>> Welcome to hydrocarbons part two.
In this video lecture, we are going to introduce the idea
of isomers with organic compounds.
Let's get started.
You know, at the simplest level, here are the isomers.
We will expand on this a bit in a future video
but this is a great foundation.
So we're going to look at really three primary types
of isomers in this video.
The first one is called constitutional, or structural.
And I've written the definitions down here for you below, right,
so here's structural isomers.
So it's compounds that have same chemical formula
but different connectivity, right?
So an example here would be like if we had C5H10, right,
so there's the same chemical formula
but we could put those five carbons in a straight chain,
or we could do four carbons with a branch,
or we could do three carbons with a branch, right?
Or let's look at - oh, whoops.
I'm sorry.
That's C5H12, that's all right.
Okay, now let's jump to C5H10.
Now for C5H10, here C5H12 these -
still is 10 so these makes sense.
Okay, we have our five carbons and carbons wants four bonds
so we just can put it together in a lot of different ways.
Here's C5H10, here we have a straight chain
with a double bond, right, or we could put the double bond here
and even that makes sense.
But here's the crazy one.
Or we could actually have a ring.
All three of these compounds are C5H10.
They have the same chemical formula
but they definitely have different connections.
So we have a couple different examples of structural
or constitutional isomers.
Now, let's look at conformational isomers.
Now, these are two different types of stereo isomerism
and with stereo isomers, right,
now we have the same chemical formula
and the same connections, all right?
But - whoops - okay.
But we have - but different spatial arrangement.
Whoops. Right?
So the same chemical formula and same connections
but different spatial orientation.
All right, so there is three different types
of stereo isomers.
We're going to just focus on two for this video.
And the first one are conformational,
this is all about rotation, all right?
So when we look at conformers, here's our definition right here
for conformers, right?
They're actually the same compound
with different rotations.
So we could have - right, so there's our C5H12 and now
if we focus, for example, right here, we could rotate, right?
So there - it's still C5H12 and it's still pentane
but these would be considered conformers.
So now I want to - I'm going to show you a simplified model here
so let's look at these models.
All right, so - whoops, there we go.
Okay, so here, we have - right, they're both C5, right?
Now to change them, right, I just rotate -
let's see how do I want to do this?
Yeah, this will work.
Okay, so now if I rotate here, right, oh, there we go.
Okay. Now I have it right.
There. We have the same - right,
we have the same compound, right?
We just rotated, right, and we could go this way
and now they're back to being the saw tooth, all right?
So these are conformers.
So right now, they would be considered identical.
If we rotate one of these, now they're considered conformers.
So they are the same compound
but when they have different rotations, they're -
the best term to describe them is conformer.
And here, I've just focused the models on the carbons only
to make it not so busy, right?
So when we have the same compound
and different rotations,
the most correct term would be conformers.
And now, the other type of stereo isomer we're going
to look at is geometric.
And here, these also come in pairs
and we have cis and trans, all right?
So when you think cis, think same side.
And with trans, think like if we had a transatlantic voyage,
we would be going across the Atlantic Ocean,
we would say across sides.
So geometric isomers only occur for two types
of organic compounds and that would be
when we have a double bond, right?
So if we look here, right,
if we think about the double bond kind of as a plane, right?
So here we see that these carbons are
on the same side so it's cis.
Here, right, one's here and we would have to go
across the double bond to get to the other one so it's trans.
All right.
So let's look at these briefly, right?
And so - we'll just put them -
we'll do them this way, all right?
So here, we see, right - in this hand, we see how the double bond
and their - the green atoms are on the same side.
And then here, they're across the double bond
and now there's no rotation.
I can't twist this to make the other one.
We can't - it's not like conformers,
where we could twist them and make them into each other.
These are unique compounds and we'll look a little closer
at that on the next page.
There's one more that we can make geometric isomers
and this is with rings with two substituents, right?
Substituents is a fancy way to say branches, all right?
So we could have our six membered ring
and we could have both branches, right, a wedge, right,
both branches going up.
And so this would be another example of cis,
they're on the same side.
Or we could have one of our branches going up
and the other one going down and so that would be trans.
All right, let's look at those models.
All right, so here, up or down, right?
Here's the two, right?
They're both up so you could see - so here's our ring.
So we have the plane of the ring and so -
whoa, there we go, right?
They are both on the top so that would be cis.
And then if we compare that with this model here,
now we see that one is going down and one is going up.
So with the plane of the ring, they're across that plane
of the ring so that's trans.
