Alkane and cycloalkane nomenclature III
Practice naming hydrocarbons with branches and rings. Also discusses classification of carbons: primary, secondary, tertiary, and quaternary carbons. Created by Jay.
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- at6:00, could it also be it be named 2-methylbutyl since you can number the chain so the methyl group can be located on the 2 carbon instead of the 3?(53 votes)
- Not as a substituent. Because we're assuming it's attached to a bigger molecule, you start numbering from the carbon that's bonded to the parent chain.(60 votes)
- Why is it named 1-methyl ethyl and not 1-ethly methyl . I thought it was named alphabetically?(17 votes)
- the ethyl is the largest chain so it will be at the end(29 votes)
- At6:42he used the prefic "neo," but never explains why. The substituent seems to follow the pattern which uses the "tert" prefix so I'm confused why it's not called tert-pentyl.(19 votes)
- Neo prefix is used for when the (CH3)3 branch is attached to another carbon/carbon chain.
Tert prefix is when the (CH3)3 branch is attached to any R group (i.e. any other substituent like Br/Cl etc).
If we look on the video for the neo-pentyl chain, if carbon number 1 were another substituent (i.e. not carbon/carbon chain) it would be tert-butyl rather than neo-pentyl.
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- i cant understand why do we use "iso"......
thanks in advance(3 votes)
- When you see a "Y-structure" (Look at the example 2 and 4 in the beginning) as a substituent attached to your longest carbon-chain, the backbone of your molecule, you should immediately think "-Iso".
Why?: It´s because, as you can see in example 2 and 4, you have two protruding methyl-groups attached from a singe carbon (The second carbon in example 2 and the third carbon in example 4), you could call it "-Sec" but whenever you see a single "Y-structure" without any other branching it is more common to call it "-Iso".
Iso in organic chemistry means "the same", or "no change" and as you can see in the "Y-structure", it looks the same from each angle.(8 votes)
- Can the neopentyl also be text-pentyl?(2 votes)
- IUPAC Name of Tert-pentyl is 1,1-dimethylpropyl
IUPAC Name of neopentyl is 2,2-dimethylpropyl :D(8 votes)
- For classifying carbons; alpha, beta and gamma are also used instead of numbering them with a degree sign. Is this statement right?(1 vote)
- The alpha carbon is the carbon attached to a functional group; the beta carbon is the next one out; the gamma carbon is the next one after that, and so on. The furthest carbon atom from the functional group is called the omega carbon. For example, linolenic acid is an omega-3 fatty acid containing three C=C bonds. (Omega is the last letter in the Geek alphabet. The 3 means that the double bond furthest from the functional group is between the third and fourth carbon atoms from the end).(5 votes)
- At12:48, isn't that carbon bonded to 4 carbons(3 votes)
- no it bonded to 3 carbons and the extra one available bond is attached to hydrogen , and this is why its called Primary(1 vote)
- At about6:00Is there any logic behind the use of the -am(yl) suffix and the neo- prefix?(1 vote)
- Amyl is the common name for a 5-carbon group. Pentyl is the *IUPAC name.*
Just as iso means a methyl group on the next-to-last carbon, neo means two methyl groups on the next-to-last carbon.(5 votes)
- At1:05, why is the longest carbon chain consisting of only 2 carbons? Why cant it be 3 carbons?(1 vote)
- You number the continuously-attached carbons of the side-chain from the carbon that is attached to the main chain.
