Organic 211L
Recrystallization
Additional Handout
Recrystallize. Why? To separate a compound from impurities.
How to recrystallize (generally): Dissolve your solute in the least amount of solvent
necessary. Remove impurities if they are insoluble by a hot filtration. Or if you have
soluble impurities (or after removing the insoluble impurities) chill the solution to
recrystallize the solid. After a solid has formed, vacuum filter the solid from the liquid
using a Buchner funnel.
a) Find a good recrystallization solvent. A good recrystallization solvent is one that does
not dissolve the solute when cold (around room temperature or colder) but does dissolve
the solute when warm (preferably near boiling). For solids, the solubility of solutes
increases as the temperature increases. A good recrystallization solvent should not be
toxic or react with your solute. A way to test for a good recrystallization solvent would be
to put a small amount of solute in a test tube and then add a set amount of solvent at room
temperature. The test tube would then be put in a beaker of boiling (or warm) water. Be
careful to not be above the boiling point of the solvent. For example, methanol boils at
64.7 oC and putting a test tube of methanol in boiling water would boil off the methanol.
Remember: Like dissolves like. One solvent may not give the properties you need. Two
(or more) solvents may be required. You could use methanol to get your sample to
dissolve and then add water (second solvent must be miscible with the first one) to
decrease the solubility of your solute.
b) Find the least amount of solvent necessary. There are two ways to find the least
amount necessary.
i) If you know the identity of the sample you are recrystallizing, you may look it up in
books or the Internet. For example, benzoic acid on Wikipedia says
http://en.wikipedia.org/wiki/Benzoic_acid{soluble (hot water); 0.34 g/100 mL (25 oC),
THF 3.37 M, ethanol 2.58 M, methanol 2.91 M}
ii) Determine it experimentally by adding the same amount of solute to many test tubes.
You would then add different amounts of solvent to each tube. For example, you could
add 0.1 grams of solute to a test tube. You would then add 5 mLs, 10 mLs, 15 mLs, etc.
of solvent. Heating the tubes up and then chilling them back down would return you
solid. You would see which tube gave you the highest percent recovery and use that
amount of solvent.
c) Recrytallize your solvent depending upon whether your solute has soluble or insoluble
impurities.
Soluble impurities – This means the impurities dissolve in the solute and do not reform a
solid upon cooling.
Insoluble impurities – This means the impurities never dissolve in the solute even with
heating and must be removed by hot filtration.
C – Insoluble impurities –
1) Fold a fluted filter paper by folding a filter paper in half. Now, fold it in half again.
Now, fold it in half again. One more time. Now open the filter paper to a half moon and
fold each visible fold in half by going back and forward.
2) ALWAYS RECRYSTALLIZE IN AN ERLENMEYER. Why? Because the solvent
leaving the flask hits the sides of the flask and returns back into the flask. Will you lose
some solvent? Sure, but not near as much as if you recrystallized in a beaker. Add the
solute into an Erlenmeyer.
3) Add the least amount of solvent into the Erlenmeyer at room temperature and add a stir
bar.
4) At the same time as step # 2, add an Erlenmeyer of some solvent onto the hotplate as
well.
5) Turn heat up on hotplate. Temperatures on hot plate do not correspond to your
solution. A higher number means a warmer plate. If you turn the number up too high, the
porcelain top WILL crack and you will be charged for a stirring hot plate ($800).
6) Heat your sample until all of your solute dissolves. THIS IS VERY TOUGH TO
CALL. When you do you know when your sample has dissolved and all that is left are
impurities? Good question.
7) Add your fluted filter paper to a powder funnel (this is a funnel with a wide neck so it
won’t plug.) Put the powder funnel and filter paper in a beaker. Pour your hot solution
carefully into the filter paper and the hot solution will go into the beaker. Your solution
will cool down going into the beaker so some solid will form in the beaker.
