>> Welcome to hydrocarbons part five.
In this video lecture we are going to look at chirality,
limiting the exploration to what's really relevant
for allied health students.
Chirality is just a very key important topic
because our biochemistry is almost entirely chiral.
And so let's get busy and see what that means.
Now remembering from our introduction to isomers
that constitutional or structural isomers, right,
that's the same chemical formula with different connections.
And so for example like here we have C6 H12.
Same chemical formula, but look very different.
And then we've studied and stereoisomers --
I think these are the most challenging.
Right? Because they have the same connectivity,
same chemical formula, right,
but different spatial arrangements.
So this is where we have to bust out our brain yoga
and visualize things in three dimensions.
And this can be where it's very helpful to get
that molecular model kit to build your ability to see.
So let's quickly review here.
Right? So confirmations are just about rotation.
Right? So these are actually the same compound
with different rotation.
And then we could look at -- Remember geometric.
That's where we have the cis trans.
And that could be alkenes or cycloalkanes.
And we covered that pretty thoroughly in a previous video
so I think that that's enough said there.
So now this is the new part.
This is where we talk about chirality.
And their enantiomers.
And so we have to have a chiral carbon
and what is a chiral carbon?
A chiral carbon is a tetrahedral carbon.
Right? And so we know what tetrahedral means.
Right? Tetrahedral means that it has four single bonds.
And each of these bonds is going to be a different group.
Right? So a tetrahedral carbon bonded to four different groups.
So let's go ahead and use our tetrahedral geometry, right,
making our three dimensional shape.
Right? The straight lines are in the plane.
The wedge is poking out.
The dash is going back.
Right? And then just the four different groups just
symbolically A, B, C, D for now.
Okay? So chiral carbons are -- They have a lot of names.
They're very important.
They can be called a chirality center.
They can be called a stereo center
since they're stereoisomers.
They can also be called asymmetric carbons.
So don't let these names bother you or fool you.
Just, you know, expand your vocabulary.
And so in the same way that we built flashcards
for like our ions and our conversion factors,
it could be helpful for you to get some flashcards going
for the different types of isomers to make sure
that you have them down.
So let's look at a couple examples of what is
and what is not chiral.
So 2-butanol, right?
Butanol is a four carbon chain.
And then coming off of carbon 2 we have the alcohol.
And then for this example I'm going to put the hydrogen here
because this is our chiral carbon.
If we look at this chiral carbon,
we see that it has a methyl group, an ethyl group,
a hydrogen and a hydroxyl.
Right? So there's four different groups bonded
to this chiral carbon.
Let's compare that to 2-propanol.
In 2-propanol here's our three carbons.
We have our alcohol coming off the second carbon again,
but when we look here, right --
So we see there are no chiral carbons
because these groups are the same.
Right? So the hydrogen's different
and the hydroxyl's different,
but since these two are the same, achiral.
So when you see A in the front it means not chiral.
Right? Like asymmetry means not symmetrical.
All right.
So now let's take a minute and let's look at some models
and help to bring some experience to this.
So here is the simplest set of models right here.
I'll hold these up for you.
There we go.
Okay. So let me turn it.
There we go.
So you can see that --
Notice that there's a mirror image, right.
So we've got a methyl group on the top
and then we see the white and the green and the orange.
All right?
So this carbon in the center is a chiral carbon
because it's bonded to four different groups, the white,
the green, the orange and the methyl group.
Now there's something very interesting
about chiral carbons.
They are mirror images, but they're like our hands
in that they're non superimposable.
If I try to overlay these and I line up like the hydrogens
and the methyls, look what happens.
Right? The oranges and the greens don't line up.
And let's see.
Yeah. So that's -- So just like our hands --
Let me hold my hands up here for you.
Oops. There we go.
Okay. Here's my hand, right?
So here's the mirror image.
Right? So the mirror plane is going down the center.
Right? So if I try to overlay my hands, right,
my thumbs aren't going to work.