If you start at C-1, you can go out only one carbon with your pencil. To get to the third carbon without removing your pencil, you would have to re-trace your steps back to C-1, and that is not permitted.(3 votes)
- At6:00, why is it 3-mythlbutyl and not 2-mythlbutyl? Isnt it shorter to name it from right to left, or is there a rule to start from the center in case of branched off substituents?(1 vote)
- The rule is that the numbering of a side-chain starts with the carbon attached to the main chain numbered as C-1.(3 votes)
So how do we name this molecule? Well, we start with the longest carbon chain. So there are seven carbons in my longest carbon chain. So I would call this heptane. And I number it to give the substituent the lowest number possible. So in this example, it doesn't really matter if I start from the left or from the right. In both examples, you would end up with a 4 for your substituent there. Now, this substituent looks different from ones we've seen before. There are three carbons in it, but those carbons are not in a straight-chain alkyl group. So if I look at it, right there are three carbons, but they're not going in a straight chain. They're branching of branching here. So this is kind of weird. How do we name this substituent? Well, down here, I have the same substituent, and I'm going to draw this little zigzag line to indicate that that substituent is coming off of some straight-chain alkane. And when you're naming a complex substituent like this, you actually use the same rules that you would use for a straight-chain alkane. So you first identify the longest carbon chain, which in this case is only two carbons. So that would be an ethyl group coming off of my carbon chain. So I'm going to go ahead and name that as an ethyl group. I'm going to go ahead number it to give my branching group there the lowest number possible. So I go 1 and 2. So what is my substituent coming off of my ethyl group? Well, that's a methyl group coming off of carbon 1. So I name it as 1-methylethyl. OK, so now, that complex substituent is named as 1-methylethyl. So I could go ahead and put that into my name. So coming off of carbon 4, I have 1-methylethyl. And I'm going to put that in parentheses. And all of that is coming off of carbon 4 for my molecule. So 4-(1-methylethyl) heptane would be an acceptable IUPAC way of naming that molecule. So if your naming your complex substituent as 1-methylethyl, that's the official IUPAC way, but there are also common names for these complex substituents. So the common name for 1-methylethyl is isopropyl. So isopropyl is the common name. And isopropyl is used so frequently that it's perfectly acceptable to use isopropyl for the name of this molecule as well. So you could have said, oh, this is 4-isopropylheptane, and you would have been absolutely correct. So that's yet another IUPAC name. So iso means same, and it probably comes from the fact that you have these two methyl groups giving you this Y shape that are the same. So that's one complex substituent, one that has three carbons on it. Let's look at a bunch of complex substituents that have a total of four carbons on them. So all these guys have a total of four carbons. And let's do that same trick with the zigzag line so we can ignore the rest of the molecule and just think about them as being alkyl groups. So how do I name these? The same steps. You find your longest carbon chain. So for this one, the longest carbon chain would be 3. It's an alkyl group so it's a propyl. And when you number it, this would get a 1, this would get a 2, and this would get a 3. So you have a methyl group coming off of carbon 1 here. So it would be 1-methylpropyl as the name of that complex substituent. The common name for that is sec-butyl. So butyl because there are a total of four carbons. Let's do the next one, so 1, 2, 3 again. So when I number it 1, 2, 3, I can see that it would be propyl once again. So I go ahead and write propyl here. And what is coming off of that group? Well, I have a methyl group coming off of carbon 2 this time, so it would be 2-methylpropyl for this complex substituent. The common name for this is isobutyl. So butyl again because there are four total carbons in this complex substituent. Iso because, once again, you have these two methyl groups, so they're like the same, so you get that Y formation. So that's isobutyl. The next one, longest carbon chain, there are two carbons in my longest carbon chain so that would be ethyl. And when I number my longest carbon chain, I can see that I have two methyl groups, and each of those methyl groups is coming off of carbon 1. So I would say this would be 1,1-dimethylethyl. So 1,1-dimethylethyl would be the IUPAC name for this complex substituent. You will also see tert-butyl. So tert-butyl is probably used even more frequently. Again, butyl because there are a total of four carbons here. So those are the three possibilities for a complex substituent with a total of four carbons. Let's look at just a few of the possibilities for complex substituents that have five carbons. There are actually much more than this, but these are the ones that are most commonly used. So let's just focus in on these two. So once again, we'll draw our zigzag line to represent the fact that this is actually connected to some straight-chain alkane. And once again, we find our longest carbon chain-- 1, 2, 3, 4. So that would be butyl. And when I number that carbon chain-- 1, 2, 3, 4-- I can see that I have a methyl group coming off of carbon 3. So it would be 3-methylbutyl for the IUPAC name. And this is also called isopentyl. So you could say isopentyl since there are five carbons now, and iso, because again, you have this methyl group and this