IUPAC Naming - Online Tutor, Practice Problems & Exam Prep

Understanding alkane nomenclature is essential for systematic naming in organic chemistry. The IUPAC system, established in 1919, uses a root or parent chain, substituents, numerical locations, and modifiers to create names. Key prefixes for carbon chains include meth-, eth-, prop-, and so on, up to dodec-. When naming, prioritize chains with more substituents and assign the lowest possible numbers to branches. Substituents are named alphabetically, and prefixes like di-, tri-, and tetra- indicate multiple identical groups. Proper formatting includes using commas and dashes to separate numbers and letters.

The different parts of an IUPAC name

So what I want to do is I want to introduce the concept of alkane nomenclature. And 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. Alright. So 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. All right? So literally some chemists would name molecules after their dog, after their ex-wife, after something they saw in the movies. I don't know. And it got really confusing because there were a ton of names that people started having to memorize as people started discovering more molecules. So in 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.

So the way that it basically works is that there are 4 different things that we look at for IUPAC nomenclature. Here you'll see that what I'm giving you is an alcohol. Okay? So 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 smaller, more manageable pieces. The first and most important thing that we always look at is the root. Okay? The root, this is also called, let me write it in black, this is also called the parent chain. Okay. It can be called the root chain or the parent chain. And that's usually just going to be your longest chain. Okay.

Alright. So 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. So then what they said is okay, well not everything is on the root chain though. There are 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. Okay? So a substituent is just a branch. Okay? So if you ever see that word, it just means there's a branch coming off the chain. So in this case, I have a carbon that is not on the on the black line and it's just sticking out, so that means that would be a branch. Okay? But then if we have branches, we need to know where that branch is 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? So 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. Okay? You have to say what's the street, what's the zip code, all that different stuff.

Then we have this one last thing called the modifier. The modifier is just basically the functional group. Okay, I'm just going to put FG for functional group and the functional group determines the actual basically a suffix of the root. Okay? 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. Okay. So like I said, this suffix has to do with the function of the molecule. Does that make sense? So 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. Okay? Of all the different components.

Learning Alkane Prefixes up to 12 Carbons in Length

Just want to look at this chart here. This chart is going to be like 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. So, 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. So, let's go ahead and do these one at a time. What would be the prefix for one carbon? And it would be meth. Okay? So, we have Meth then the second one, so 2 carbons would be eth. Okay. And you guys can maybe start saying these with me because maybe you know some of them from general Chemistry or you learned them in class. Then we have prop. Okay. Then we have but. Then we have pent. Then we have hex. Right? Now, this one gets people sometimes when you start at 7. It's not sept, it's hept. Okay. Hept, oct, non, dec. Okay, so those aren't so bad. Now, 11 and 12 are a little bit weird. 11 is going to be undec, so that one actually makes sense. It's like 1 and 10. And then 12 is dodec. Okay? And I don't know if you can just somehow remember that that's 12 plus 10, then that will help. Okay. So those are the prefixes that you need to know. These are going to be the ones tell you basically how long that root or parent chain is.

Naming the root chain

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? Okay. So, if there's a tie between the longest chains, then what we're going to do is choose the chain that gives the most substituents. Okay. What that means is, if you have the choice between two, let's say, 10 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 that if you can break down your substituents into more, the chances are more likely that those substituents will be smaller and easier to name. Okay? 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. Okay? So, does that make sense so far? That's where we're starting. That's rule number one. Let's go ahead and move on to the next rule.

How to determine the direction of the root chain

So rule number 2, once you've already figured out the longest chain, is that we have to decide the direction of that chain. And the way we do that is to start from the closest substituent. Okay? So remember that substituent just means branch. Alright? And 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. Okay? So unlike if you're reading, you always 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. Okay? So 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 3 carbons. On the other side, you also get to the first substituent in 3 carbons. Okay? Well then we're going to compare the next closest substituents. Okay? So then 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. Okay? But then there's even a more nightmare situation, which is what if there's still a tie? What if all throughout the substituents are at 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 or is it the right or is it the bottom or the top using alphabetical order.

