Naming alkenes examples
Naming Alkenes Examples. Created by Sal Khan.
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- at the end, can the numbering of the carbons go clockwise instead of counterclockwise? if not then why not?(55 votes)
- Khan explained that either ends of the double bond could be 1 but if an Alkyl molecule is on one side that has to be 1 and the other side of the double bond has to be 2.(60 votes)
- @4:00- cant we write Hept-2,4-diene,huh??(21 votes)
- hepta-2,4-diene whenever you have a di, tri, poly etc., you must put "a" after your hept in this case. (so not hept-2,4-diene OR hept-2,4-ene)
When you write it this way: 2,4-heptadiene, it should clarify.(36 votes)
- I have several questions about naming carbon structures:
1) What if I have double bonds in an alkyl group? How do I manage the yl/ene suffixes? Do I choose the chain that is longest, or is there a preference for the one with most double bonds?
2) How do I name multiple cyclo-groups that are fused like a web? Like this <=><> or this <=|=>.
3) I named a complicated structure for practice, is it correct? 1-cyclobutane-16-ethyl-4-(2-(4-hexylcycloheptyl)ethyl)triiso-4-ene.
Thank you so much!!!(13 votes)
- 1. You pick the longest continuous chain of C atoms that goes through as many of the double and triple bonds as possible. Double and triple bonds have the same priority for naming, so you start numbering from the end nearest to a multiple-bonded carbon. If there are double or triple bonds in the side chains, you must name these as alkenyl or alkynyl groups.
Thus, you might have a molecule such as 4-ethenyl-5-methylhept-4-ene-1-yne. Ethenyl is named before methyl because e comes before m, and ene is named before yne because e comes before y.
2. The numbering of polycyclic compounds is a bit complicated. Let us take a simple example. The compound known as "hexahydroindan" consists of a cyclononane ring with a bond between C-1 and C-5. If you draw its structure, it looks like a cyclohexane ring joined side-by-side with a cyclopentane ring. Atoms 1 and 5 are shared by the two rings (they are called bridgehead carbons).
To name the compound, you start at one of the bridgehead atoms and number by going around the largest ring possible that leads to the other bridgehead ( the 6-membered ring). Then you continue around the next smaller ring (the 5-membered ring) back to C-1. The basic name is made from the number of rings and the total number of C atoms that include the bridgehead carbons and the bridging carbons. Thus the basic name is bicyclononane.
Now we have to specify the size of the fused rings. To do this, we name the compound as bicyclo[x.y.z]nonane, where x, y, and z are the number of C atoms in each path between the bridgeheads, listed in descending order. The 6-membered ring has 4 C atoms between the bridgeheads; the 5-membered ring has 3 C atoms between the bridgeheads; and the two bridgeheads themselves are joined by zero carbons. So the full name is bicyclo[4.3.0]nonane.
Other isomers could be the [5.2.0] and [[6.1.0] structures. You could also have tricyclic isomers such as tricyclo[3.2.2]nonane. Could you draw these structures?
The systematic names can get quite complicated. Look up the systematic name for "cubane", for example.
Hope this helps.
3. Sorry, I have no idea what structure you are trying to name.(7 votes)
- At4:04, wouldn't it be hept-2-4-diene?(5 votes)
- The correct name is hepta-2,4-diene. The base name for the main chain is hepta . But the "a" is dropped when it occurs before a vowel. Thus we get hept-2-ene instead if hepta-2-ene, but hepta-2,4-diene instead of hept-2,4-diene. The numbers are ignored. So in the first case we have "a" before "e" but in the second case we have "a" before "d" .(9 votes)
- can someone please give me all the rules in order that explain how to name alkanes.(3 votes)
- Rule 1) Find the longest chain of carbons. (It does NOT have to be straight.) This goes at the end of the name.
Rule 2) Identify any side branches. These go just before the longest chain name.
Rule 3) If a side chain occurs more than once, indicate how many time it occurs. This goes just before the branch name. di - two, tri = three, tetra = four. After that the prefixes are the same as for chain-length.
