Extraction
is a technique that separates compounds (usually solids) based on solubility.Furthermore,
two solvents are employed, and the two solvents are "immiscible" - that
is, they do not mix with each other.Two
different liquid layers will form (like oil and vinegar in salad dressing).The
denser liquid will be the lower layer.(If
the two liquids have the same density, you'd have a "lava lamp".)
Let's
say we have a mixture of solid salt (NaCl - ionic) and solid naphthalene
(C10H8 - nonpolar).When
we take the solid mixture and slurry it in a mixture of water (strongly
polar) and diethyl ether (CH3CH2OCH2CH3
-
weakly polar), the salt dissolves in the water, and the naphthalene dissolves
in the ether.Diethyl ether happens
to be less dense than water, and will float on top.We
could pipet off the top layer, and evaporate the ether to isolate the solid
pure naphthalene.Or we could use
a special piece of glassware to drain off the lower layer, separate from
the top layer.The top layer could
then be drained into a different beaker.
This
type of glassware is called a "separatory funnel", seen seen on the lower
left
here in the Chemkit, and here
in use.The mixture to be purified
is dissolved in a mixture of two solvents, and shaken to ensure complete
dissolution.Needless to say, the
stopper is in place, and the stopcock is closed during the shaking.After
brief shaking, the funnel is usually inverted under a hood, and the stopcock
is opened carefully to vent the mixture.This
is done because extraction solvents are often low boiling (~40oC),
and body temperature is ~37oC.Quite
a lot of vapor pressure will build up in the sep funnel, and venting releases
this pressure.Since the vapors are
often toxic, please always vent in the hood.The
sep funnel can then be clamped, or set in a small ring, or clay triangle,
and then the stopcock is opened, and the lower layer can be drained.Needless
to say (?), the stopper must be removed for the liquid to flow out freely.When
you see the interface between the two liquids come near the stopcock, close
the valve.You have just separated
the two liquids.Polar solids have
dissolved in the more polar liquid, and nonpolar solids have preferentially
dissolved in the less polar solvent.By
the way, we usually (indeed, must) choose two liquids of very different
polarity - otherwise the liquids would not be immiscible, they'd be miscible!We
wouldn't have two layers.
Reality
Check:The dissolving process is
often not an all-or-nothing proposition.For
example, a 90% of a solid might dissolve in solvent "A" and 10% in solvent
"B".[The technical term for the
percentages is "partition coefficients"].This
is not a problem because the extraction process is never done just once.The
dissolve-shake-drain cycle is done at least twice in practice.Convince
yourself that given a 90% partition coefficient, two extraction cycles
will lead to a 99% pure product.
ACID/BASE EXTRACTION
In
our actual laboratory exercise, we will separate three solids, none of
which dissolves much in water - yet one of our layers in the extraction
will be aqueous!We will reversibly
and selectively change the polarities of two of the components by making
use of their acid properties.All
three components dissolve in dichloromethane (methylene chloride, CH2Cl2)
and are
Benzoic Acid - weak
acid - pKa ~5
b-Naphthol
- weaker acid - pKa ~10
Biphenyl - Not at
all acidic - pKa ~40
None of these compounds
in their acid form is soluble in water.However,
if you remember your CHM 112 (Henderson-Hasselbach equation), you'll remember
that at a pH of about 8, benzoic acid is 99.9% converted to its conjugate
base, the water-soluble benzoate.At
pH ~ 8, b-naphthol
and biphenyl are still protonated; they are still water insoluble.
Dissolve all three
components in CH2Cl2.Shake,
in a separatory funnel, with aqueous sodium bicarbonate (pH ~ 8).Two
components stay in the methylene chloride layer, while the third component,
as its sodium salt, dissolves in the water layer.By
draining off one liquid layer separate from the other liquid layer into
two labeled beakers, you have separated one solute from the other two.
The remaining two
components in the CH2Cl2 are the phenol and the hydrocarbon.At
a pH of ~13, b-naphthol
is 99.9% converted to its conjugate base, the water-solubleb-naphtholate.
Take the methylene
chloride solution containing b-naphthol
and biphenyl.Shake, in a separatory
funnel, with aqueous sodium hydroxide (pH ~ 13).One
component stays in the methylene chloride layer, while the second component,
as its sodium salt, dissolves in the water layer.By
draining off one liquid layer separate from the other liquid layer into
to labeled beakers, you have separated one solute from the other one.
Remember:each
of the extraction processes (bicarb and hydroxide) is done twice.Each
extraction will have two different layers.If
you have one layer, you have mixed water with water.
At the end, you will
have three beakers.One has water
as the solvent and benzoate as a solute.Another
has water as the solvent and naphtholate as the solute.The
third has CH2Cl2 as the solvent and biphenyl as the
solute.
OBTAINING SOLIDS
The extraction process
is now over.We need to get back
our solid samples.Remember from
recrystallization lab that benzoic acid is not soluble in cold, pH neutral
water.Chill the benzoate beaker,
and slowly add aqueous HCl.Lots
of fizzing will result from the NaHCO3 being neutralized.The
pH will drop, and benzoic acid will solidify.Obtain
a Buchner funnel, etc. (on the left in the
photo in case you forgot) and vacuum filter your product.This
technique will also work for obtaining solid beta-naphthol.
We will get our solid
biphenyl by the following method.First
we need to dry the solvent.[Wait
- I thought all liquids were wet ?!?]Welcome
to organic lab.When you "dry a solvent",
it means you are removing traces of water from the solvent.Just
because the liquids are immiscible doesn't mean that a little water doesn't
end up in the CH2Cl2.Add
three spatula tips full of solid anhydrous sodium sulfate to the beaker
that holds the CH2Cl2 solution.Swirl
gently and decant the liquid into a 25 mL round-bottom flask.The
solvent is now dry.
Next we will dry the
solvent, in the conventional sense of evaporating it, or boiling it off.You
could leave the flask in the hood and come back next week, or set up a
simple distillation … or … we can use the "Rotovap".The
Buchi rotary evaporator is a research lab piece
of equipment that quickly removes the solvent from a solution.