The idea is if you've ever put your shoes on the wrong foot,
right -- You ever put your right foot in to a left shoe?
It doesn't fit.
It's because our feet are chiral.
All right?
Now let's look at those models a little more closely.
All right?
Here's the 2-propanol.
Okay? So here's the -- Right?
The three carbons.
And the alcohol group.
Now we can put this together and we'll see
that we could have a mirror image.
All right?
But the difference here is if I flip this one
over they're imposable.
I can line everything up.
Right? We can line up the hydroxyl groups.
We can line up the hydrogens.
And we can line up the methyls.
Right? That's why we said 2-propanol is not chiral.
We can always show a mirror.
We can show a mirror image of anything, right,
but it's superimposable.
Let's look at one last set of models.
This would be the 2-butanol.
So let me make sure you're oriented right.
So here's our one, two, three, four carbon chain.
Right? So we have the methyl and the ethyl and the hydrogen
and the alcohol group.
So when we put these together, right, here's our mirror image.
Let's see if I can rotate that.
As you can see, we have the mirror image.
Right? And if I try to impose them, if we line up the methyl
and the ethyls, notice how the hydroxyl
and the hydrogen don't overlap.
And if we flip it and we line up the hydrogens
and the alcohol groups now we've got methyls and ethyls, right.
So let's go back to our notes now.
When we look at enantiomers, right,
they always come in pairs.
And the pairs are the mirror images.
So and we'll build that as we keep going through the notes.
All right.
So there's an introduction to chirality.
And now the first thing -- skill that we want to build is
when we look at a compound we want to be able
to see the chiral carbons.
And this can be particularly challenging for our class
because we're looking at rather complex molecules.
And so it can be a little tricky.
So if we look here, right -- So CH3, CH2, CH2.
So none of those are chiral
because we have more than one hydrogen.
But when we get right here, right --
And this is where it can be a good idea to draw
in any missing hydrogens if we think that it's chiral.
So when we look here we can see, oh, here there's three carbons.
There's a propyl group.
And then here's four carbons.
That's a butyl group.
And then a hydrogen and a hydroxyl.
So this carbon is chiral.
A little star.
It's bonded to four different groups.
CH3. CH2. CH3.
None of those are an option.
If we look right here, and we add the hydrogen, right,
now we see we have a methyl and an ethyl
and the hydrogen and the bromo.
So four different groups.
So there's our chiral carbon.
So basically we just want to look for the branches.
But we don't want to just --
We don't want to get complacent, though.
We have to keep our critical thinking
because if we come here we're like, "Oh, there's a branch,
but notice that this carbon -- These are the same."
So this carbon is not going to be chiral
because it has two methyl groups coming off of it.
Remember that a chiral carbon has to have four unique groups.
But if we keep moving along here would be our chiral carbon.
It has the methyl, the ethyl,
and then this big honking thing here, right, the biggie,
and then last but not least, of course, the hydrogen.
So there would be our chiral carbon.
Here we don't see branches we know.
Not to worry.
We get here.
We look around.
This is definitely going to be a chiral carbon.
Right? So there's the ethyl.
And then another biggie and another biggie right there.
And then of course the hydrogen.
So that's the first chiral carbon,
and then this one's pretty tricky
because there's actually two.
Here's the second chiral carbon.
Right? There's our methyl and our ethyl
and then a super biggie and then of course the hydrogen.
All right?
So we had two centers here.
And then, oh my goodness, cholesterol.
There's eight chiral carbons.
All right?
So here's the challenge.
You know there's eight.
This would be a great place to pause the video for a minute
and see if you can find all eight.
And then we'll compare answers.
All right.
Let's see how you did.
Right? These are the same.
CH2, CH2, oh there's a branch.
This is definitely a chiral.
Okay? And then when we get
to this ring the rings can be very tricky.
