WEBVTT

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[MUSIC - CLAUDE DEBUSSY, "DEUX
 ARABESQUES"]

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PROFESSOR: Reaction work-up 1--

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extracting, washing, and drying.

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As all chemists
quickly learn, it

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is very rare for a
chemical reaction

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to provide only the one compound
that you're looking for.

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Even highly selective reactions
generate the desired product

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as a crude mixture, containing
byproducts and underreacted

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starting materials and reagents.

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Liquid-liquid extraction is
the most common technique

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for separating a compound
from a complex mixture.

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In an extraction, you
can take advantage

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of differing solubilities
to selectively separate

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the different components.

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By carefully planning
out an extraction

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and washing sequence,
otherwise known as a work-up,

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the desired product can
usually be separated

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from the unwanted impurities.

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This video will illustrate
the proper technique involved

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in working up a reaction.

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In an ideal world, each chemical
reaction in the laboratory

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would provide only the desired
product in its pure form.

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In reality, the product
is almost always generated

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as part of a messy mixture of
byproducts and side products.

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In short, garbage.

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Liquid-liquid extraction
is a useful technique

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used to isolate the
desired compound

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from a complex mixture.

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In an extraction, the
mixture is partitioned

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between two immiscible solvents
in a separatory funnel,

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otherwise known
as a "sep" funnel.

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The key is that the two
solvents are mutually insoluble,

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so they form two
distinct layers.

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One of the solvents is
almost always water,

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and the other is an organic
solvent of your choice.

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The different components
of the mixture

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are selectively distributed
in one solvent or the other,

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depending on their
different solubilities.

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In this way, you
can take advantage

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of differing solubilities to
selectively transport solutes

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from one layer to another.

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In most cases, but not all,
neutral organic molecules

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prefer the organic
phase, while charged

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molecules and inorganic salts
prefer the aqueous layer.

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By carefully planning
out an extraction

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and washing sequence, otherwise
known as a reaction work-up,

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the desired compound can
usually be separated from most

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of the unwanted impurities.

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This brings up an
important point.

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What is the difference between
an extraction and a wash?

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These terms describe similar,
but fundamentally different,

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operations.

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An extraction involves
pulling the desired compound

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out of a mixture of compounds.

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Whereas, in a wash, you are
pulling unwanted impurities

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away from the desired product.

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In other words, you
always keep the extracts,

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and trash the washes.

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This might sound confusing,
but it will hopefully

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become clear as we continue.

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The first step in an extraction
and washing sequence--

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once you have decided on
the appropriate solvents,

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of course--

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is filling the sep funnel.

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It is a good idea to use an
iron ring and a nice, cushy cork

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ring to safely support
the sep funnel.

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Before you add any solvent,
close the stopcock,

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and place a clean, dry
flask under the sep funnel.

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This will prevent you from
dumping precious material

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all over the dirty bench.

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The flask will save you if the
funnel leaks, or the stopcock

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somehow opened itself.

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When the stopcock
is safely closed,

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and the flask is in place,
pour your two solvents

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into the sep funnel.

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Use a sep funnel large
enough that you don't

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fill it more than 3/4 full.

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If it is too full, you won't
be able to efficiently mix

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the solvents.

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This brings us to
the next step--

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mixing and venting.

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It is important to mix the
two solvents very well.

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This increases the
contact surface area

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and allows the distribution of
solutes to occur very rapidly.

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But use caution--
mixing will frequently

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cause pressure to build up
inside of the sep funnel.

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Pressure buildup is
especially common

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when you are using
particularly volatile solvents,

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like diethyl ether, or
compounds that can generate gas,

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like sodium bicarbonate.

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Under acidic conditions,
sodium bicarbonate

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will release gaseous
carbon dioxide.

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Before any vigorous
mixing, it is a good idea

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to gently swirl and
invert the sep funnel.

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Then, invert the sep funnel,
holding the stopper tightly.

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Allow the liquid to drain
away from the stopcock,

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and point the tip of
the sep funnel away

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from yourself and other people.

