Understanding Cycloalkanes and Their Naming

Alright, continuing on in our lectures, we're going to talk about Chapter 4 now. And we just talked about alkanes in Chapter 3, so now we're going to meet cycloalkanes, which are ring structures. They're going to be alkanes that are cyclic. And we're going to talk a lot about their stereochemistry, how they're going to be arranged in three-dimensional space. Just to have some examples, there are lots of organic compounds that contain rings. And here are two examples. Prostaglandin is a hormone in the body that is used for quite a few things. And cortisone is an example of a steroid. Now, steroids have three six-member rings. So we see one six-member ring here, one here, one here, and a five-carbon ring as well. So those are just some examples. Now, the first thing we're going to do is we're going to talk about how to name these cycloalkanes. So these are sometimes called allocyclic compounds. I won't use that term anymore this semester, but that's allophatic cyclic compounds. But cycloalkane is the most common way of referring to these saturated cyclic hydrocarbons. Now, our alkanes have the general formula CnH2n plus 2. But our cycloalkanes have this formula right here, CnH2n. The reason for that is as we close these rings, we're missing two carbons off the end carbons because they're in a closed structure. So just remember that this is the general formula for cycloalkanes. And what they're showing you here is they're showing you stick and ball or line, not line, I'm sorry, stick and ball models of cyclopropane, cyclobutane, cyclopentane, and cyclohexane. And they're also showing you the skeletal drawings. The skeletal drawings are just polygons, and what you'll see is you've already probably picked up. The way to name these things is simply to add cyclo in front of the general alkane name. So if you have a three carbon alkane that's cyclic, it's cyclo. propane. Normally a four carbon chain would be butane, but since it's cyclic we say cyclobutane. Pretty easy to pick up on how those are generally named. Now the actual IUPAC naming steps are going to be a little bit different. So the first thing you're going to do is you're going to count the number of carbons in the ring and you're going to count the number in the largest substituent. Now what they're showing you here are two different examples. So what you're going to have to take into account is sometimes these cyclic alkanes are going to be the parent molecule, and sometimes they're going to actually be a substituent. It just depends. So let's look at this first example here. So we have a methyl group here, CH3, and then we have a 1, 2, 3, 4, 5 carbon cyclic molecule. So we have more carbons in our cyclic molecule than we do our alkane. Here, or our alkyl group. So what we do is since we have more carbons in our cyclic molecule, that is the parent. So what you're going to do every time is you're going to look and see which has more carbons. The chain or the cyclic portion of the molecule. In this case, it's the cyclic portion. So what we would say is this is going to be a cyclopentane molecule. That's our parent and it's methyl substituted. So we say methyl cyclopentane. You say, wait a minute, where's the one? In this case, if you only have one substituent, you don't need to write the one dash out here in front of it. It's implied that that substituent is going to be on the first carbon. Now, if for some reason you have two or more, then you have to number everything. But if you only have one substituent, you don't need to write the one out there. It's sort of a redundancy. But if you write it, it's not wrong either. It's totally okay. So again, in this example, we have an alkyl-substituted cycloalkane. Now, let's look at the opposite of that. Here, we have a 3-carbon cyclic molecule, and we have a 4-carbon chain. Well, 4 carbons is obviously more carbons than 3, so in this case, we're going to choose the 4-carbon chain as our parent. So this is going to be a butane and it's going to have a cyclopropyl. substituent on it. So if you remember how to name the alkyl groups you do the cycloalkanes the same way. When they're substituents you just drop the a and e and add yl. So in this case we have a butane that has a cyclopropyl group attached to it in the first carbon position. So this would be one cyclopropyl butane. Now for carbon chains you go ahead and you have to to put all the numbers in there so just remember that you don't have to do it on a cyclic alkane when it's the parent if there's only one substituent so we but over here we have to write that in so this is a cycloalkyl substituted alkane so you'll have to do that every single time you see those two things in combination not too tricky though okay so now we're going to have to number the substituents when we have more than one, when we have an alkyl substituted cycloalkane. So let's look at this example over here. So over here we have a six carbon ring, so that's going to be of course cyclohexane. And what they're going to do is they're going to show you the difference between numbering with this example and this example. And so if we start numbering with either one of these methyl groups, it doesn't matter because they're equal of course. So we would say 1, 2, 3, 4, 5, and 6 going that direction. And if we started with the methyl group on this one, for example, and we counted 1, 2, 3, 4, 5, 6, that would give us two different substitutions. So over here, it would give us 1, 3-dimethylcyclohexane. Over here, this numbering would give us 1, 5-dimethylcyclohexane. Well, The goal is to have the overall lowest number of substituents, not the number of substituents, but the lowest overall numbering of your substituents so that you have the lowest carbon number that you can get. So one in three is much better than one in five. So this is the preferred method of counting. Now here's another example over here. This one has three substituents, so things get a little bit more tricky. So we're going to look at the correct way and then the incorrect way. So if we look at this group we see these two things side by side and then we see this methyl group over here. So what you want to do is you're generally going to count, start counting where these substituents are kind of grouped together. So if we count 1, 2, 3, 4, 5, 6, 7, we know that this is a 7 carbon cyclic molecule so the parent and suffix are going to be cycloheptane. And then for this molecule, we would have 2-ethyl-1-4-dimethyl-cycloheptane. So for our numbers, we get 2, 1, and 4. Now, if we come up over here and we look at this one, if you count starting at this ethyl group, you