Right? If we look at this carbon we see when we branch out,
right, this is a CH2 and then this carbon is bonded
to four other carbons.
So that pen's a little fat.
We're going to go with this.
So there's our second.
CH2, CH2 -- Any time we see a branch, right?
So if we look at here, right, this goes --
Notice that these two carbons are different.
This is a CH2 and then this carbon is busy bonded
to a lot of other carbons.
So this is also chiral.
Same thing with this one here.
Right? Methyl group.
Six membered ring.
Five membered ring.
Lots of differences.
So not -- Even though they're carbons,
the carbons are different.
Okay. Same thing here.
Right? Part of a fused ring.
Part of another fused ring.
By itself hydrogen.
Okay. So we've got one, two, three, four, five, six.
We've only got a few left right here.
Right? Methyl and then these other carbons are different.
And then, last but not least,
right here [inaudible] hydroxyl group.
All right?
So there's the eight chiral carbons of cholesterol.
And this would -- This is our most challenging.
We won't get any more difficult than cholesterol.
And we'll practice this in class together as well.
So just know that this is our goal.
Right? But right now if you're more comfortable working
on like A, B and C that's okay.
That's a great place to start,
and with a bit more practice you'll be able to look
at more complicated molecules and identify all
of the chiral carbons.
All right.
Now let's have a little review here,
a little nomenclature review.
And then weave in there what we've been learning
about chirality.
So we're going to draw the skeletal line structure
for 3 ethyl, 4 methyl hexane.
And then we're going to star any chiral carbons.
Right? So remember we want to start
at the end of the name, hexane.
That tells us we have six chiral carbons.
Our six carbons.
And then if we number these, okay,
coming off of carbon 3 is that ethyl group.
That's two carbons and a methyl group.
All right.
So let's go across and see if we can find chiral carbons.
So CH3, CH2.
Oh. This one looks like it has possibilities,
but this is where we have to step back.
Notice that carbon 3 has two ethyl groups coming off.
So since those are the same group, carbon 3 can't be chiral.
But let's move on to carbon 4.
Ah ha. Here's our chiral carbon.
Right? We have our methyl and our ethyl and then our biggie.
And then of course we can't forget sweet little hydrogen.
All right.
So hopefully you starred chiral carbon 4.
There's our chiral carbon.
And now we're going to look a little more closely
at enantiomers and see how to draw them.
So, as I mentioned on the first page, right,
when we have one chiral carbon we're going to end
up with a pair of enantiomers.
And when you look at the names of different drugs or compounds
if they have chiral centers you will see RS, little L,
little D, plus or minus or DL.
And they always come in sets of two because they're pairs.
And so this -- We're not going to be responsible
for naming these, but when you read a name if you see any
of these terms at the beginning
of the name it's letting you know
that there's chiral carbons.
And then here's the butanol.
I did my best with the models and we'll work together in class
that we see that we have the mirror image.
And so we can use the perspective formula
that we've been used to drawing to show the chiral carbons.
And we're going to introduce a new type of drawing now.
This is called the Fischer.
And so with perspective formulae you can think of it
as like a side view looking at a profile.
And with the Fischer projection we can think
of it as a direct view.
And so from now on when you see structures written
in this cross form the horizontals are wedges.
Okay? So the horizontals poke out of the paper with the wedge.
And I like to think of it
as like they're reaching out to give us a hug.
Horizontals are hugs.
Right? They're coming out of the paper.
And then the dashes, the vertical lines are the dashes,
and those are going back in to the paper.
So the verticals go back in to the paper.
Okay? So it just minimizes having
to draw the wedges and the dashes.
So this is now called a Fischer projection.
All right?
So when we are talking
about chiral carbons we could draw them using the perspective
formula or the Fischer projection and we want
to be able to draw -- Right, since they come in pairs,
if we're given one enantiomer we want
to be able to draw the pair.
Right? So we're going to have our pair of enantiomers.
All right.
So now this would be a good minute --
I want to show you something with my hands.