So that's our two examples
of geometric isomers for cis and trans.
And those are right here, right?
And so that's doing with the difference
in spatial arrangement.
All righty.
Okay, let's move on to the next page
and look a little more closely at the cis trans isomers,
the geometric isomers.
It's very important to realize that they cannot interconvert.
They are unique compounds, right?
So here, we see the dash and the wedge
so that would give us our trans.
And then here, there's two wedges
so that's why it's cis, okay?
So that's an important part of the name.
And then with the alkenes,
think about the plane of the double bond.
So here, right, we have them on the opposite sides,
the substitutions are on the opposite of the double bond
so that's why we have trans.
And then here, they're on the same side so cis.
And then I included a little bit of information here, right?
See, they are different compounds,
the melting points are different.
The boiling points are different.
They can have differences in water solubility,
all different kinds of things.
One thing to notice though,
especially with the double bonds, is that it's important
that there's two different groups, right?
So if we look at this compound right here, all right,
there's no cis trans, all right?
Because when we look
at the carbons forming the carbon carbon double bond,
notice that these two groups are the same, all right?
So here, right, we have those hidden hydrogens
that we don't see.
So we have our methyl group and our hydrogen
and our methyl group and our hydrogen, right?
So we have two different groups.
So it's possible to have cis trans.
So - and the same way that I mentioned that with rings,
the rings can only have two substituents
so not all rings can have cis trans isomers.
The same thing with double bonds, not all compounds
with double bonds can be cis trans.
And so why do we care about this?
Well, it's pretty trippy.
Right now, the fact that you can even see this video is taking
advantage of the interconversion of cis trans isomers.
This rhodopsin is a compound that's part of our vision
and right here, we can see that there's a cis double bond
and when the light energy, right, the image that we want
to see brings in light energy, there's a chemical reaction
that converts this double bond to a trans double bond.
And through that rotation, it triggers the nerve impulse
and that gives us vision.
So cis trans isomers are an important aspect of chemistry
and they're helping us learn more about it.
All right, so we have a lot of information here.
Let's practice it to make sure that we know how
to apply this knowledge.
So let's look at here, right?
So let's go back through, right?
So we -- you got to know the term structural isomer, right?
Same chemical formula, different connections.
So right now would be a great time to pause the video
and see how well you can do
at drawing the five structural isomers for C6H14.
All right, let's see how you did.
For me, I like to start with the straight chain.
So there's two, four, five, six, okay?
So there's our six carbons.
And then basically just start shortening the chain.
So we'll take this first carbon off, then put it as a branch.
And then we have a couple options.
It could go to this inner carbon or that inner carbon.
So that gives us two more possibilities.
Sometimes we can get carried away and try to keep going
but then we would just have the identical compound flipped
like a pancake.
So we started with six carbon chain
and then here's our five carbon chain.
Now we can go to a four carbon chain
and that would leave us with two branches.
So those two branches could go on the same carbon
or they could go to two separate inner carbons.
So these would be the five structural constitutional
isomers for C6H14.
All righty.
Now, let's work on the next topic.
So this would be another good time to pause.
What are the relationships between compounds A, B and C?
[ Background noise ]
All righty.
So the first thing I want
to do is double check our carbon count.
And so we see carbon one has six carbons
and so does compound B, right?
A and B, both have six.
And so does C, all right?
So now we have the same chem -
all three have the same chemical formula.
But what we notice about A and B, right,
is they're actually the same compound.
There are six carbons in a row,
we just have this difference in rotation.
So we would describe these as conformers, right?
The same compound but shown with different rotation.
And this could be helpful when you're working in groups
or are working with someone studying outside of class.
Sometimes people will draw the same compound
but do different conformers and I don't want
that to cause you any headaches or frustration.
Just go oh yeah, we just showed different rotations,
they're the same compound.
But here, C is definitely different.
It has different connectivity.
The sixth carbon is a branch.
It's not part of the main chain so the relationship between A
and B and C would be structural or constitutional isomers.
All righty.
Now let's see if we can put all of this together.
So now we have - here is our reference compound,
compound one.
And we have six compounds to compare, two through seven.
So the idea here, we look - we would look at one and two
and say what's the best description
of their relationship.
And then ask that question five more times.
So this would also be a great place to pause the video
for a moment and see how well you do
and then we can check your work.
Okay, let's see how you did.
So when we look at the reference compound,
we notice that we have two ledges and they're coming off
of carbon one and carbon three, all right?