Rule 4) When necessary, indicate which carbon the side-branch comes from by counting from the end of the main chain that gives the lowest possible number.(13 votes)
- At4:00, it says that hept-2,4-ene is the correct way to write it, but I thought the other way(2,4 heptene) was the correct way. Is there any real difference between them?(2 votes)
- One of many good answers for a very similar question on this page can be found here:
- Once you name the molecule according to the rings and double bonds you still need to follow the IUPAC Naming system. The last molecule is mostly right, but should be named: 1,2,4-trimethyl.... The reason is that the IUPAC naming system wants the substituent ordered with the lowest possible numbers. The 1,2,4 version is clearly lower than the 1,4,6 version. It's something easy to mix up though. Same molecule, different order in numbering, however.(1 vote)
- I'm just learning this from the videos, but based on the explanation I thought if you had cyclohexene the double bond would have to be between the 1- and 2- carbons, so 1,2,4-trimethyl would put methyl groups on either end of the double bond instead of putting them at just one end of the double bond.(6 votes)
- So, how do you number the carbons if the double bond is in the middle of a single chain?(1 vote)
- Counting the carbons is the same, even if there is a double bond. The difference would be the ratio of hydrogens to carbons.(2 votes)
- No question, but a I recommend something - you should name the last cyclo-structure 2,3,5-tricyclohex-1-ene. Because you should always get the smallest numbers in front of the prefixes and therefore you should count clockwise in this case.(0 votes)
- The rule is that you want the lowest numbered substituents, not the lowest sum of all substituents. So while 1+2+3+5 < 1+1+4+6, 1+1 < 1+2. Therefore 1,4,6-tricyclohex-1-ene is correct, but as the -1- is implied it is 1,4,6-tricyclohexene.(3 votes)
- I might be wrong but do we ignore where the double bond is if its a cyclo? for example you hace 1,4,6-Trimethylcyclohene instead of 1,4,6-Trimethylcyclohex-1-ene. The first 1 is where the first methyl group is but its also where the double bond begins. do we ignore it?(1 vote)
- The double bond takes priority in the numbering of an alkyl-substituted cyclohexene.
The two alkene carbon atoms must be numbered 1 and 2, and you number around the ring in the direction that gives the lowest numbers.
C-6 is always next to C-1.
You don't have to include the number of the double bond in the name, because its carbons are always C-1 and C-2, but you do have to number the positions of the substituents.(3 votes)
Everything we've named so far has been an alkane. We've seen all single bonds. Let's see if we can expand our repertoire a little bit and do some alkenes. So let's look at this first carbon chain right here. And actually, here I drew out all of the hydrogens just to remind you that everything we were doing before with just the lines, it really was representing something like this. When you start having the double bonds, and we'll explain it in more detail later on, it actually starts to matter a little bit more to draw the constituents, because there's actually different ways that you can arrange it. Because these double bonds, you can imagine, they're more rigid, you can't rotate around them as much. But don't think about that too much right now. Let's just try to name these things. So like we always do, let's try to find the longest chain of carbons. And there's only one chain of carbons here. There's one, two, three, four, five, six, seven carbons in that chain. So we're going to be dealing with hept, that is seven carbons. But it's not going to be a heptane. Heptane would mean that we have all single bonds. Here we have a double bond, so this is going to be an alkene. So this tells us right here that we're dealing with an alkene, not an alkane. If you have a double bond, it's an alkene. Triple bond, alkyne. We'll talk about that in future videos. This is hept, and we'll put an ene here, but we haven't specified where the double bond is and we haven't numbered our carbons. When you see an alkene like this, you start numbering closest to the double bond, just like as if it was a alkyl group, as if it was a side chain of carbons. So this side is closest to the double bond, so let's start numbering there. One, two, three, four, five, six, seven. The double bond is between two and three, and to specify its location, you start at the lowest of these numbers. So this double bond is at two. This is actually hept-2-ene. So this tells us that we have a seven carbon chain that has a double bond starting-- the ene tells us a double bond. Let me write that down. So this double bond right there, that's what the ene tells us. Double bond between two carbons, it's an alkene. The double bond starts-- if you start at this point-- the double bond starts at number two carbon, and then it will go to the number three carbon. Now you might be