do 1, 2, 3, 4, 5, 6, 7. Well, Now we're going to have a 1-ethyl, that's not too bad, but then we have a 2,6-dimethylcycloheptane. Well, 1, 2, and 6, that's way higher than 2, 1, and 4, so this is not going to be an option. Now, if you start counting at this methyl group over here, and start counting towards those other substituents, 1, 2, 3, 4, 5, 6, 7, now we have a... 3 ethyl, 1,4-dimethyl. So now look at those numbers. It's not about how we number the ethyl, it's how we number everything overall. So we've got a choice of 2, 1, and 4, 1, 2, and 6, and 3, 1, and 4. Well, 1, 2, and 4 is much better than those other options. So generally when you have two groups clustered together you're going to try to count such that they both have the lowest numbers and usually that's going to be one and two like this example. Okay, so what I'd like you to do is let's try to name these. Let's try to name these. So you can pause me and give them a shot on your own. Alright, so hopefully you gave those a shot. What I did was I went to the whiteboard and I copied and pasted that molecule. Now... I also drew out, you can see on the right hand side, I drew out that group. So if you're not sure what that is, you may want to go back, but this is an isopropyl group. Isopropyl. So we have to name. Using this group, so we're going to use the I in the ISO now you could also name this one methyl ethyl totally, okay to do that too totally cool, but I want to use the kind of common name of Isopropyl to name this so just for funsies. I went ahead and wrote out the Kind of the IUPAC name for isopropyl one methyl ethyl and let's try to name this molecule so now We have a choice here. We can start counting around this molecule, and I think the first thing we have to realize is we have two substituents, two substituents only. So we have a choice of 1 and 2, or we have a choice of 1 and 2. That's a very skinny little writing there, but we have the choice between 1 and 2 like this. So what we're going to have to do is we have to find a way to do a tiebreaker to figure out which one of these carbons gets to be number one and which one gets to be number two. Well the trick on this is you're comparing a, let me, sorry let me get a different, I'll make this thicker one, that's annoying. Alright, so we have the choice between a methyl group and methyl. underlining this or isopropyl or this is also an ethyl group. So remember whenever you're doing the IUPAC names like the one methyl ethyl we're basing our substituent on the ethyl not the methyl. The ethyl is methyl substituted but we're prioritizing and alphabetizing by the ethyl. So we're basically comparing I and E to M. What that means is we're going to choose the E or the I because they of course come first in the alphabet. So we won't see 1 and 2 like this. This carbon will be 1 and this carbon will be 2. So we have basically a 1 isopropyl. I need to get rid of this mouse pad, it's causing me issues. 1-isopropyl-2-methylcyclohexane. Hopefully you can see that. Oh, it's kind of ugly. All right. 1-isopropyl-2-methyl-cyclohexane. Or we can also say 1-1-methyl-1-methyl-ethyl-2-methyl. And I'm not going to write the rest of it out. So the 2-methyl-cyclohexane. Both of those are totally legit. Alright, so let's look at the next one. Alright, hopefully you've given yourself a chance to work on this one. I'll transfer this and we'll work on this one together. Alright, so what I've done here is I've transferred the molecule. And what I did was this bottom group right here, I went ahead and came over here and drew it out. So that you could see what it really looks like. It's actually a t-butyl group. If you don't remember those, you have to go back and refresh yourself. But t-butyl group, or you could also name that 1,1-dimethylethyl. It's whatever floats your boat on that. The IUPAC name is fine, or the accepted common name. So now, how are we going to number this molecule? This looks much like what we did earlier with the examples. So, One way we're going to number this is to do 1, 2, 3, and 4. Okay, and then the other way we can number this is, and of course these numberings would go all the way around the molecule, not just stop there. But we're really concerned with our substituents, we can number this 1, 2, 3, and 4. And then, of course, we could do a lot of other numberings, but we really just want to keep our numbers as low as possible, remember, on our substituents. So we wouldn't want to go around this way and count, really. Nor would we want to start from this carbon here and count around like this because that's going to make the numbers incredibly high. So really these two options that we have currently are the best choices. And you probably instantly knew that this option in red is not the way we should number these. The way I have them written in black is the best way to number them. So we have 1, 2, and 4. So what I'm going to do is, if you realize that was the numbering, that's fantastic. And then what I'm going to do is pause this not to take up precious recording time with my writing. I'm going to put the name out there. So we know we have what? A 1-tertbutyl group or 1-parentheses-1,1-dimethyl-ethyl group. We have a methyl group and then we have a bromine group. And we have carbons 1, 2, and 4. So I'm going to pause it and write out the name. And something I forgot to do is you should go ahead and number all the way around just so that you get the number of carbons. So this is a cycloheptane, right? We've got seven carbons. So we would have four bromo, one t-butyl, two methyl, cycloheptane. And remember, as far as the butyl, t-butyl group goes, we don't alphabetize using the t. We use the b in butyl. branched substituents that have tert or sec in them, we don't alphabetize with those, but we do use iso for like isopropyl. Now I'm going to pause this and write out the other name using the IUPAC name for the t-butyl group. Alright, so here's the other name. You have 4-bromo-1-1-1-dimethylethyl. 2-methylcycloheptane. So the actual arrangement of the substituent stays the same. Remember Br, Bromo, and then Bu for the butyl or ethyl. Remember whenever we have one of these complicated substituents where the substituent itself is substituted, we alphabetize by the substituent. So B, then E, then M. and then our parent and our suffix. So not too bad. Let's go back to our slides and see if that's what we got. So current slides worked examples. And that is exactly. Now what they did is they wrote out tert-butyl. You can write out tert or you can just put the t. Either one of those are perfectly acceptable. All right. So this is a good place to stop the first video.

