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Course: Organic chemistry > Unit 3
Lesson 1: Naming alkanes- Representing structures of organic molecules
- Naming simple alkanes
- Naming alkanes with alkyl groups
- Correction - 2-propylheptane should never be the name!
- Common and systematic naming: iso-, sec-, and tert- prefixes
- Naming alkanes with ethyl groups
- Alkane with isopropyl group
- Organic chemistry naming examples 2
- Organic chemistry naming examples 3
- Naming a cycloalkane
- Naming two isobutyl groups systematically
- Organic chemistry naming examples 4
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Naming alkanes with ethyl groups
Naming conventions for alkane chains that have an ethyl group branch. Created by Sal Khan.
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At2:32, It wouldn't be possible to have a 6-ethyldecane since you would start from the end nearest the ethyl group on both. So there are two 5-ethyl groups(3 votes)
So the 5-ethyl was named so because it is closest to the end, and the 6-ethyl is NOT 5-ethyl because you have to keep counting from the end you used to count the first ethyl group? Is that why it is 5,6-diethyldecane rather than 5,5-diethyldecane? I'm not sure I explained it well enough...(15 votes)
Shouldn't the E be capital for 5 ethyldecane(1 vote)
No. IUPAC says that the names of elements and compounds are not proper nouns (like Darren and Portugal), so they are not capitalized in English unless they start a sentence. Hence:
5-Ethyldecane is a hydrocarbon, and 5-ethyldecane is also a branched hydrocarbon.(14 votes)
How do you work out which "methyl" or "ethyl" group to use? I don't understand(3 votes)- You pretend that you are a rat in a maze. You will receive a reward if you can identify the longest continuous chain of carbon atoms. You name that main chain (decane?)
Everything hanging off from the side is a side-chain. You may have methyl groups at C-3 and C-7, with ethyls at C-5, C-6, and C-7. That makes it a triethyldimethyldecane (ethyl comes before methyl in the naming).
Then you use numbers to locate every group on the main chain.
The name is 5,6,7-triethyl-3,7-dimethyldecane.(8 votes)
- the ethyl chain that sticks out of the 5th carbon, why don't we start counting the carbons where that alkyl meets with the 5th carbon? so instead of being ethyl, why is it not pentyl ?(1 vote)
In systematic naming, the base name always uses the longest possible chain of carbons. If you named this molecule with a pentyl substituent instead of an ethyl substituent, you would not be using the longest possible chain of carbons as your base.(8 votes)
What we would have done if the group attached to 6th Carbon atom had been a methyl or butyl.
Please help as soon as possible.(2 votes)- The names would have been 5-ethyl-6-methyldecane and
5-butyl-6-ethyldecane.(3 votes)
What if we had a 4 carbon chain attached to the '6th' Carbon instead of ethyl and 1 C attached to that 4 C chain at, say, position 1(attached C)?
Now, wouldn't there be 2 chains(longest) with 10 C each,
and they are asymmetrical?
How would we name this?
I think the structure would be something like this :
........../\/\
/\/\/\/
.......|
......./\/\
(don't mind the dots__place holder)(1 vote)
So as far as I've been able to follow, you would indeed create two possible 10 carbon chains which could act as the main chain. However instead of getting two competing names it would still have only one correct name under IUPAC rules. As for which decane chain to use, we would use the one with more branching occurring. In one chain we have two alkyl groups branching off, the ethyl chain and the new four carbon chain you added. In the other we have three branching groups, an ethyl group, a methyl group (1 carbon chain), and a butyl group (4 carbon chain). Using this chain the name should now be: 5-butyl-6-ethyl-4-methyldecane
Hope that helps.(3 votes)
Isn't. 4,5, diethyl redundant? The fact that there are ethyl groups on carbons 4 and 5 imply that there are two of them.(2 votes)
You're right that it is sort of redundant, but it's in the rules and we didn't make them up we just have to follow them.(1 vote)
What if the second ethyl group was attached to the 3rd carbon from left?
Will the naming be 3,6 diethyl decane?(1 vote)
What does the suffix -yl mean?
Answers appreciated.(1 vote)- Means it’s a carbon chain that’s bonded to another carbon chain.(2 votes)
So if my understanding is right, in order to name the second alkyl group, you have to use the same numbering convention? I mean, I can't count the carbons from the right for the first alkyl group and from the left for the second alkyl? Am I right?(1 vote)- You are right.
You start numbering the whole chain from one end.
Then, you label the groups according to those numbers.(2 votes)
2:32Video transcript
- [Instructor] I think we're
ready now to tackle some more or even more complicated examples. So let's draw something crazy here. So let's see, let me draw a chain, let me draw it like that. And so, like we've done
in all of the examples, you wanna find the longest chain. We could count from here, one, two, three, four,
five, six, seven, eight, or maybe it's one, two,
three, four, five, six, seven. No, or maybe it's one,
two, three, four, five, six, seven, eight, nine, 10. That is our longest chain. Let me make that in green. So our longest chain here is in green. So this backbone has 10 carbons in it. The prefix for 10 is dec, it is a alkane, since it has all single bonds. So we can write decane for the backbone. And then it has a group right here, it has a group right here. And this group consists
of one, two carbons attached to the backbone. The prefix for to carbons is eth, so this is an ethyl group. The yl is 'cause it's a group attached to the main alkane chain. So we call this ethyl decane. But we have to specify where
the ethyl group is attached and we wanna give it as
low of a number as possible so we start counting on
the side closest to it. So it's one, two, three, four, five. So this is 5-ethyl decane. Now let's complicate
this a little bit more. So let me just copy and paste this. Copy and then paste. So I have pasted it there. And let me complicate this
molecule a little bit more. Let me add another ethyl group to it. Let me add another ethyl group to it. So let's say it is, we have another ethyl group over there. Now, what is this going to be? Well, the longest chain is still going to be that thing in green, so it's still going to be a decane. It's still going to be a decane. But now we have two ethyl groups, one on the five carbon, one, two, three, four, five, and then one on the six carbon. And then one on the six carbon. So what we write here is, you might be tempted to write
5-ethyl, 6-ethyl decane, which really wouldn't be wrong, but it would just be maybe more letters than you wanna write. Instead, you write, instead you write, five comma six, diethyl, diethyl diethyl decane. The five, six tells us the two carbons on the main backbone that the
ethyl groups are attached to and the di says that we have two ethanes, two ethyl groups, I should
say, not ethane groups. Two ethyl groups, one over here, and one right over here.