to name substituents in alphabetical order. Remember substituents would be like ethyl, methyl. I don't care about which one's bigger. I just care about which one has a lower letter in the alphabet.
Also, prefixes don't count towards this, so if I have dimethyl or trimethyl, I don't actually count 'd' as the alphabetical order. I would count methyl, 'm', as the alphabetical order. Remember that prefixes don't count.
Then finally, this is the part that I was saying, we're going to use commas to separate numbers from numbers and we're going to use dashes or hyphens, whatever you want to say, to separate letters from numbers.
Up here why did we use a dash or a hyphen? Because there's a number and a letter. Then we would use a dash because there's a number and a letter next to each other. If it was a number and a number, I would use a comma. The biggest thing is that you don't want to have spaces. I'm just going to – that c isn't very clear, commas. Cool.
All right, so now this one you guys are on your own. I want you guys to provide the following IUPAC name. You're just going to do this entire thing on your own and go on to the next video when you're ready for the answer. 

In short:

  • Always name your substituents in alphabetical order
  • Commas separate numbers from numbers, and dashes separate numbers from letters (i.e. 1,2-dimethyl)

Example: Provide the IUPAC name for the following alkane

8m

Does this make sense? You just learned how to name simple alkanes. Give yourself a pat on the back!

Alkane Nomenclature Additional Practice Problems