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Slowly open the stopcock
to allow venting.

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Now, you're ready
to really mix it up.

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Hold the sep funnel firmly,
and shake it vigorously

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for several seconds.

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Once again, invert the
funnel and vent it carefully.

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Repeat the shaking and venting
until you no longer hear

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gas escape when you
open the stopcock.

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Then, set your funnel down,
and let the layers settle.

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Ideally, two layers with
a nice, clean interface

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should form very quickly.

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Unfortunately, things don't
always go so smoothly.

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A thick, cloudy layer,
called an emulsion,

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can sometimes form between
the two solvent layers.

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An emulsion is a colloidal
mixture of the two solvents,

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and is frequently caused by
the presence of fine particles

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in the solution.

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Getting rid of an emulsion
can take some time,

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so if you know that you
are likely to generate one,

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your best bet is to take pains
to prevent it ahead of time.

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For example, gently swirling
and inverting your sep funnel

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will rarely cause
an emulsion to form.

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You will have to spend more time
mixing because swirling is not

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as efficient as shaking,
but it may be worth it.

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Don't worry, if you unwittingly
generated a monster emulsion,

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you still have a few options.

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First, you can
just sit and wait.

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It might clear itself up,
even if it takes a few hours.

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If you don't have all day
to finish your extraction,

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try swirling the mixture gently,
and stirring the emulsion

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with a glass rod.

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If this doesn't work,
add several milliliters

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of saturated sodium
chloride solution

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to the funnel, and swirl to mix.

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The sodium chloride
increases the ionic strength

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of the aqueous
layer and decreases

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the solubility of the
organic solvent in the water.

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As a last resort, you may
need to use vacuum filtration

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to filter your entire mixture
through a pad of Celite.

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When you are carrying
out an extraction

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and washing sequence,
it is very important

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that you keep track of
which layer is which,

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and what compounds are
dissolved in each layer.

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This means that you constantly
need to be asking yourself,

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which layer is on the top?

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Which layer is on the bottom?

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One way to keep track of
this is to know something

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about the densities of the
solvents you are using.

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In every case, the
solvent with the lowest

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density will be on top.

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And the solvent with the highest
density will be on the bottom.

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This is helpful because
you can generally

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assume that dilute
aqueous solutions have

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a density around 1 gram per
milliliter, similar to water.

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The organic solvent
will be in the top layer

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if it has a density less
than 1 gram per milliliter,

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such as hexane, diethyl
ether, or ethyl acetate.

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The organic solvent
will be the bottom layer

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if it has a density
greater than 1

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gram per milliliter, such
as methylene chloride

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or chloroform.

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But be careful-- high
concentrations of solutes

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can sometimes drastically
affect the density of a solvent.

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If you're having
trouble figuring out

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which layer is
which, you may need

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to perform a quick
solubility test.

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Take a couple of drops
of the layer in question,

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and add them to a small amount
of water in a test tube.

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If the drops dissolve without
turning the water cloudy,

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then the drops were
from the aqueous layer.

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If the solution turns
cloudy, or the drops

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form an insoluble
layer on the bottom,

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or an insoluble
layer on the top,

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then the drops are
from the organic layer.

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But even if you think you
know which layer is which,

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do not discard any of
the layers until you

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are absolutely
sure that you have

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isolated all of the material.

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Once you have a good idea
of where your material is,

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it's time to
separate the layers.

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Before you do anything, place
a clean, dry labeled beaker

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under the sep funnel,
and remove the stopper.

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If you don't do this,
you'll have some trouble

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getting any liquid to drain.

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With the stopper off,
open the stopcock,

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and allow the bottom layer
to drain into the beaker.

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Take it slow when you
get near the interface,

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so that you can close the valve
precisely in between the two

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layers.

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Once you have drained the bottom
layer, you have a few options.

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If you still need to perform
a wash or an extraction

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with the top layer, then
leave it in the funnel.

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Pour in the second solvent,
and proceed to mix and vent.

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If you are done with the top
layer, then pick up the funnel,

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and pour it into a second
clean, dry labeled beaker.

