Have you ever
wondered why vodka is such a strong alcoholic drink
compared to other beverages? That's because they
use a process known as distillation once
or even multiple times to increase the
concentration of ethanol in the drink. Today, we'll be talking
about how distillation works. You can do this in your
organic chemistry lab, and let's take a look at
the setup I've drawn here. First, in green, you have what's
called the distilling flask. This is where you put in
your mixture of compounds that you want to separate out. Next, in orange, you
have your oil bath. You want to use an oil bath,
because oil won't evaporate when you heat it up, and
it's good for maintaining a constant temperature
throughout this process. Shown up there in red,
you have the thermometer. You need to be able to
measure what temperature your compounds are
boiling out at. And shown in yellow
is the condenser. With a condenser, water has
to cycle in and then out. This keeps the condenser
cool, and the reason the condenser needs to be
cool is because distillation involves a series
of vaporizations and condensations. So initially what
happens is, you have some liquid in
your distilling flask, and as it gets heated
up, it turns into a gas. Then, because the
condenser is so cool, it'll condense down
the distilling flask, ultimately landing here
in the pink receiving flask as a liquid. This also needs to be kept cold. So what I've shown here
in blue is the ice bath. And again, it's kept cold
for the exact same reasons. You want this whole right
side of the setup to be cold, so that the liquid can
readily condense back into its pure form. There's one more tiny thing
we haven't labeled yet, and that's this. This is a vacuum adapter. Why would you need a vacuum in
distillation, you might ask. Isn't it enough to
just heat it up really hot to get it to
evaporate to a gas? Well, no actually,
because sometimes you have compounds that have
very, very high boiling points. If you have such a
high boiling point, it can be difficult to distill. But lowering the pressure
of the entire system makes it easier to
vaporize substances, because at lower pressure
there isn't much of a force pushing back down
on the liquid, which makes it so much easier
for it to vaporize upwards into the gas phase. Let's take a
two-component mixture. The first thing
you have is hexane, and I'll show that
here in the flask. The second thing
you have is toluene. This is a pretty
conjugated aromatic ring, which is why it has a
higher boiling point, and I'll also show
that here in the flask. How do we monitor what's going
on during a distillation? Usually, you'd want to plot
this out in the form of a graph. We're plotting
temperature versus time, with temperature on the
y-axis and time on the x-axis. And what happens first
is, initially, you're just heating up the system. So the temperature's
rising slowly but surely. When you approach the
boiling point of hexane, around 68 degrees,
you see a plateau. Why is it that
you see a plateau? Let's quickly review what
happens during a phase change. The temperature stays constant. What you'll see here as the
hexane is going from liquid to gas is it gets
vaporized up here. You'll see this temperature
register in the thermometer, and then it'll condense back
down into the liquid phase since this side is so
cool, and there you'll be collecting these
two drops of hexane. What happens after we've
collected that flask? What you'll see again is
an increase in temperature. And make sure to switch
out your receiving flask. You'll see me magically
change the color of the flask to show that this is a brand
new flask for collecting pure toluene. You have this toluene
now at 110 degrees, again hitting a plateau, because
once more that represents a phase change going
from liquid to gas, then condensing back into
liquid again and dripping into this new receiving flask. So ultimately what
we've collected are these two flasks, one with
toluene and one with hexane. And there we were able to do a
pretty successful distillation. What happens if instead you
have a three-component mixture? Well, this works
pretty much the same as a two-component
mixture, except that you'll see more plateaus in your graph
of temperature versus time, which I'll draw
here on the side. The first compound
that we have here is acetone, which
looks like this. The next one is
cyclohexane, which has a slightly
higher boiling point. And lastly, you
have acetic acid. This has a higher boiling
point than the other compounds, because it's capable
of hydrogen bonding, meaning there's more forces
between the acetic acid molecules, making it
harder to pull them apart into the gas phase. So again, let's
look at our graph. First, what you might see
again is a slight increase at the beginning when you're
just heating up your flask. But as soon as you
hit 56 degrees, you'll see that plateau
as acetone vaporizes and condenses. Then, you'll see the
temperature increase again until 81 degrees, where
it'll hit a plateau, and that represents cyclohexane
vaporizing and condensing. Lastly, you'll see
another increase. And finally, you'll be
able to get acetic acid. Let's take our final example
and answer the question, how is it that they get
vodka and other drinks to be so strong. To produce strong drinks, this
is the kind of distillation they might need to do. As you can see,
ethanol and water, their boiling points
aren't too different from each other, only
22 degrees Celsius. Do you think that
will affect what happens during the
distillation process? Well ideally, what
you'd want to see in your graph of
temperature versus time is like what we've
seen in the past. You'd hope that
what would come out is something that looks like
an increase in temperature, followed by a plateau
at 78 degrees, followed by another sharp
increase, and finally a plateau at 100 degrees. However, that's not
really what happens. Instead what you
get is something that looks kind of
like this purple graph. You might get this
increase at the beginning, but instead here
you have this slope. Why is this is a problem? Well, before you might have been
able to get the pure ethanol, meaning the pink line,
and pure water separate. But here, they're kind of
mixing into this purple. So anything you see
between these two points isn't really pure
ethanol or pure water, it's some mixture of the two. And if you're still
getting a mixture, it means your alcohol
still isn't very strong. So how can we fix this problem? You might think, what happens
if I distill the compound again? If you were to distill
this compound again, the next time you
might get something that looks kind of
like this orange line. This would be a
little bit flatter, maybe it would get a little
bit steeper than before, but it still wouldn't
be very pure. And if you kept distilling
it over and over again, you might eventually
reach what you had hoped to get
in the beginning, that ideal blue and
pink separation. But just doing a simple
distillation multiple times can be extremely
time consuming, so is there another
way we can do it? There's actually
another setup that will allow us to effectively do
multiple distillations at once. This is known as
fractional distillation. If you look at this
picture in the right, the only thing different
between this picture in the picture on
the left is that you have a fractionating
column here. This column can be filled
with a number of substances, such as beads or other
things, but just for fun I'm going to fill it
with some stars. So you see, you want to
completely fill this column. And how does that affect what
goes on during distillation? If you had your two
compounds again-- let's just say that you
have purple representing two compounds in
here-- what happens is that instead of just
going straight up, vaporizing once, and then condensing
again, it will go up, vaporize, and then maybe condense
onto one of these stars. And from there, it'll
vaporize again then condense into another of these stars. So as it goes up and up
past the packaging material, it's going through so many
vaporizations and condensations so that when it finally does
reach the receiving flask, it'll be much purer
than what you would get through just one
simple distillation. So let's summarize what
we've talked about today. We talked about how you would
set up a simple distillation and how that's great for
separating out compounds with pretty big boiling
point differences, say a difference bigger
than 25 to 30 degrees Celsius, and fractional
distillation, which is great for
separating out compounds with smaller differences
in boiling point.