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Entgegen-Zusammen Naming Scheme for Alkenes Examples
相關課程
- Based on what we learned in the last video, let's see if
- we can name these two molecules here.
- Let's start with this blue one.
- So the first thing we notice, it is an alkene.
- It has a double bond right there.
- So we can, right from the get go, write that it's going to
- end with an ene.
- And then we need to figure out what the longest
- carbon chain is.
- And it looks like it's this one here.
- One, two, three, four, five, six, seven carbons.
- So it's going to be hept as our prefix on the alkene.
- Let me write it out a little bit further to the left.
- And then we have to figure out where the double bond is.
- And we always want to start numbering our carbons from the
- direction that bumps into the double bond first. And so the
- double bond is closer to this end of the
- chain than that end.
- So we're going to start numbering here.
- So one, two, three, four, five, six, seven.
- And so the double bond starts on the three carbon.
- So it is hept-3-ene.
- And we also have a functional group.
- We have a methyl group sitting on the number four carbon.
- So it is 4-methylhept-3-ene, which is correct.
- But we still haven't considered what's going on, on
- either side of the double bond.
- Whether the higher priority functional group is closer to
- each other on both sides of the double bond or whether
- they're farther away from each other.
- And to do that we first have to identify the higher
- priority group on each side of the double bond.
- So on the right side of the double bond-- let me circle it
- in magenta on each side-- so on the right side of the
- double bond, we only have one functional
- group, right over here.
- You could view this as an ethyl group.
- That's pretty obvious.
- There is no other functional group bonded to the carbon.
- There's only a hydrogen over here.
- Now on the left side it's a little bit less obvious.
- We have two functional groups from the point of
- view of this carbon.
- You have the methyl that we already pointed out, and you
- also have these three carbons right over here.
- You could view that as a propyl group.
- And what we need to do to identify the highest priority
- group is to use the Cahn-Ingold-Prelog namings or
- priority scheme that we learned several videos ago.
- And there you literally go from this carbon, you look at
- what it's bonded to, and you compare the atomic numbers.
- But in both cases it's a carbon to a carbon.
- This is a carbon to a carbon.
- So their atomic numbers are the same.
- So then you go one bond further away and you see which
- one is bonded to a higher atomic number atom.
- This carbon bonds to a carbon, which is a
- higher atomic number.
- This carbon only bonds to three hydrogens.
- This one does two hydrogens and one carbon.
- Because it's bonded to another carbon, it takes priority.
- This propyl group is a higher priority functional group.
- So now when we're trying to decide whether it is entgegen
- or zusammen, we look at these 2 groups.
- And we see that they are sitting on the same side of
- the double bond.
- They are both above the carbons.
- They are closer to each other.
- So this molecule is zusammen.
- Which on some levels, you can think of as the same thing as
- cis, but cis and trans stops applying when you start having
- more than two functional groups.
- In this case, we have three.
- So we would call this Z-4-methylhept-3-ene.
- And that's because the higher priority functional groups are
- on the same side of the double bond.
- Now let's do this one over here.
- And someone pointed out, rightly, that I had misspelled
- zusammen in the last video.
- It's actually spelled like this, zusammen.
- I had spelled it with two s's and one m.
- I guess you can forgive me.
- I don't speak German.
- But anyway, I thought I would point that out.
- Now let's try to label this thing right over here.
- So the first thing, this once again is an alkene.
- Let's identify the longest carbon chain here.
- So it looks like one, two, three, four, five, six, seven,
- eight carbons.
- Double bonds are closer to the left hand side.
- So we will start numbering here.
- One, two, three, four, five, six, seven, eight.
- So just the main chain is oct-- let me make sure I have
- some space here-- it is oct-3-ene.
- And then we have, well we have one functional group sitting
- off of the main chain.
- We have this bromine sitting right over there
- on the third carbon.
- So we would call this 3-bromooct-3-ene.
- And now we have to figure out is it entgegen or zusammen.
- So if we look on the carbon on the right hand side, it's
- pretty obvious that this is the only functional group.
- We just have a hydrogen there.
- So let me circle it in the magenta.
- And then on the left hand side we have two functional groups.
- We have this [UNINTELLIGIBLE]
- bromo or we have a bromine sitting right there.
- And then we have this propyl group.
- And because this propyl group is bigger, you might be
- tempted to say it takes higher priority.
- But remember, in the Cahn-Ingold-Prelog system, you
- give higher priority to the atom that has a
- higher atomic number.
- Bromine has an atomic number of 35.
- Carbon has an atomic number of only 6.
- So Bromine is actually higher priority.
- So this is the higher priority functional
- group right over here.
- So now for deciding whether it's entgegen or zusammen, we
- see that our higher priority groups are apart.
- They're on opposite sides of the double bond.
- This one is on top.
- This one is below.
- We are apart.
- So this is entgegen.
- Or we would write this is E-3-bromooct-3-ene.
- And E is for-- just as a bit of a refresher-- it's for
- entgegen, a word that I enjoy saying, entgegen.
- There you go.
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