Leah here from leah4sci.com and in this video we’ll introduce the concept of chirality
looking at enantiomers, stereosisomers, chiral centers and more.
You can find this entire series along with my practice quiz and cheat sheet by visiting
my website at leah4sci.com/Chirality.
Chirality falls under the topic of Stereochemistry specifically we’re looking at stereoisomers.
Don’t forget that isomers are molecules with the same molecular formula but something
is different about them, constitutional, rotational, conformational, different types of isomers.
And the stereo portion tells you that you have the same molecular formula, same connectivity
meaning the order of atoms but the difference is their specific orientation and space, meaning
where they’re locked into.
Now this can include cis-trans isomers over double bond or a ring, or stereoisomers which
I think of as chiral isomers – R and S.
When I think of chirality, I think of handedness for the molecule.
Many people are right handed, some people are left handed and different tools and machines
are meant to be operated with one hand or the other, the right or the left hand.
Imagine taking one of those big fancy gloves that’s meant for your right hand and putting
it on to your left hand, it just doesn’t work.
We’re now looking at the top of my right and left hand and they look about the same.
Thumbs in the same place, fingers in the same place, when I turn it over, they still look
the same.
So while hands are the same though, no matter how I turn them, imagine there’s mirror
right here between them.
Right now my thumbs are mirrored, fingers are mirrored, there’s still an imaginary
mirror between them , there’s still an imaginary mirror, no matter how I turn them it appears
that there’s an imaginary mirror between them.
Now what happens if I try to superimpose them?
My thumbs are sticking out, so if I turn it, now my fingers are on opposite direction.
If I flip my hands so that my thumbs and fingers are aligned, all my palms are on opposite
direction, no matter what I do, there’s no way for me to perfectly superimpose my
right and left hand making them mirror images to each other.
So we agree that hands are mirror images to each other, and if these are your right and
left hand, thumb down on the table, they are the same but they’re mirror images of each
other.
And the only way that you can superimpose a hand would be if you took a thumb and added
it to the other side assuming the fingers are the same, took a thumb and added it to
the other side.
Now you see that the left and right hand are exactly the same, in fact there is a plane
of symmetry going right down in the middle and that is the key.
If there is symmetry then your two hands will be superimposable and they will not be unique.
The right, two thumb up will fit a left two thumb in and the other way around.
When thinking of chirality we’re typically looking of chiral carbon every now and then
you’ll see a molecule that is chiral as a whole or a Nitrogen for example with four
substituents and a positive charge.
99% of a time it’ll be a chiral carbon.
A chiral carbon must have 4 unique substituents.
If they’re not unique, it’s like having two thumb hand and they’re be superimposable.
In order to have 4 unique substituents we’re looking at 4 bonds to carbon, that means it
must be sp3 hybridized and tetrahedral in its shape.
So for example, if I show you a carbon here and I draw it in 3-dimensions so that I have
two bonds in the plane of the page, one up, one down to the left, I have one coming out
down but out of the page and one down into the page.
This is just one of the methods that you use to draw a chiral molecule but the key here
is to recognize we have 4 substituents and so they’ll have to be unique.
We’ll show A, B, C, and D. I’m using these colors so that we can follow along with the
model kit where the yellow represents the white.
4 unique substituents on an sp3 hybridized tetrahedral carbon gives me a chiral carbon
in the center.
This central or chiral carbon is referred to as the stereocenter or the asymmetric carbon
or the asymmetric center.
In organic chemistry it helps to look and hold the 3d structure.
If you don’t have a kit yet, get one on amazon by going through leah4sci.com/Kit and
if you purchase through my link, I do get a small commission.
So notice here we have a central carbon atom with 4 unique substituents.
This is considered the chiral or asymmetric center and it’s tetrahedral because it’s
sp3 hybridized.
So notice that I have two, apparently same looking structures where each one has a chiral
center and 4 substituents.
If you look at the two molecules you can see an imaginary mirror between them because they’re
perfectly reflecting each other.
And as I turn them, they are still perfectly reflecting each other.
Since these are chiral with an sp3 central carbon and 4 substituents, because they have
mirror between them, they’re considered enantiomers or mirror images and no matter
how I turn them I can always find that mirror between them.
But now if I try to superimpose them, let’s see what happens.
