Correction - 2-propylheptane should never be the name!
About this video
This short video corrects an earlier example. A name like 2-propylheptane cannot be correct, because a propyl group on the second carbon of a heptane chain means the true longest chain is longer and runs through the branch. The correct backbone is nonane, and the name becomes 4-methylnonane. The lesson is to always check for the longest chain before naming.
You'll be able to
- spot when a chosen chain is not the longest and re-check the backbone.
- recognise why 2-propylheptane is an invalid name and rebuild it as 4-methylnonane.
- explain the rule that the base name must come from the longest carbon chain.
Key words
- Longest chain rule
- The base name must come from the longest continuous carbon chain in the molecule.
- Propyl group
- A three-carbon branch attached to a larger chain.
- Locant
- The position number of a branch on the backbone.
- Correction
- Re-checking a structure against the naming rules to find where a first attempt went wrong.
Transcript
In the last video, we tried to draw a 2-Propylheptane. And we did our best attempt at drawing it, but it was pointed out that this wouldn't even be called a 2-Propylheptane to begin with. So you actually should never see something called 2-Propylheptane. Let me show you what I'm talking about.
So when do you see something like this, you might immediately say-- and the way we drew it was actually correct, it just wouldn't be called 2-Propylheptane. So you say heptane. So that is a seven carbon alkane. No double bond, so one, two, three, four, five, six, seven.
And then on the second carbon, so you have one, two, three, four, five, six, seven, we have a propyl group. Propyl, that is three carbons. So on the second carbon we have a propyl group. That's three carbons. So that is one, two, three. And so the way we drew it was just like this.
And so if someone gave you 2-Propylheptane, this would be what you would draw. But you wouldn't call this 2-Propylheptane. Because remember, if you're given the molecule you look for the longest chain and the longest chain here is not the heptane chain. It is not one, two, three, it is not this thing in magenta-- in this kind of mauve color.
It's this chain where you start over here. If you start over here, one, two, three, four, five, six, seven, eight, nine, you actually get a longer chain. So this would actually be the backbone of this molecule right over there. That right over there would be the backbone, and so you would number it.
You start numbering closest to the group that's attached, so one, two, three, four, five, six, seven, eight, nine. So you have nine carbons in your backbone, so we're dealing with nonane. We're dealing with nonane. And you have a methyl group: one carbon attached to the fourth carbon of our main backbone.
So this is going to be four methyl, this is our methyl group right here, 4-Methylnonane. So it was brought up, I think the user name is Minoctu, and they correctly corrected me, that there would never be such a thing as 2-Propylheptane. I just made that up. If someone were to, kind of, label this molecule they would call it 4-Methylnonane and ask you draw it.
But either way, both of these would point you in the right direction. This would just be the incorrect name for it, because you'd be looking at-- if someone gave you this molecule and you named it this way, that would be incorrect. So I apologize for this. This is 4-Methylnonane.
If you do heptane you're not finding the longest chain.
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