Naming benzene derivatives introduction - Chemistry, Class 12 Video Lecture

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.