EXPERIMENT #2
Purification Of An Unknown Solid By
Recrystallization And Identification By Melting
Point
Recrystallization is a procedure that is often used to 'purify'
organic compounds which are obtained in an impure state from a
chemical reaction or extraction process. In recrystallization
the compound to be purified, a solid at room temperature, is
dissolved in a hot solvent, filtered (usually gravity) and
allowed to cool. The purified compound will then crystallize out
of the solution. Separation of the purified compound from the
'mother liquor' is achieved by filtration (usually vacuum)
followed by air drying. Typically purity is 90% after one
recrystallization step (99%, 99.9% after two and three
recrystallizations respectively).
For recrystallization to occur the following requirements must
be met:
1. The solvent must not react with the desired compound
2. The purified compound must be easily removed from the
solvent (usually by filtration)
3. The purified compound must be very soluble in hot and
almost insoluble in cold solvent
4. The impurities must either be very soluble in cold solvent
or almost insoluble in hot solvent
When recrystallizing mixtures contain small quantities of
colored impurities, small amounts of selective absorbents such
as charcoal are often added to the hot solution before
filtration, to remove the colored impurities. It is important to
avoid excess amounts of absorbents since they have a tendency to
also absorb appreciable amounts of the substance being purified.
To ensure good recovery it is also necessary to avoid excessive
amounts of solvent. Usually 10% more than the minimum amount of
solvent is used (The minimum amount of solvent required may be
determined by using tabulated solubility data).
During the filtration process it is necessary that both the
solution to be filtered and the apparatus are kept hot. If
appreciable cooling occurs, crystals may form prematurely (in
the filter) which results in significant yield reduction.
In many instances crystallization does not occur spontaneously.
To help induce crystallization it may be necessary to scratch
the side of the container with a stirring rod (both in and out
of the solution). Alternatively, crystallization may be induced
by the 'seed' method, this involves adding a few very small
crystals of pure compound to the cooled sample.
Experimental Procedure:
Step 1: Recrystallization
You will receive a small sample of material of an impure solid
from TABLE 1. Each of these materials can be successfully
recrystallized from hot water (so you will not be going through
the process of determination of the proper recrystallization
solvent). Heat your solvent close to the boiling point. Add some
of your impure unknown to an 250 ml Erlenmeyer flask (Never keep
empty glassware on hot plate, because the glass will crack!).
Transfer some hot water to the flask containing the unknown.
Heat this mixture on hot plate. Add just enough hot water till
all your unknown has dissolved in hot water. Allow the water to
continue heating for 3 to 5 minutes. Try not to add more than 20
mL of solvent. Remove the mixture from the hot plate and
carefully add a spatula tip full of decolorizing carbon
(charcoal).
At this point you will perform a “hot filtration” over a steamed
fluted piece of filter paper. If you need to, this is the point
where you will boil off excess solvent. After the filteration
step, this material needs to be removed from the hot plate and
allowed to cool slowly. Once this flask has cooled off, you
should see crystals form. This material will then get chilled in
an ice bath for ~ 10 mins. The contents (recrystallized
unknown) of the Erlenmeyer flask is filtered with a Büchner
funnel.
You should allow yourself enough time to recrystallize the
material and allow it at least 30-40 minutes to dry in the oven
before you attempt to perform a melting point/mixed melting
point. The oven should be at least 80OC to drive off as much
water vapor as possible (at least 20 minutes).
Step 2: Melting Point Determination
Determine the melting point of your unknown, heating the unknown
at no more than 4 degrees per minute near the suspected melting
point. Please be aware that there are only 6-7 Meltemps per lab
room so you will need to share. However, a Meltemp can have
three melts going on simultaneously (in parallel tracks) so it
can be useful to do pure and mixed samples side by side.
When you look through the eyepiece
of the Meltemp, this is what you
will see: three tubes with material
side by side.
When you attempt to ascertain the
identity of your unknown, you want
to mix your material 1:1 with
materials that you suspect of being
your material. If you chose
correctly your material and the
known will melt at the same point.
If your material is not the same as
the chosen known, you will see a melting point depression below
the melting points of both of the materials (the eutectic
point). Also, allow the Meltemp to cool down to below 70 degrees
before you attempt to perform another melt attempt.
TABLE OF POSSIBLE UNKNOWNS:
Name
Benzamide
Benzoic Acid
Acetanilide
Succinimide
Cyanoacetamide
Glucose Pentaacetate
Adipic Acid
Benzilic Acid
Salicylic Acid
Cinnamic Acid
Melting Point(oC)
128
122
114
126
120
131
152
150
157
133