Name the longest carbon chain

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. So first of all, let's figure out what the longest carbon chain is. Go ahead and think about it for a second and then get back to me. Alright. 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 7 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 7 carbons. Oops. So what I'm doing right here is I'm actually still going through rule number 1. I haven't even gotten to 2 yet. 2 is direction. 1 is just which chain is going to be the longest one or is going to be my root chain. So 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 if there's a tie between two different ways you can 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. Okay? So what I want to do is I want to erase both of these, or I want to erase one and we're going to see how many substituents each one gives. So 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 1. Now what we're going to do is we're going to erase the yellow and I'm going to draw the blue. So 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, 2. I have something here and I have something there. Okay? And 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. So is the blue going to be my first carbon or is the red going to be my first carbon? Do you guys know we're on to rule number 2. So do you guys remember how to decide that? What we would do is we would start from the closest substituent. Okay? And the closest substituent, if I go to blue, I would have to go to 2, 3, 4, 5. My first substituent starts at 5 if I start numbering from the blue direction. Okay? So 5 is the number to beat. Now if I start numbering from the red direction, I already have a branch coming off. So which of these is going to give me the lower number? 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. And what that means is that this is going to be 1, 2, 3, 4, 5, 6, 7. 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 2 and at the 3. And that is way better than having substituents at the 5 and at the 6, 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. Okay. 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.

Name the longest carbon chain

All right, so let's solve this one. So, basically, I had a few different options. I could have maybe taken the yellow route and done this as my longest chain. Were there any other longest chains that I could have made? Actually, no. There was no other way to make a 6 carbon chain. Now what you guys might be thinking is well Johnny, what if I did it like this, the blue direction like that? Wouldn't that be a little bit different? And actually no, that would be the same exact chain. And the reason is because, I'm just going to erase this, notice that both of these groups at the ends are both identical, and both of the groups at the ends are identical here. Okay. So what that means is it doesn't really matter if you pick the bottom one or the top one. That's still the same thing. So if you drew your longest chain like this, that's fine. That's the same thing as drawing this. Okay? It just doesn't really matter as long as you have those 6 in order because like I said, the groups on both ends are exactly the same. Okay? Cool. So that's the first thing. So actually, the first part, rule number 1, was easy. Figuring out the longest chain was easy. Now we just have to figure out the numbering. Okay? Or the direction. So should this be my number 1 or should this be my number 1? Okay. So should I have red as my 1 or blue as my 1? So what I would do is I would say okay, let me start counting and see when I get to my first substituent. So for red, I would get to 2 and I would already reach a substituent. You guys agree with that? Cool? So that's hard to beat. Red looks like red might win. Now let's look at the blue side. I get to 2 and 2 is also a substituent. So wait, what does that mean? Does that mean we give up? No. It does not mean that. It means that there's a tie, so we have to go to the next rule. The next rule says, okay, well, if there's a tie, then move to the next substituent. So that means that now I have to keep counting from red and say how long does it take me to get to the next substituent? Well, 3 and then 4 would be the next one. Okay, so 4 is the number to beat. Now with blue, 3, I'm already there. So which one's going to win? Blue. So it turns out that this is going to be a 6 carbon chain, so this is going to be hexane and it's going to be numbered in this direction. Does that make sense with this being my 1 and this being my 6? Is that cool, guys? Hopefully, that made sense. All right, let's go ahead and move on to the next rule.

How to identify and locate branches (substituents)

All right. So this is the page where we really get into naming. Okay. We're going to learn how to put everything together and make a full alkane name by the end of this page. So rule number 3, this is after we've already identified the longest chain and the direction of that chain, rule number 3 is to designate numerical locations of substituents. Okay? So what that means is that I'm going to want to actually say okay, is this substituent on 2 or is it on 3? Okay. Now when there's more than one identical substituent, so let's say that I have 2 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. So the prefix for 2 is di, the prefix for 3 is tri, and the prefix for 4 is tetra. Okay. So, di, tri, and tetra. And, when we do represent these chains, these branches, they aren't true alkanes. And the reason is because they're always going to be missing an H because of the fact that they are on a branch. So for example, let me just give you an example. Here I have a long chain and then I have a 2 carbon chain coming off of it. Okay? The molecular formula of this 2 carbon chain is CH2CH3. K? 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 3 but when it's a substituent or a branch, it has a 2. So 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 the a y l suffix. So basically alkanes become alkyls. And 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 y l suffix for the for the alkane. Okay? So that means that this would be called ethyl, not ethane. Okay? The actual branch off right there. Okay. And that means that we're missing 1 hydrogen. Okay? So 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 root chain, determine the direction of the root chain and then identify and locate all the substituents. So 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.

Name the longest carbon chain and substituents

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. Okay? 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. So 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. Okay? 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. Okay. So that one's just that one's just a loser. Red sucks. Okay.

