Leah here from leah4sci.com and in this video we're going to discuss Polarimetry as an introduction
to Optical Activity. You can find my entire Chirality video series along with a practice
quiz and cheat sheet by visiting my website leah4sci.com/chirality. In the video series,
we discuss how R & S are chiral molecules that are mirror images or enantiomers to each
other. And if you have a solution that is just R or just S, that is considered to be
an optically active solution but when you mix them together into a single solution you
get what is called a Racemic mixture. But what is it called Optically Active? If you
look at the word, optic should remind you of your eyes or light, what does chirality
have to do with light? or the better question R&S is what we discussed on paper but how
do you actually test this in a lab? or how do you actually determine if a solution is
pure or racemic? One way to tell molecules apart is by identifying how they rotate plane
polarized light in either direction. Which brings us to Polarimetry.
Polarimetry is how we measure optical activity. But the actual device is called a polarimeter
and if you look at the word carefully you get polar which has to do with light and meter
which has to do with measuring. Light is how we measure optical activity of chiral molecules.
Think back to General Chemistry or even Physics when you learned the topic of plane polarized
light. Remember that light can exist as a particle or a wave. And if we look at it as
a wave, it oscillates up and down, up and down but in one direction. So notice here
it oscillates up and it oscillates down, but if we were to look at it head down, all we
would see is straight up and down and up and down and that's in one direction. This is
sometimes referred as monochromatic light, because we're looking at light in one color
and one wavelength. But if you look at something like a light bulb that is lighting up a room,
it happens because the light waves are coming out in every direction so each wave that's
coming out is oscillating in one specific plane, but overall the light is going in every
direction and that's how it lights up a room. When we want to use light to measure optical
activity, we needed to be going in one direction because if it goes in one direction and something
makes it turn, we can clearly see that turn happening. But how do we get plane polarized
light in just one direction? What we need to do is place a filter in front of the light
so imagine that this is a filter that has a tiny little slip in one direction, so when
the light is hitting it in every which way, only the light oscillating up and down in
the direction that allows it to pass through the filter, is able to continue on. This is
how we isolate light traveling in just one direction so that we can test it on the chiral
molecules. Here, light in every direction but here it's polarized oriented in one direction
or simply plane polarized light. Why is this so important? Because a chiral molecule, whether
"r" or "s" is going to have the ability to turn light in one specific direction. This
is characteristic of each type of enantiomer because the way that it turns light is gonna
be very specific to the "r" version or "s" version of that specific molecule and change
from molecule to molecule. For example, r 2-bromobutane will be different than r 2-bromopentane,
each will have its own characteristic signature of how it turns light. Imagine you're looking
right here and you're seeing light coming at you in just one direction. And when the
light that is oscillating up and down hits that molecule, the light is going to turn
very very slightly so that now it oscillates at a slightly different angle, still up and
down but notice there is an angle right there and if it hits more of the same molecule,
it can rotate more and more and more. If a chiral molecule rotates light to the right
or clockwise, that would be a positive rotation and that is called "dextrorotatory" or "d-rotary".
Where d-rotary means it turns light to the right, in fact, d-glucose is actually called
dextrose because it is d-rotary. If a chiral molecule makes this light turn to the left
or counterclockwise, then this molecule would be considered a negative rotation, negative
counterclockwise or "levorotatory" or "l-rotary".
Be very careful when you're memorizing these definitions because positive and negative,
"d" and "l" have nothing to do with "r" and "s". When we look at r and s, we rank the
substituents, we put number four in the back, cross it out, draw a path from one to two
to three, that is strictly a ranking system for a molecule on paper, but you can have
some molecules that are r-positive (dextrorotary) and other r molecules that are negative or
levorotatory. So "r" and "s" does not equal positive or negative, they are completely
different ranking systems. So let's put this all together and understand how the polarimeter
works. Picture the polarimeter as a small tube that has a liquid sample in it and that
sample contains your chiral solution. On one end of the tube, we have the light coming
in and just imagine that the light is coming in that one direction where it's oscillating
at whatever degrees it starts out but consider that to be your zero degrees, your starting
point. So if we look at it head on, going in, say this right here is considered my zero
degrees. As the light enters the solution, traveling in that plane path, that one plane,
it's going to hit the chiral molecules and solution and when it hits a chiral molecule,
it turns slightly and it turns slightly and it turns slightly, and by the time the light
comes out the other end, having hit enough of that one type of chiral molecule, it will
have turned. So imagine that when it comes out, instead of seeing the light in the up
and down position at zero degrees, it now comes out at some other angle because the
entire plane of polarization is still plain polarize, it's still one direction but that
direction is now at a slightly different angle. When you have a pure chiral sample, meaning
a solution that is optically active, it's going to turn it in one direction or the other,
where enantiomers being opposite are going to turn it in opposite directions, for example,
if we look at 2-bromobutane, we can put the bromane forward on carbon-2 or back on carbon-2
where the one on the left is R 2-bromobutane and the one on the right is "S". This specific
molecule, based on lab experiments, has been proven when put into a polarimeter, the "r"
sample of 2-bromobutane comes out to be a negative value, so what would the s sample
be? Remember enantiomers are equal but opposite so whatever number you get for r, you're going
to get the exact number for s, but the positive value, which means that r is l-rotary and
s is d-rotary for this specific example.
Don't worry, we'll look at math on the next video, what happens if I put a solution that
has 50% r and 50% s into the same polarimeter? As the light is traveling through, the R molecules
are attempting to turn it to the left, but the S molecules are attempting to turn it
to the right. So the light will hit r and turn to the left and hit s and turn to the
right and r and s and r and s and because the concentrations of both are the same because
it's a 50/50 mixture, the net result is going to be a zero degree change, it comes out exactly
as it went in because a solution that is racemic is considered optically inactive. Not because
the molecules don't work but because they're fighting against each other and every right
turn gets cancelled out by a lefter sabotaging each other for a net result of no change.
It's not just racemic mixtures that give you no turn, a chiral molecules are also considered
optically inactive because if they have no chiral center they don't turn right, so you
wouldn't know if it's racemic or simply a chiral if you had a zero degree rotation.
But if the mixture is not quite racemic, meaning you have r and s but you have more of one
or the other, you're still going to get some rotation and this brings us into the math
for absurd and specific rotation, which were going to cover in the next video.
You can find this entire video series, along with the chirality practice quiz and cheat
sheet by visiting my website, www.leah4sci.com/chirality