And here are two examples. Prostaglandin is a hormone in the body that is used for quite a few things. And cortisone is an example of a steroid. Now, steroids have three six-member rings. So we see one six-member ring here, one here, one here, and a five-carbon ring as well.

So those are just some examples. Now, the first thing we're going to do is we're going to talk about how to name these cycloalkanes. So these are sometimes called allocyclic compounds. I won't use that term anymore this semester, but that's allophatic cyclic compounds.

But cycloalkane is the most common way of referring to these saturated cyclic hydrocarbons. Now, our alkanes have the general formula CnH2n plus 2. But our cycloalkanes have this formula right here, CnH2n. The reason for that is as we close these rings, we're missing two carbons off the end carbons because they're in a closed structure. So just remember that this is the general formula for cycloalkanes. And what they're showing you here is they're showing you stick and ball or line, not line, I'm sorry, stick and ball models of cyclopropane, cyclobutane, cyclopentane, and cyclohexane.

And they're also showing you the skeletal drawings. The skeletal drawings are just polygons, and what you'll see is you've already probably picked up. The way to name these things is simply to add cyclo in front of the general alkane name. So if you have a three carbon alkane that's cyclic, it's cyclo.

propane. Normally a four carbon chain would be butane, but since it's cyclic we say cyclobutane. Pretty easy to pick up on how those are generally named. Now the actual IUPAC naming steps are going to be a little bit different.

So the first thing you're going to do is you're going to count the number of carbons in the ring and you're going to count the number in the largest substituent. Now what they're showing you here are two different examples. So what you're going to have to take into account is sometimes these cyclic alkanes are going to be the parent molecule, and sometimes they're going to actually be a substituent.

It just depends. So let's look at this first example here. So we have a methyl group here, CH3, and then we have a 1, 2, 3, 4, 5 carbon cyclic molecule.

So we have more carbons in our cyclic molecule than we do our alkane. Here, or our alkyl group. So what we do is since we have more carbons in our cyclic molecule, that is the parent. So what you're going to do every time is you're going to look and see which has more carbons. The chain or the cyclic portion of the molecule.