So now I have my numbering determined. Now I have to identify and locate all the substituents. The easiest way to do this is just to number the entire chain. So I'm just going to say 2, 3, 4, 5, 6, 7, 8, 9. How many substitutions do I have? I have 2. I have 1 on the 3 and 1 on the 5. Okay? So the way we're actually going to name these, I'm just going to put here subs. Okay. 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. So one of the substituents 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. Okay? 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.

Proper name ordering and punctuation

Now we pretty much have all the pieces that we need to put a name together. I just need to teach you 2 additional rules. Alright? So 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. Okay? So remember, substituents would be like ethyl, methyl. I don't care about which one's bigger, I just care about which one has the lower number and a lower letter in the alphabet. Okay? And also, prefixes don't count towards this. So if I have dimethyl or trimethyl, I don't actually count as the I don't count as the alphabetical order. I would count Methyl, M, as the alphabetical order. Okay? So 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. Okay? So up here, why did we use a dash or a hyphen? Okay. Because there's a number and a letter and then we would use a dash because there's a number and a letter next to each other. But if it was a number and a number, I would use a comma. Okay? The biggest thing is that you don't want to have spaces. Okay? I'm just going to clarify that it isn't very clear. Commas. Cool. Alright. 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.

Provide the IUPAC name for the following alkane

Alright guys, so this one was actually surprisingly tricky right from the very beginning because of the fact that there are a lot of different chains that could be the longest chain. So I think one of the obvious chains that you might have thought of would be that this would be the longest chain. Okay? So that's possible. The only thing is that there are actually other combinations as well that would also be just as long. I could also make my chain go up like this. That would be another possibility. That would also be 7. And then, this is going to get ugly, I could also do this. Okay. Do all of these make 7 carbon chains? Alright. So what I'm going to have to do is go 1 by 1 and see which one gives me the most substituents. Right? Let's go to the yellow one first. How many substituents do I get? I get 1 up here, 23. Okay. Now you guys might be wondering why did I assign that as 23 if they're coming off of the same carbon? Because they're 2 different branches. Okay. So each branch gets its own number. So this would be 3 substituents. Okay? Three branches. So that's the number to beat. Let's see if any other chains can beat that. So let's do the one that I made in blue. That would have been like that. How many substituents do I get for this one? What I get is one here and then I actually just get one really complicated one here. Is that good? No. This one's actually worse because this one just gives me 2 substituents, and they both—I mean, and it's less than 3. So this one's not going to be my answer. So let's finally do the one that I did in purple. So I'm doing this. How many substituents do I get now? I'm actually going to get, let's see, 1, 2, 3, 4. Yikes. Okay. So this is actually going to be my longest chain because this is the one that gives me the most branches. Okay. Now this is probably the reason that you guys got this question wrong. If you got the question wrong, it's probably because you had the wrong chain from the very beginning. Okay? Now what I want to do is go ahead and start using the other rules in terms of direction. Okay? So is this going to be my one carbon or is this going to be my one carbon? Okay. Well, let's go ahead and start numbering. So my first substituent would start on 3 for red. My first substituent would start on 3 for blue. So there is a tie. Okay? But then that tie is easily broken on the next round because on the next round my next substituent would be on 4 for red, but my next substituent would still be on 3 for blue. Why? Because I have 2 substituents on the 3 position. Does that make sense? So I would start off and I would have that one cancel out with that one, but then I would have this one win, and this one would lose. Okay because it's closer. So blue should be my number 1. Does that make sense? Cool, so now I know that I'm numbering in this direction. That helps a lot. Now I just need to get the substituent numbers and stuff. So this would be 3, this would be 4, and this would be 5. Are you guys cool with that? So now what we have to do is put all of this together and get everything down on paper. So what's the root name? I like to separate stuff out. The root name is going to be heptane. Okay. What are the substituent names? Okay. Well, in this case, I want to put locations and substituents. Okay. So how many different types of substituents do I have? I know I have 4 substituents, but in terms of types, how many do I have? Actually, only 2 different types. I have methyl groups, which I have 3 of. I have 3 methyls. 1, 2, 3. And then I have 13 carbon chain, which is going to become what we call a propyl group. Okay? So what that means is for substituents, I actually just have a trimethyl. See how that works? I put them all together. Instead of naming them out individually, trimethyl methyl saves a lot of space. And then I'm also going to have a propyl. Okay? Now all I have to do is get the number locations of all of these. Okay? So I'm going to put sub out here to give me some more space. Alright? So what are the number locations of the trimethyl? Well for trimethyl, I actually have to be explicit about every single location. Why is that? Because if I don't put exactly where each of the methyls is, what I'm going to wind up getting is a very confusing structure where I don't know where they go. So it looks like 2 of the methyls are on the 3 position and one of the methyls is on the 5. So the way this is going to look is it's going to be 3, 3, 5-trimethyl. Alright? Tricky, right? But that is the way that we name it because now I know that 2 are on the 3 and one is on the 5. Now I know that one of you guys is asking, but Johnny, couldn't I just put 35? Wouldn't that be fine? Why do I have to put 335? No, you can't because then I wouldn't know where's the third one. Is it on the 5 or is it on the 3? So you need to say that 2 are on the 3 and 1 is on the 5. Alright? Then let's do the propyl. That one's easy. That one's on 4. Alright, so now I'm done with my roots. I'm done with my substituents with my root and substituents. Now all I need to do is put it together in the right name. So obviously this is going to be a long name, and I have to figure out what's the correct order to say all of these. You could have all of these steps right, but if you put the name in the wrong order on your exam, you still get the question wrong. So we have to be very meticulous about each step. So now when we're ordering this, we have to go in alphabetical order and prefixes don't count. So that means what are we going to talk about first? What's going to be the first substituent that we lead off on? It's going to be trimethyl. And the reason is because of the m. K? M beats p. So that means that I'm going to say that this is 335. I'm going to run out of room probably, so I'm going to try to write a little smaller. Trimethyl. I'm actually going to take myself off the page. Okay? 3,3,5-trimethyl. Then we've got 4-propyl because that one is the next one. Notice that every time that I have a number and a letter separated, Oops. Okay. Yeah. Every time that I have a number and a letter separated, I'm using a dash. Okay. So then the next thing would just be for a propyl and then I've just got to say what the ending is, and the ending is heptane without a space. So then it would just be heptane. Isn't that interesting where the m beat the p in alphabetical order? Alright, so if you got that answer, then awesome job. If you didn't, that's okay because I know that this is tricky. It's your first time. Alright, so now let's go ahead and move on to the next topic.