8) Some solid will form in the fluted filter paper on the powder funnel. You take the extra
hot solvent from step # 4 and pour some of it (as little as possible) on the fluted filter
paper while over the beaker to wash it into the beaker. This beaker will not contain more
than the least amount of solvent possible.
9) Concentrate your sample. How? Assume you put 10 mLs in step # 3. Also, assume you
added 5 mLs in step # 8. You would put your beaker on a hot plate and boil off the
solvent (IN A HOOD) back down to 10 mLs.
10) Chill your beaker down to 5 oC or so. The colder the better.
11) Vacuum filter your solid through a Buchner funnel (shown below). The vacuum will
be furnished by attaching a hose to the filter flask and attaching it to a water aspirator.
Turn the water on full to get the full pressure. WATCH FOR LEAKS OF WATER ON
THE FLOOR. The filter paper must fit EXACTLY in the Buchner funnel and cannot go
around the sides (or your product will escape to the filtrate.) The liquid that comes
through is called the filtrate or MOTHER LIQUOR. We will not use standard taper
joints as shown in the drawing below but will use a FILTER ADAPTER.
Filtrate or
Mother
Liquor
12) Let your solid dry for a week or more (depending upon solvent.)
13) DISCARD THE MOTHER LIQUOR IN THE APPROPRIATE PLACE.
C) Soluble Impurities
1) ALWAYS RECRYSTALLIZE IN AN ERLENMEYER. Why? Because the solvent
leaving the flask hits the sides of the flask and returns back into the flask. Will you lose
some solvent? Sure, but not near as much as if you recrystallized in a beaker. Add the
solute into an Erlenmeyer.
2) Add the least amount of solvent into the Erlenmeyer at room temperature and add a stir
bar.
3) Turn heat up on hotplate. Temperatures on hot plate do not correspond to your
solution. A higher number means a warmer plate. If you turn the number up too high, the
porcelain top WILL crack and you will be charged for a stirring hot plate ($800).
4) Heat until all the solute dissolves. This is much easier to determine than with insoluble
impurities.
5) Chill your beaker down to 5 oC or so. The colder the better.
6) Vacuum filter your solid through a Buchner funnel (shown above). The vacuum will
be furnished by attaching a hose to the filter flask and attaching it to a water aspirator.
Turn the water on full to get the full pressure. WATCH FOR LEAKS OF WATER ON
THE FLOOR. The filter paper must fit EXACTLY in the Buchner funnel and cannot go
around the sides (or your product will escape to the filtrate.) The liquid that comes
through is called the filtrate or MOTHER LIQUOR. It will contain your impurities.
7) Let your solid dry for a week or more (depending upon solvent.)
CALCULATIONS: Percent recovery
To calculate percent recovery, you use the following equation.
Amount Recovered
Percent
Recovery
Amount Started With
You should know two solubilities. The solubility of the solute when in cold solvent and
the solubility of the solute when in hot solvent.
For example: Assume benzoic acid dissolves 1 g/ 5 mLs of solvent when the solvent is at
98 oC.
Assume benzoic acid dissolves 1 g/ 45 mLs of solvent when it is at 4 oC.
Question # 1 – Assuming you use the solubility numbers above and that you have 5
grams of material to recrystallize. What is the least amount of solvent necessary?
We use the hot solubility numbers. 1g/ 5 mLs. We multiply by 5 (since we have 5 grams)
and 25 mLs would be the least amount of solvent necessary.
Question # 2 – What is the maximum percent recovery?
Now, we use the cold solubility numbers. 1 g/ 45 mLs
If we used 25 mLs in question 1 as our least amount necessary, we would set up the
equation below and solve for X.
X
1g
25 mLs
45 mLs
X = 0.55 grams. This is the amount of material still dissolved in our solution. So
remembering the percent recovery equation, 0.55 grams still dissolved, and our starting
amount of 5 g, we solve for percent recovery.
5 g - 0.55 g
Percent
Recovery
5g
Percent Recovery = 89 %