asking, well, what if I had more than one double bond here? So let me draw a quick example of that. Let's say I have something like, one, two, three, four, five, six, seven. So this is the same molecule again. One, two, three, four, five, six, seven. The way we drew it up here, it would look something like this. What if I had another double bond sitting right here? How would we specify this? Well, once again we have seven carbons. One, two, three, four, five, six, seven. So we're still going to have a hept here. It's still going to be an alkene, so we put our ene here. But we start numbering it, once again, closest to the closest double bond. So one, two, three, four, five, six, seven. But now we have a double bond starting at two to three, so it would be hept-2. And we also have another double bond starting from four and going to five, so hept-2,4-ene. That's what this molecule right there is. Sometimes, this is the-- I guess-- proper naming, but just so you're familiar with it if you ever see it. Sometimes someone would write hept-2-ene, they'll write that as 2-heptene, probably because it's easier to say 2-heptene. And from this, you would be able to draw this thing over here, so it's giving you the same amount of information. Similarly over here, they might say 2,4-heptene. But this is the specific, this is the correct way to write it. It let's you know the two and the four apply to the ene, which you know applies to double bonds. Let's do a couple more. So over here, I have a double bond, and I also have some side chains. Let's see if we can figure out how to deal with all of these things. So first of all, what is our longest chain of carbons? So we have one, two, three, four, five, six. Now we could go in either direction, it doesn't matter. Seven carbons or seven carbons. Let's start numbering closest to the double bond. The double bond actually will take precedence over any other groups that are attached to it. So let's take precedence-- well, over any other groups in this case. There will be other groups that will take precedence in the future. But the double bond takes precedence over this side chain, this methyl group. But it doesn't matter in this case, we'd want to start numbering at this end. It's one, two carbon, three carbon, four carbon, five carbon, six carbon, seven carbon. So we're dealing with a hept again. We have a double bond starting from the second carbon to the third carbon. So this thing right here, this double bond from the second carbon to the third carbon. So it's hept-2,3-ene-- sorry, not 2,3, 2-ene. You don't write both endpoints. If there was a three, then there would have been another double bond there. It's hept-2-ene. And then we have this methyl group here, which is also sitting on the second carbon. So this methyl group right there on the second carbon. So we would say 2-methyl-hept-2-ene. It's a hept-2-ene, that's all of this part over here, double bonds starting on the two if we're numbering from the right. And then the methyl group is also attached to that second carbon. Let's do one more of these. So we have a cycle here, and once again the root is going to be the largest chain or the largest ring here. Our main ring is the largest one, and we have one, two, three, four, five, six, carbon. So we are dealing with hex as our root for kind of the core of our structure. It's in a cycle, so it's going to be cyclohex. So let me write that. So it's going to be cyclohex. But it has a double bond in it. So it's cyclohex ene, cyclohexene. Let me do this in a different color. So we have this double bond here, and that's why we know it is an ene. Now you're probably saying, Hey Sal, how come we didn't have to number where the ene is? So if you only have one double bond in a ring, it's assumed that one end point of the double bond is your 1-carbon. When you write just cyclohexene, you know-- so cyclohexene would look just like this. Just like that. You don't have to specify where it is. It's just, one of these are going to be the double bond. Now when you have other constituents on it, by definition or I guess the proper naming mechanism, is one of the endpoints of the double bond will be the 1-carbon, and if any of those endpoints have something else on it, that will definitely be the 1-carbon. So these both are kind of the candidates for the 1-carbon, but this point right here also has this methyl group. We will start numbering there, one, and then you want to number in the direction of the other side of the double bond. One, two, three, four, five, six. So we have three methyl groups, one on one. So these are the-- let me circle the methyl groups. That's a methyl group right there. That's a methyl group right there. That's just one carbon. So we have three methyl groups, so this is going to be-- it's at the one, the four, and the six. So it is 1, 4, 6. We have three methyl groups, so it's trimethyl cyclohexene. 1, 4, 6-trimethylcyclohexene. That's what that is, hopefully you found that useful.