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- If I were to come over to your place and say, name this
- molecule, the first thing you would do is you'd say, well,
- look, we have a double bond, So we're
- dealing with an alkene.
- We have four carbons in the longest chain.
- There's actually one chain here.
- So it's one, two, three, four carbons.
- So the prefix will be but-, and regardless of which end
- you count from, the double bond starts at the number two
- carbon, so this is but-2-ene.
- I'd say, well, that's very good, and that is correct
- based on everything we've learned.
- But what about what about this molecule?
- What about the molecule that looks like this?
- And you'd go through the same process.
- You'd say it has a double bond.
- It's an alkene.
- Our longest chain has four carbons:
- one, two, three, four.
- Regardless of which end you start counting from, the
- double bond starts at the number two carbon: one, two,
- three, four.
- So once again, you say, hey, Sal, that's also but-2-ene.
- And I'd point out that these are each fundamentally
- different molecules.
- And the reason why they're fundamentally different
- molecules is because this double bond can't rotate.
- It's got that pi bond there for that second bond that
- keeps this carbon-carbon bond rigid.
- So it's not like the molecule can switch configurations
- between this-- it's not like it can switch conformations
- between this and that.
- It cannot rotate around this double bond, so these are
- different molecules.
- These are different molecules that will behave different
- chemically.
- And because they behave different, we have to have
- different names for them.
- Now, there's two different naming conventions.
- One is the simpler naming convention, and it works when
- we have one functional group on each of the carbons in the
- double bond.
- And then in the next two videos, we'll talk about the
- slightly more advanced naming scheme that will work when we
- have more than one functional group.
- So the first way to name it is if you have
- your functional groups.
- So let me circle the functional groups.
- So in this top one, our functional groups are on
- opposite sides.
- They're apart.
- They're on opposite sides of the carbon-carbon double bond.
- This is on the top, This is on the bottom.
- In this molecule right here, our functional groups are on
- the same side.
- So when our functional groups are on different sides, we
- could call it trans.
- We could call it trans, literally, I believe, Latin
- for, apart, or Latin for opposite.
- I'm not a Latin scholar, so forgive me.
- But the functional groups are apart, so we call it
- trans-but-2-ene.
- The other convention to use comes from the German forepart
- and there we call it entgegen.
- And instead of writing out entgegen, we write E in
- parentheses.
- So we would call them-- let me just use different colors
- since this molecule down here is blue.
- So we would call this (E)-but-2-ene.
- These mean the same thing.
- (E)-but-2-ene.
- But this is now kind of the standard convention.
- We'll see this notation where you use entgegen, or (E), is
- actually more powerful.
- It can extend to when we have more than one functional group
- on each carbon.
- Let me just make it clear that the (E) stands for entgegen,
- which is German for apart, or at least I believe it is.
- I can't speak German so I'll just have to take people's
- words for it.
- Now, in this situation where our functional groups are on
- the same side, you could call this cis-but-2-ene.
- Cis, I believe, comes from the Latin for together, or you
- could call it (Z)-but-2-ene.
- And you could imagine this (Z) comes from
- the German for together.
- And it stands for zusammen, which is German for together.
- And that's all you have to do.
- And in the next few videos, we'll do a bunch of examples
- here so we can really figure out whether to label something
- with either an (E) or a (Z).
- But I wanted expose you to the cis- and trans- naming scheme,
- because that's sometimes used for simpler molecules where
- you only have one functional group on each side.
- But I want you to realize that they're really trying to make
- the same differentiation.
- They're really just trying to decide whether the functional
- groups on one side of a double bond and on the other side of
- a double bond, whether they're on the same side, cis-, or
- (Z), or zusammen, or on different sides, trans- or
- (E), or entgegen.