So this would be cis one comma three dimethyl, cyclopentane.
And when in doubt, name it.
Right out the name, that can also help.
All right.
So let's compare with compound two, right?
One, two, three, four, five, so we still - right,
we still have one three substitution
but now notice it's trans, okay?
So everything is the same except we have cis and trans
so this would be a geometric isomer.
Let's try compound one and three.
We still have the same chemical formula but notice now,
both branches are coming off the same compound, right?
We have different structures so this would be structural isomer,
same chemical formula but different connectivity.
Let's look at compound four.
Now this one, right, it's rotated but this is
where we use our brain yoga, right?
And we see that it's still, right, one,
three dimethyl except it's trans.
So this would be a geometric isomer compared to compound one.
All righty.
Let's keep going here.
Compound five, we go one, two, three, four, five.
So if we look quickly, we're like oh, look, it's cis.
And it's true that it's cis
but it's cis one comma two, not one comma three.
So these would be structural isomers, same chemical formula
but different connectivity,
or one two substitution versus a one three.
All righty.
Let's look at compound six.
One, two, three, four, five.
Ah-ha, so same chemical formula, same connectivity but we're back
to that trans so another example of a geometric isomer.
And then last but not least, we have compound seven
and we look here, one, two, three, four,
five and so this is - they're both cis
so cis one comma three dimethyl cyclopentane, okay?
So this one is identical, right?
It's just been, right, just rotated
in the plane of the paper.
All righty.
Okay, so we're going to have a little bit more practice.
So isomers tends to be a very challenging subject
and so I want to give you a lot of guided practice
so we can build your confidence so you're ready
to work independently.
So this will be our last page of practice.
This would be another good opportunity to pause
so I'll just pause here once and give you a chance
to answer these questions and then restart the video
and then we'll work through the rest of them together.
All right.
Let's see how you did.
Okay, so if we look at A through E
and we start counting the carbons, okay, six there,
six there, another six, another six there as well.
Okay, so all of these compounds are C6H14.
They all have the same chemical formula, all right?
So which ones are structural isomers?
Well, now we want to look.
So, we notice here that A is the only one that's a pure straight
chain so it stands alone.
If we look at B and C,
we see that they both have a five carbon chain
with a methyl group coming off of carbon two, all right?
So these are also - these two are the same compound.
And now if we look at D,
now we see we only have a four carbon chain
and then there are two branches coming off of the second carbon.
So this is a unique compound.
And last but not least, we look at compound E
and it also has a five carbon chain but notice now
that the branch is coming off of carbon three, all right?
So amongst these five compounds, we have four unique compounds
so we would say that A is a structural isomer with B and C
and with D and with E, right?
So these are our - as I boxed them,
these would be the structural isomer groups
and then we only have one conformer.
Here, when we have B and C, we notice that around carbon four,
we have a rotation, all right?
Don't get confused by the pancake flip.
If we rotate - I'm going to rotate this back
so you can see it, okay?
So this would be B and C so notice, right,
that it's just the rotation right here.
So B and C are the conformers, right?
All right, and then for cis and trans, making sure that we know
when these terms apply.
So a ring with only two groups and two wedges,
that's going to be cis.
Here, we have a double bond.
We draw in the plane.
When we look at these two carbons, they have - right,
a methyl group and a hydrogen, an ethyl group and a hydrogen.
So this is eligible for cis and trans and we see
that the groups are across the double bond so that's trans.
Trans can be a little tricky when we draw it
in the saw tooth, right?
When we're first introducing it, we showed it like this, right?
So it looks a little easier to see but normally,
when we see it written, it will be written like this.
So you just got to, you know, do a little brain yoga.
All right.
How 'bout here, right?
There are two substitutions but notice they're coming off
of the same carbon so this is not applicable.
There is no cis trans here.
This one - this one's pretty tricky, right?
So we have this group and this group on the same side
so this double bond would be cis.
But now if we look at the second double bond, they're across.
So we have one cis double bond and one trans double bond.
That's about as tricky as we'll get.
And then let's look here, here's our double bond but notice,
right, we have the same groups.
So this does not meet our criteria for cis trans.
And the same thing, here's a double bond but with the ring,
cis and trans is not applicable there as well.
All righty.
Okay. So that wraps up our introduction to isomers.
This is a very challenging section but it's very important
because it's going to have great impact on biochemistry.
So this would be a great time
to work those post video practice problems
to reinforce your understanding.