All right.
So when we look at our hands we have the mirror image, right.
So the mirror plane is coming right through the middle.
Okay? So notice that like the mirror's in the center.
My thumbs are closest.
My pinkies are farthest away.
And my fingertips are at the top.
Right? So when we do a mirror image, right, whatever's closest
to the mirror stays closest to the mirror.
And now if I poke my thumbs towards you notice I have the
mirror image on both hands
and both thumbs are pointing towards you.
If I put my thumbs back both thumbs are back.
So we have to -- We have to think about --
We've all looked in mirrors, but we might not really have thought
about what a mirror image means.
So basically a mirror image means, right,
if my pinkies are farthest
from the mirror they stay farthest from the mirror.
So now that we've practiced with our hands a little bit,
let's see if we can apply this to the paper.
All right?
So if we're drawing the mirror image, right, if something's
at the top it stays at the top.
So the CH3 group is going to stay at the top.
Right? And then think.
The hydrogen would be like the pinkie.
Right? It's going to continue to poke out.
Everything -- These were all in the plane of the paper.
They stay in the plane of the paper.
The wedge stays a wedge.
The OH is close to the mirror.
It stays close to the mirror.
The bromine is on a dash.
It stays on a dash.
And it's also there.
So these are the pair of enantiomers
for this compound coming off of that chiral center.
I think the perspective formula is probably the trickiest.
The Fischer is a little more straightforward.
So with the Fischer -- Now this is our chiral carbon right here
in the center.
Okay. So all we're worried about is the orientation
around the chiral center.
So at the top here, this ethyl group,
we can just write this as we always would.
We don't have to make that in to a mirror image
because it's not chiral.
And then right at the top and the bottom, right,
the fingernails stay at the top, the wrist stays at the bottom.
There's the hydrogen.
Right? So the mirror image comes
from this methyl group that's closest to the mirror.
And the bromine was furthest from the mirror.
All right?
So now this is our second pair of enantiomers.
So in our class you'll be responsible
if you're given one enantiomer then you want to be able
to draw its partner, the other pair.
And that's where we'll stop with that part.
We're going to look a little more closely at enantiomers
because they have such an important
biological significance.
Another way that enantiomers are described is they are called
optical isomers and so optical
in this sense has to do with light.
So enantiomers are very fascinating.
They have the same boiling point, melting point,
density, refractive index.
That's how they -- That's how they bend light.
Right? How soluble they are in water.
But where they differ is how they interact
with polarized light.
Right? So that's different.
This is rotating light.
And so quite literally Louis Pasteur was sitting there
with some tartaric acid and tartaric acid exists
as a pair of enantiomers.
And he noticed that the light --
There was something happening with the light.
And so ultimately we were able to do experiments
where we could take light and we could put it
in a polarizing monochromator and then a polarizing filter
and here we had one enantiomer.
Not the pair.
But if we just had one enantiomer we could measure
that the light was rotated.
Right? So here it was vertical up and down.
And now we see that it's been rotated.
Right? So the light is rotated.
And so what we want to note -- The last thing we really need
to know about enantiomers is they come in pairs
and one enantiomer is going to rotate light clockwise
and the other enantiomer is going
to rotate light counterclockwise.
And that's why they get called optical isomers.
So as you continue on your journey through the science
and the allied health field if you come
across the term optical isomer I don't want it
to throw you for a loop.
I want you just to understand that it's talking
about this phenomenon of enantiomers.
It is possible to have a one to one mixture of the enantiomers
and that has a special term called a racemic mixture.
So let's make sure that --
And this is the last thing that you need to know
about enantiomers and then we have one last page
of practice problems.
Right? So here's glucose, right?
That will be important when we study biochemistry.
D glucose rotates the plane polarized light plus
52.7 degrees.
So what would be the rotation for L glucose?
Right? Well, we know that they rotate equal and opposite.