To place one on top of the other I need these substituents to line up so let’s flip it
and line up the red and the green, we’ll look at what happen, the blue and white do
not line up.
If I try to switch it so the blue and white are lined up, well look what happened to the
red and green, they don’t match.
Just like with my hands, something is sticking out, remember those thumbs and fingers.
Here the only way to line up two substituents is to have the two other substituents be opposite.
In fact, the only way I can superimpose these two is if I take some of the bonds and physically
break them moving these substituents around just like the example of moving a thumb to
the other side, and now look what happens.
The blue is lined up, the white, green, and red, they are perfectly lined up.
They’re still chiral but they they’re two of the same chiral molecules rather than
mirror images of each other.
Let’s prove it by trying to reflect them in each other.
Look at that, the green and blue are reflected, but the red and white are opposite, they don’t
not lined up.
If I try to put a mirror between the green and white, now the blue and green don’t
line up.
So no matter what I do I will not be able to show these as mirror images because if
they’re chiral and they’re the same they’re not an isomer of each other, right?
They’re either the same or enantiomers.
So we have the options for two different types of molecules when it’s chiral.
Going back to the hands example, if you have a right hand and a right hand, they’re exactly
the same.
They’ll fit the same gloves, they’ll be superimposable with each other, we’re gonna
call them, the same.
If we have a left hand and a left hand, once again they are considered as the same and
are superimposable.
Superimposable tells you that you can take one right hand and put it on top of the other
right hand and they’ll look exactly the same or take one chiral molecule and another
molecule of the same chirality put one on top of each other, they’ll look exactly
the same.
There are two scenarios where you’ll see a superimposable molecule.
If you have two chiral molecules and they’re exactly the same meaning they’re not mirror
images, they’re considered superimposable or if you have an achiral molecule, a molecule
that is not chiral because it has plane of symmetry.
For example if you have a carbon with three rather than four unique substituents for example
this central carbon has a methyl, a hydrogen, another hydrogen, and an OH Hydroxy.
In this case, having two hydrogen atoms means this molecule has a plane of symmetry and
if it’s not easy to see here, build it in your molecule and rotate it in this direction
so they have the hydrogens on the right and the left.
We have two hydrogens coming out of the page, one to the right, one to the left.
There is an OH going down towards the back, there is CH3 going straight up in the plane
of the page and look at that, symmetry right down the middle.
So if your molecules are chiral and the same, or if they’re achiral with a planar symmetry
they’re going to be superimposable.
Now let’s see what happens if instead of 4 unique substituents I have only 3.
We’ll take off the green one and swap it for a white.
So let’s start with creating mirror images.
I have the red and blue, red and blue.
They are perfect mirror images of each other.
But how is it possible to have mirror images of each other if I can take the same molecule
and superimpose it on the other one.
How is the superimposable molecule also mirror image?
And the answer is, two of the same substituents makes this molecules symmetrical, Notice there’s
a line of symmetry going right down the center.
Again a line of symmetry no matter how I turn it, I can find that symmetry and that makes
this molecule achiral because this molecule has a tetrahedral sp3 carbon in the center.
But two of the same substituents for a total of three unique, it’s not chiral.
And if it’s not chiral, it doesn’t have a mirror image, it’s going to be superimposable,
and another molecule of its kind.
And just because it’s superimposable doesn’t make them enantiomers, it just makes them
superimposable because they’re the same exact thing.
If you have two molecules that are chiral but they’re opposite chirality meaning right
handed and left handed then they are considered enantiomers.
Enantiomers are chiral molecules that are non superimposable because they’re opposite
to each other.
And another common term you’ll hear for this is mirror images because one reflects
into the other.
In Organic Chemistry we classify chiral carbons or chiral atoms as being R or S. In Biochemistry
and sometimes in Organic Chemistry we’re looking at Organic Molecules.
We’re going to use the D and L configurations which tells you how it rotates plane polarized
light.
And what we’ll not gonna discuss in this series just keep in mind R and S does not
equal to D and L directly because some chiral molecules will be D, some R will be L and
the other way around.
So how do you tell if the molecule is R or S in Organic Chemistry?
That’s exactly what we’ll discuss in the next video.
You can find this entire video series along with the chirality and stereochemistry practice
quiz and cheat sheet by visiting my website leah4sci.com/Chirality.
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