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- I mentioned in the last video that 99% of the aromatic
- compounds that you'll see in a chemistry or organic chemistry
- class is either going to be benzene or a molecule derived
- from benzene.
- So what I want to do in this video is just familiarize you
- with some of these molecules and how to name them.
- So you know what you're looking at.
- Or I guess so you could name what you're looking at.
- So you've seen this multiple times.
- Benzene just looks like this.
- It's a six carbon ring with three double bonds.
- These three double bonds like that.
- And that's not it's only configuration.
- Sometimes it's shown to be in resonance with
- this form right here.
- So the double bonds all flip around the circle, like that.
- Or sometimes it's simply drawn like this.
- And I talked about this in the last video.
- Just to show that really neither of these
- configurations is exactly right.
- That these pi electrons, they're just moving around the
- entire ring.
- So and sometimes you'll just have the hexagon with a circle
- on the inside to show that the pi electrons are just floating
- around the entire ring.
- Now if I were to add something to the benzene ring, it's
- pretty straightforward to name it.
- So say I have this molecule right here.
- So let's see I have that.
- That's benzene.
- But let's say this carbon over here, it has
- one, two, three bonds.
- If I didn't draw anything else, you'd just assume that
- there's also a hydrogen here.
- But maybe there's no hydrogen there.
- Let me do this in a different color.
- Maybe you have a chlorine there.
- Well this is just chlorobenzene.
- If that was a bromine, it would be bromobenzene.
- So pretty straightforward.
- Let me draw it.
- Let me actually change it a little bit.
- Let's say you had a chloro there.
- And let's say you had, oh I don't know.
- Let's say you had a bromo over here.
- Let's say you have a bromo over there.
- Draw the bromo.
- So you can just start numbering.
- You can actually start numbering in either place.
- Let's start with the bromine.
- It's alphabetically in front.
- So this would be 1-bromo, 2-chlorobenzene.
- Pretty straightforward.
- Now it gets a little bit more involved.
- The IUPAC, which we know is kind of the group that named
- most things.
- Everything else we've seen so far, they came up with a
- separate naming mechanism from the common names.
- But benzene, it's so ingrained in the organic chemistry
- community, that all of the benzene or benzene derivative
- molecules, they just kind of said, well you know what,
- we're just going to use the names that everyone uses.
- So here's a couple of common benzene derivatives.
- So if you have this molecule right here.
- Let me-- if you have this molecule right here-- I'll
- actually draw the pi electrons as a circle.
- Actually I'll draw it with the double bonds.
- So let's say you have this molecule right here.
- And over here you have an OH.
- You have an OH right there.
- This is called a phenol.
- So it's not called a benzene anymore.
- And if you had a molecule that looked like this,
- so just like a phenol.
- So essentially it is a phenol.
- But let's say you have a bromine right there.
- What you do is you start numbering at the group that's
- making this is a phenol.
- So you start numbering there.
- One and then you get to the two.
- So this is 2-bromo.
- This is 2-bromophenol.
- And unfortunately this is one of those things that you just
- kind of have to memorize.
- That a phenol is really just a benzene ring with an OH group.
- Another one that is probably a good idea to memorize.
- I mean at least for me in organic chemistry, the most
- confusing thing is when someone says the name of a
- molecule that sounds like they expect you to understand.
- But you don't understand it.
- So it's, I guess, a good idea to understand as
- many names as possible.
- So if you just have a benzene ring, and you just have a
- methyl group attached to that benzene ring,
- this is called toluene.
- And once again let's say you had a
- fluorine right over there.
- This would be one, two, three fluorotulene.
- I always have trouble saying that.
- So it's one, two, three.
- You start at the group that's kind of
- making this the toluene.
- So let me make it a little bit clearer.
- So this would be 3-fluorotoluene.
- This would all be in one word.
- I just wrote it up here to save some space.
- Now a couple of other ones that you'll see.
- And once again, as you know, if you watched many, many Khan
- Academy videos, I hate memorizing things.
- But these are just names.
- And these aren't systematically derived.
- The chemistry community just uses them, because that's the
- way they've heard them.
- So it is one of those things you kind of have to memorize.
- So if you have a benzene ring, where the functional group
- that is kind of defining the benzene ring, it's essentially
- becoming an amine.
- We haven't actually covered videos on amines yet.
- And I'll do that in a future video.
- But it's essentially, it's replaced one of the hydrogens
- in ammonia.
- Let me draw ammonia.
- Ammonia has three hydrogens and one lone pair.
- Here we've replaced one of the hydrogens.
- Actually there's two hydrogens here.
- We've replaced one of the hydrogens with a benzene ring.
- This type of thing you call an aniline.
- Right there.
- And the same idea.
- If I put a fluoro here, this would be 2-fluoroaniline.
- So you use whatever the base molecule is, and then you just
- name it, really the way we've named a lot of things before.
- And I'll just introduce you to two more, just so that you
- have them under your belt.
- So that you'll have seen them before if anyone introduces
- them to you.
- And they're really introducing groups that we'll study in
- more detail in the future.
- But if you have a benzene ring once again.
- So we have a benzene ring.
- And then you have a carboxyl group.
- And we'll talk more about carboxyl groups in the future.
- But it creates carboxylic acid.
- This thing right here, this hydrogen, it can actually be
- released quite easily.
- And we'll talk about that in the future.
- This right here is called benzoic acid.
- And this name, actually there is a little bit
- more logic to this.
- You have the benz part for the benzene.
- So let me make this in different colors.
- The benzene part, that gives us the benz right there.
- And then in general, whenever you have a carboxyl group like
- this, it becomes a carboxylic acid.
- And you tend to add the -oic acid to the end.
- So this actually has some logical naming to it.
- Now the last one I'll introduce you to is very
- similar to this.
- Instead of having an OH right here, we just have a hydrogen.
- It would be an aldehyde.
- So let me draw that.
- And you could almost imagine what
- that's going to be called.
- It's benzaldehyde.
- So let me draw it.
- So you have your benzene ring.
- And then instead of this carboxyl group, you have an
- aldehyde group.
- You have your carbon.
- There's implicitly a carbon here.
- Let me make that, just in case it's the first time
- you're seeing it.
- You have a carbon there.
- Instead of an OH group, you have just a hydrogen.
- And so once again you have a benzene.
- So it actually makes sense to put the benz over there.
- And then you have the aldehyde group.
- Then you have an aldehyde group right over here.
- So this is called a benzaldehyde, or this molecule
- is called benzaldehyde.
- And we'll study amines and aldehydes and carboxylic acids
- in much more detail in future videos, and actually see
- reactions that involve them.
- But I just wanted to expose you to this.
- And so when you see these kinds of names, you don't
- become intimidated.