So if D glucose is going
in the clockwise direction then L glucose is going to go
in the counterclockwise direction,
but because they're enantiomers the value will be the same.
Just right?
So D glucose goes clockwise and L glucose goes counterclockwise.
All right.
And then to make sure you understand the definition
of a racemic mixture, right --
So if we had plane polarized light and we put it
through a racemic mixture of D and L glucose
that means it's one to one.
What happens to the light?
Basically there's no net rotation.
All right?
So half of the sample will rotate the light clockwise
and the other half of the sample will rotate the light
counterclockwise and so they cancel out.
Okay. And now let's look at our last page
and I've been mentioning and emphasizing that there's a lot
of biochemical applications.
So let's get a sense of those to see why we care
so much about chirality.
All right?
It's because the enantiomers will have different --
In most cases the enantiomers have different
biological effects.
Right? So here's our buddy vitamin C. Let's look
at vitamin C and go ahead and star the chiral carbon.
All right.
Let's see how you did.
Right? So there it is.
This would be our chiral carbon.
Right? This can't be the chiral carbon
because it has two hydrogens.
And, oh, actually we have two chiral carbons, right,
and then this would also be a chiral carbon here.
Okay. So we have two chiral carbons.
That's awesome.
All right?
And we're going to put our emphasis on this one right here.
So in this -- And here's our mirror plane.
Okay? So this is the vitamin C that's a nutrient.
Right? When people aren't feeling well they'll say, "Oh,
I should take a little bit of vitamin C, help me feel better.
It's a good antioxidant."
Right? The enantiomer has little to no biofunction.
Okay? So only one enantiomer really supports our nutrition.
The other one is just along for the ride.
Another example would be thalidomide.
So I'll give you guys a moment to star the chiral carbon here.
Right? So it has to be tetrahedral.
There we go.
It's right here.
Okay. So there's our chiral carbon.
Now thalidomide has a very interesting history.
Here's the pair of enantiomers.
This enantiomer here relieves nausea
and was prescribed I think late fifties, early sixties in Europe
for women that were pregnant
because they had morning sickness
and this enantiomer did a great job of relieving nausea.
Unfortunately in the body it isomerized into its enantiomer
and the enantiomer is a teratogen.
And what a teratogen does is it causes mutations in fetuses.
And so shortly after --
Within less than a year of them prescribing thalidomide
to relieve nausea, infants were being born
with the flipper hands and feet,
with these mutations from the thalidomide.
So it was quickly taken off the market.
Ultimately, though, thalidomide is being used
in cancer treatments.
I have a friend who was diagnosed
with blood cancer back before 2000 and none of us thought
that she would even see the new millennium
and she is still alive and in part she's due
because she does take thalidomide.
But she's not going to get pregnant
and she's not going to have any kids.
Over time, though, she has started to feel numbness
in her fingers and toes.
So there is something about this enantiomer
that affects our limbs.
And there's scientists out there looking at it more closely.
One last example of chirality.
THC. The psychoactive ingredient of cannabis.
Here's the long name.
So here I mentioned about the Rs and the Ss.
That let us know that there's chiral carbons.
Here's how it rotates light.
Because it has chiral carbons, there's an enantiomer,
but the active form is 13
to 230 times more active than its enantiomer.
And the interesting thing
about cannabinoids is they are also produced
in our body as endocannabinoids.
So they're -- So as we explore the medical value
of cannabis we'll be looking more closely at this.
But for here we'll just practice starring the chiral carbons.
So I'll give you a moment.
And as we look along, right, any time we see the straight chain
or double bonds no chirality there.
We've got matching methyl groups here.
No chirality here either.
But this carbon and this carbon, right,
there's our two chiral carbons on THC.
All right.
So that wraps up our video on enantiomers.
I know it's a lot, but hopefully with those last couple
of pages you see how important this is to our biochemistry
and you'll take a few minutes now
to work the practice problems to reinforce your understanding.