In this case, it's the cyclic portion. So what we would say is this is going to be a cyclopentane molecule. That's our parent and it's methyl substituted. So we say methyl cyclopentane. You say, wait a minute, where's the one?

In this case, if you only have one substituent, you don't need to write the one dash out here in front of it. It's implied that that substituent is going to be on the first carbon. Now, if for some reason you have two or more, then you have to number everything. But if you only have one substituent, you don't need to write the one out there. It's sort of a redundancy.

But if you write it, it's not wrong either. It's totally okay. So again, in this example, we have an alkyl-substituted cycloalkane.

Now, let's look at the opposite of that. Here, we have a 3-carbon cyclic molecule, and we have a 4-carbon chain. Well, 4 carbons is obviously more carbons than 3, so in this case, we're going to choose the 4-carbon chain as our parent. So this is going to be a butane and it's going to have a cyclopropyl. substituent on it.

So if you remember how to name the alkyl groups you do the cycloalkanes the same way. When they're substituents you just drop the a and e and add yl. So in this case we have a butane that has a cyclopropyl group attached to it in the first carbon position. So this would be one cyclopropyl butane. Now for carbon chains you go ahead and you have to to put all the numbers in there so just remember that you don't have to do it on a cyclic alkane when it's the parent if there's only one substituent so we but over here we have to write that in so this is a cycloalkyl substituted alkane so you'll have to do that every single time you see those two things in combination not too tricky though okay so now we're going to have to number the substituents when we have more than one, when we have an alkyl substituted cycloalkane.

So let's look at this example over here. So over here we have a six carbon ring, so that's going to be of course cyclohexane. And what they're going to do is they're going to show you the difference between numbering with this example and this example. And so if we start numbering with either one of these methyl groups, it doesn't matter because they're equal of course. So we would say 1, 2, 3, 4, 5, and 6 going that direction.

And if we started with the methyl group on this one, for example, and we counted 1, 2, 3, 4, 5, 6, that would give us two different substitutions. So over here, it would give us 1, 3-dimethylcyclohexane. Over here, this numbering would give us 1, 5-dimethylcyclohexane. Well, The goal is to have the overall lowest number of substituents, not the number of substituents, but the lowest overall numbering of your substituents so that you have the lowest carbon number that you can get. So one in three is much better than one in five.

So this is the preferred method of counting. Now here's another example over here. This one has three substituents, so things get a little bit more tricky. So we're going to look at the correct way and then the incorrect way.

So if we look at this group we see these two things side by side and then we see this methyl group over here. So what you want to do is you're generally going to count, start counting where these substituents are kind of grouped together. So if we count 1, 2, 3, 4, 5, 6, 7, we know that this is a 7 carbon cyclic molecule so the parent and suffix are going to be cycloheptane.

And then for this molecule, we would have 2-ethyl-1-4-dimethyl-cycloheptane. So for our numbers, we get 2, 1, and 4. Now, if we come up over here and we look at this one, if you count starting at this ethyl group, you do 1, 2, 3, 4, 5, 6, 7. Well, Now we're going to have a 1-ethyl, that's not too bad, but then we have a 2,6-dimethylcycloheptane. Well, 1, 2, and 6, that's way higher than 2, 1, and 4, so this is not going to be an option.

Now, if you start counting at this methyl group over here, and start counting towards those other substituents, 1, 2, 3, 4, 5, 6, 7, now we have a... 3 ethyl, 1,4-dimethyl. So now look at those numbers.

It's not about how we number the ethyl, it's how we number everything overall. So we've got a choice of 2, 1, and 4, 1, 2, and 6, and 3, 1, and 4. Well, 1, 2, and 4 is much better than those other options. So generally when you have two groups clustered together you're going to try to count such that they both have the lowest numbers and usually that's going to be one and two like this example.

Okay, so what I'd like you to do is let's try to name these. Let's try to name these. So you can pause me and give them a shot on your own.

Alright, so hopefully you gave those a shot. What I did was I went to the whiteboard and I copied and pasted that molecule. Now... I also drew out, you can see on the right hand side, I drew out that group.

So if you're not sure what that is, you may want to go back, but this is an isopropyl group. Isopropyl. So we have to name. Using this group, so we're going to use the I in the ISO now you could also name this one methyl ethyl totally, okay to do that too totally cool, but I want to use the kind of common name of Isopropyl to name this so just for funsies.