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Here is a little bit of sep
funnel etiquette to follow.

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When you are
separating the layers,

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always drain the bottom
layer through the stopcock,

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and pour the top
layer through the top.

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This will minimize
recontamination

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of your material.

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Before we move on to proper
post extraction procedures,

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let's walk through
a sample reaction

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work-up, similar to one you
might use in the laboratory.

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Let's say that you performed
an acylation reaction

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to generate phenyl acetate.

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At the end of the
reaction, you were

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left with a solution containing
a lot more than the material

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you were looking for.

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So what do you do?

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First, you know
that phenyl acetate

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is soluble in diethyl ether,
and that diethyl ether

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is insoluble in water.

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Therefore, in your work-up,
you will use diethyl ether

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as the organic
layer, and wash it

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with different aqueous solutions
to remove the impurities.

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Now, it is time to
plan out the washes.

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It is important
that you use what

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you know about the impurities
to decide on appropriate aqueous

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washes for your mixture.

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Keep in mind that the washes
should only be 10% to 50%

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of the volume of the solution
that you are washing.

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And make sure to repeat
each wash two or three times

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to wash away as much of the
impurities, as possible.

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Your crude mixture contains
acetic acid, a fairly strong

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acid, and two weak bases--

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triethylamine and DMAP.

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To get rid of the
acetic acid, you

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need to wash the solution
with a mild base that

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will deprotonate the acid
and pull the charged acetate

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ion into the aqueous layer.

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Saturated sodium
bicarbonate works very well

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for this purpose.

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Add the sodium bicarbonate
solution and mix well.

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Make sure that you
vent frequently

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to release the carbon dioxide
gas that is generated.

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Finally, drain away
the aqueous solution

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that now contains
the sodium acetate,

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and repeat the wash
two more times.

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To get rid of the weak bases--

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triethylamine and
DMAP-- you need

00:13:27.470 --> 00:13:31.190
to wash with a dilute
solution of a strong acid that

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will protonate the
bases and pull them

00:13:33.260 --> 00:13:35.480
into the aqueous layer.

00:13:35.480 --> 00:13:39.980
A 10% aqueous solution of
HCL works well for this.

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Once again, add the wash,
and mix and vent well.

00:13:45.390 --> 00:13:47.280
Drain the aqueous
layer containing

00:13:47.280 --> 00:13:50.400
the protonated amines,
and repeat two more times.

00:13:54.160 --> 00:13:57.840
So far, this looks like a pretty
effective reaction work-up.

00:13:57.840 --> 00:14:00.840
But there is one more
thing to keep in mind.

00:14:00.840 --> 00:14:03.360
Water is slightly
soluble in ether--

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not to a large extent, but there
is some water hanging around

00:14:06.870 --> 00:14:08.850
in the organic layer.

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One way to pull
some of the water

00:14:10.530 --> 00:14:14.760
out is to wash it with saturated
sodium chloride, otherwise

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known as brine.

00:14:16.620 --> 00:14:19.440
The high ionic strength
of the salt solution

00:14:19.440 --> 00:14:23.460
decreases the solubility of
the aqueous layer in the ether,

00:14:23.460 --> 00:14:26.070
effectively washing
away some of the water.

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Voila.

00:14:30.610 --> 00:14:33.430
Now that you have done all
you can with your sep funnel,

00:14:33.430 --> 00:14:36.250
it's time to dry your
organic layer even more

00:14:36.250 --> 00:14:37.510
with the drying agent.

00:14:40.120 --> 00:14:43.240
Magnesium sulfate is
one popular drying agent

00:14:43.240 --> 00:14:45.850
because it is quick
and effective.

00:14:45.850 --> 00:14:49.090
It is also a very fine
powder, so take care

00:14:49.090 --> 00:14:52.210
that you don't leave any of
your desired material absorbed

00:14:52.210 --> 00:14:55.180
onto the surface
when you discard it.

00:14:55.180 --> 00:14:57.790
Keep in mind that
magnesium sulfate is also

00:14:57.790 --> 00:14:59.240
slightly acidic.