methyl group looking like a Y. They're like the same thing. Or I've seen this called isoamyl before. So isoamyl or isopentyl are acceptable IUPAC names as well. What about this one on the right? Longest carbon chain-- 1, 2, 3. So that would be propyl. And numbering it 1, 2, 3, immediately it is obvious that you have two methyl groups coming off of carbon 2. So it would be 2,2-dimethylpropyl, otherwise known as neopentyl since once again you have five carbons for these. So again, there are many more, and we'll stop with those. And so that gives you an idea about how to approach naming complex substituents. And of course, when you name complex substituents, you have to use them when you're naming straight-chain or cycloalkane molecules. So let's look at a cycloalkane molecule, and let's see how to name this guy. Well, I have four carbons in my ring, and I have four carbons in this group. So tie goes to the cycloalkane. So remember from the last video, if you have an equal number of carbons in your ring as with your chain, you're going to name it as an alkyl cycloalkane. The cycloalkane wins the tie. So there are four carbons, so this'll be cyclobutane. So let's go ahead and write cyclobutane here. And once you've determined that you're going to name it as a cycloalkane, then you have to look at this complex substituent and say, OK, well, that's 1, 2, 3, so that would be propyl. And then when you number that complex substituent 1, 2, 3, obviously there is a methyl group coming off of carbon 1. So you would write 1-methylpropyl. And if you wanted to, you could identify that 1-methylpropyl as coming off of carbon 1 of your cyclobutane. So you could put this in parentheses and write 1-(1-methylpropyl)cyclobutane. Or you could just leave the one off, and say (1-methylpropyl)cyclobutane, because, again, it is implied. What is the common name for this complex substituent? So 1-methylpropyl, and we go back up here, and we find 1-methylpropyl was also called sec-butyl. So we could also have named this molecule sec-butylcyclobutane. So let's go ahead and write that. So sec-butylcyclobutane is a perfectly acceptable IUPAC name as well. So it just depends. Do you want to do it the official IUPAC way, or do you want to memorize some of these common names? Let's do one more thing in this video. Let's classify carbons. OK, so we're going to something called classification of carbons. This is a topic that comes up over and over again throughout inorganic chemistry course, so the earlier you learn this concept the better off you are. So classification of carbons. So let's say I have carbon connected to three hydrogens, and then I also have it connected to one other carbon in some R group. I want to know how to classify this carbon. So this carbon is connected to one other carbon. So therefore, we say it is primary. So that is a primary carbon right there. Let's take off one of the hydrogens, and let's put on another R group. So I'll make it R prime to distinguish it from the first R group. So this time, if I wanted to know the classification of this carbon, it's connected to two other carbons, so it is said to be secondary. So it is a secondary carbon like that. And I'm going to once again take off one of the hydrogens. So I'll make it an R double prime group. And now if I wanted to classify my central carbon, now this is connected to 1, 2, 3 other carbons, so it is said to be tertiary. So that is a tertiary carbon like that. And finally, I have one more example, of course. I take off the last hydrogen. So now I have R, R prime, R double prime, and R triple prime. So what is the classification of this carbon now connected to four other carbons? So it is said to be quaternary. So that is a quaternary carbon right here. All right, so if I'm trying to think about where some of these common names come from I can see, oh, well, right here I have-- if carbon's bonded to two other carbons, well, that would be secondary. So S-E-C for my prefix. So let's go back up here, and let's see if we can find those examples. So here I have this carbon bonded to two other carbons. So this carbon was said to be secondary, so I think that's where this comes from. I've never seen that explained in a textbook or anywhere, but it just makes sense. So it's ignoring the fact that this carbon is actually attached to a ring. It's saying this carbon on my complex substituent is bonded to two other carbons, so it is secondary on that complex substituent. What about tertiary? So carbon bonded to three other carbons is said to be tertiary. So if I go back up here again I can say, well, that would make sense, because if I look at this carbon, it's bonded to three other carbons, right? So I could say that is a tertiary carbon. And once again I'm ignoring the fact that this carbon is actually bonded to another carbon on the ring. So if you just look at the complex substituent, that carbon is said to be tertiary, which I think is where the name comes from. Let's do one more example of assigning classification of carbons to this molecule. So let's look at this carbon right here. This carbon is bonded to one other carbon and three hydrogens. So this carbon is said to be primary. This carbon right here is bonded to two other carbons, so it is said to be a secondary. This carbon right here is bonded to three other carbons so it is tertiary. This carbon is bonded to one other carbon so it is primary. This carbon is bonded to three other carbons, so it is tertiary. And all of the carbons on the ring right here are bonded to two other carbons, so they are all said to be secondary. So that's a very important skill to develop classifying your carbons. Again, this will come up in future videos with different functional groups.