I went ahead and wrote out the Kind of the IUPAC name for isopropyl one methyl ethyl and let's try to name this molecule so now We have a choice here. We can start counting around this molecule, and I think the first thing we have to realize is we have two substituents, two substituents only. So we have a choice of 1 and 2, or we have a choice of 1 and 2. That's a very skinny little writing there, but we have the choice between 1 and 2 like this. So what we're going to have to do is we have to find a way to do a tiebreaker to figure out which one of these carbons gets to be number one and which one gets to be number two.

Well the trick on this is you're comparing a, let me, sorry let me get a different, I'll make this thicker one, that's annoying. Alright, so we have the choice between a methyl group and methyl. underlining this or isopropyl or this is also an ethyl group.

So remember whenever you're doing the IUPAC names like the one methyl ethyl we're basing our substituent on the ethyl not the methyl. The ethyl is methyl substituted but we're prioritizing and alphabetizing by the ethyl. So we're basically comparing I and E to M.

What that means is we're going to choose the E or the I because they of course come first in the alphabet. So we won't see 1 and 2 like this. This carbon will be 1 and this carbon will be 2. So we have basically a 1 isopropyl. I need to get rid of this mouse pad, it's causing me issues.

1-isopropyl-2-methylcyclohexane. Hopefully you can see that. Oh, it's kind of ugly.

All right. 1-isopropyl-2-methyl-cyclohexane. Or we can also say 1-1-methyl-1-methyl-ethyl-2-methyl.

And I'm not going to write the rest of it out. So the 2-methyl-cyclohexane. Both of those are totally legit.

Alright, so let's look at the next one. Alright, hopefully you've given yourself a chance to work on this one. I'll transfer this and we'll work on this one together.

Alright, so what I've done here is I've transferred the molecule. And what I did was this bottom group right here, I went ahead and came over here and drew it out. So that you could see what it really looks like.

It's actually a t-butyl group. If you don't remember those, you have to go back and refresh yourself. But t-butyl group, or you could also name that 1,1-dimethylethyl. It's whatever floats your boat on that. The IUPAC name is fine, or the accepted common name.

So now, how are we going to number this molecule? This looks much like what we did earlier with the examples. So, One way we're going to number this is to do 1, 2, 3, and 4. Okay, and then the other way we can number this is, and of course these numberings would go all the way around the molecule, not just stop there. But we're really concerned with our substituents, we can number this 1, 2, 3, and 4. And then, of course, we could do a lot of other numberings, but we really just want to keep our numbers as low as possible, remember, on our substituents.

So we wouldn't want to go around this way and count, really. Nor would we want to start from this carbon here and count around like this because that's going to make the numbers incredibly high. So really these two options that we have currently are the best choices. And you probably instantly knew that this option in red is not the way we should number these.

The way I have them written in black is the best way to number them. So we have 1, 2, and 4. So what I'm going to do is, if you realize that was the numbering, that's fantastic. And then what I'm going to do is pause this not to take up precious recording time with my writing.

I'm going to put the name out there. So we know we have what? A 1-tertbutyl group or 1-parentheses-1,1-dimethyl-ethyl group. We have a methyl group and then we have a bromine group. And we have carbons 1, 2, and 4. So I'm going to pause it and write out the name.

And something I forgot to do is you should go ahead and number all the way around just so that you get the number of carbons. So this is a cycloheptane, right? We've got seven carbons. So we would have four bromo, one t-butyl, two methyl, cycloheptane. And remember, as far as the butyl, t-butyl group goes, we don't alphabetize using the t.

We use the b in butyl. branched substituents that have tert or sec in them, we don't alphabetize with those, but we do use iso for like isopropyl. Now I'm going to pause this and write out the other name using the IUPAC name for the t-butyl group.

Alright, so here's the other name. You have 4-bromo-1-1-1-dimethylethyl. 2-methylcycloheptane.

So the actual arrangement of the substituent stays the same. Remember Br, Bromo, and then Bu for the butyl or ethyl. Remember whenever we have one of these complicated substituents where the substituent itself is substituted, we alphabetize by the substituent. So B, then E, then M. and then our parent and our suffix.

So not too bad. Let's go back to our slides and see if that's what we got. So current slides worked examples. And that is exactly. Now what they did is they wrote out tert-butyl.

You can write out tert or you can just put the t. Either one of those are perfectly acceptable. All right. So this is a good place to stop the first video.

Transcript for:Understanding Cycloalkanes and Their Naming

Transcript for:
Understanding Cycloalkanes and Their Naming