Here’s what students ask on this topic:

What is the IUPAC naming system and why is it important in organic chemistry?

The IUPAC naming system, established in 1919, provides a standardized method for naming chemical compounds. This system is crucial in organic chemistry because it eliminates confusion caused by the use of common names, which were often arbitrary and inconsistent. The IUPAC system uses a set of rules to name compounds based on their structure, including the identification of the longest carbon chain (root or parent chain), the presence of substituents (branches), their numerical locations, and functional groups (modifiers). This systematic approach ensures that each compound has a unique and universally understood name, facilitating clear communication among chemists worldwide.

How do you determine the parent chain in IUPAC nomenclature?

To determine the parent chain in IUPAC nomenclature, you need to identify the longest continuous chain of carbon atoms in the molecule. This chain is referred to as the root or parent chain. If there are multiple chains of equal length, choose the one with the most substituents. The parent chain forms the base of the compound's name, and its length is indicated by a prefix (e.g., meth- for one carbon, eth- for two carbons, etc.). Identifying the correct parent chain is essential for accurately naming the compound.

What are the rules for numbering the carbon chain in IUPAC nomenclature?

In IUPAC nomenclature, the carbon chain is numbered to give the substituents the lowest possible locants (numbers). Start numbering from the end of the chain closest to the first substituent. If there is a tie, proceed to the next substituent until a difference is found. If the substituents are symmetrically located, use alphabetical order to determine the direction. The goal is to assign the smallest possible numbers to the substituents, ensuring clarity and consistency in the compound's name.

How do you name substituents in IUPAC nomenclature?

In IUPAC nomenclature, substituents are named based on their structure and position on the parent chain. Common substituents include alkyl groups, which are named by replacing the -ane suffix of the corresponding alkane with -yl (e.g., methyl for CH₃, ethyl for C₂H₅). When multiple identical substituents are present, use prefixes like di-, tri-, and tetra- to indicate their quantity. Substituents are listed in alphabetical order in the compound's name, and their positions are indicated by numbers separated by commas and hyphens (e.g., 2,3-dimethylpentane).

What are the common prefixes for carbon chains in IUPAC nomenclature?

In IUPAC nomenclature, the prefixes for carbon chains indicate the number of carbon atoms in the parent chain. The common prefixes are: meth- (1 carbon), eth- (2 carbons), prop- (3 carbons), but- (4 carbons), pent- (5 carbons), hex- (6 carbons), hept- (7 carbons), oct- (8 carbons), non- (9 carbons), dec- (10 carbons), undec- (11 carbons), and dodec- (12 carbons). These prefixes form the basis of the compound's name, reflecting the length of the longest continuous carbon chain.