00:14:59.240 --> 00:15:01.270
So it may not be
suitable for use

00:15:01.270 --> 00:15:03.265
with compounds that
are sensitive to acid.

00:15:05.770 --> 00:15:09.070
To dry the organic layer
with magnesium sulfate,

00:15:09.070 --> 00:15:12.690
add a small amount of
the powder and swirl.

00:15:12.690 --> 00:15:16.710
Incrementally add more drying
agent, and swirl until you

00:15:16.710 --> 00:15:20.010
get the snow globe effect.

00:15:20.010 --> 00:15:23.790
There should be free,
unclumped powder in the flask,

00:15:23.790 --> 00:15:26.120
even after letting it
sit for a few minutes.

00:15:29.510 --> 00:15:32.960
Sodium sulfate is another
common drying agent.

00:15:32.960 --> 00:15:35.690
Although it is slower
than magnesium sulfate,

00:15:35.690 --> 00:15:37.820
it is effectively neutral.

00:15:37.820 --> 00:15:41.480
And, in its granular form, will
absorb less of your desired

00:15:41.480 --> 00:15:45.080
material onto its surface.

00:15:45.080 --> 00:15:47.960
Drying the organic layer
with sodium sulfate

00:15:47.960 --> 00:15:50.570
is similar to using
magnesium sulfate,

00:15:50.570 --> 00:15:52.970
except you may need
to look a bit closer

00:15:52.970 --> 00:15:56.150
to see whether there is
free, unclumped drying agent

00:15:56.150 --> 00:15:57.800
in the flask.

00:15:57.800 --> 00:16:01.400
It is also a good idea to let
the solution dry for 5 or 10

00:16:01.400 --> 00:16:05.440
minutes before removing
the drying agent.

00:16:05.440 --> 00:16:09.130
To remove the drying agent,
perform a gravity filtration

00:16:09.130 --> 00:16:11.510
with fluted filter paper.

00:16:11.510 --> 00:16:14.610
Once you have filtered
the entire solution,

00:16:14.610 --> 00:16:17.570
be sure to rinse the
drying agent very well

00:16:17.570 --> 00:16:21.500
so that you don't leave any
product stuck to the surface.

00:16:21.500 --> 00:16:24.920
It is a good idea to rinse
the drying agent three times

00:16:24.920 --> 00:16:28.368
with clean, dry solvent,
adding each of these rinses

00:16:28.368 --> 00:16:28.910
to the flask.

00:16:32.590 --> 00:16:34.650
Once you are confident
that you have transferred

00:16:34.650 --> 00:16:37.980
all of the desired material
to the round-bottom flask,

00:16:37.980 --> 00:16:40.200
you are ready to
proceed to the rotavap,

00:16:40.200 --> 00:16:43.470
and concentrate the
solution in vacuo.

00:16:43.470 --> 00:16:46.650
For detailed information on
proper use of the rotavap,

00:16:46.650 --> 00:16:50.772
you can watch the second
reaction work-up video.

00:16:50.772 --> 00:16:54.090
[CHIMING]

00:16:54.090 --> 00:16:56.700
Remember, this video
is intended to help

00:16:56.700 --> 00:16:59.610
you prepare for lab by
providing a demonstration

00:16:59.610 --> 00:17:02.310
of the proper
experimental technique.

00:17:02.310 --> 00:17:04.260
It is not intended
as a replacement

00:17:04.260 --> 00:17:08.310
for reading your lab manual
or the supplementary material.

00:17:08.310 --> 00:17:10.710
In order to become a
great experimentalist,

00:17:10.710 --> 00:17:13.140
it is important that you
understand both theory

00:17:13.140 --> 00:17:14.579
and technique.

00:17:14.579 --> 00:17:15.839
Now it's your turn.

00:17:15.839 --> 00:17:16.379
Good luck.

00:17:19.379 --> 00:17:36.229
[MUSIC - CLAUDE DEBUSSY, "CLAIR
 DE LUNE"]