Cerritos Community College
Organic Chemistry 211 Laboratory
Purification of Impure Acetanilide
(Recrystallization)
Objective: Impure acetanilide will be purified and isolated by the means of recrystallization.
Introduction: Recrystallization (AKA: crystallization) is a purification technique that is based
on the difference in the solubilities of the desired compound and the impurity (impurities)
contaminating the desired compound, as a function of varying temperature. This makes the
choice of the right solvent a critical one. As a rule of thumb, at high temperatures (i.e. the boiling
point of the solvent) compound of interest and the impurities should be overly soluble in the
solvent, but at lower temperatures, the compound of interest should be quite insoluble in the
solvent of choice.
A quick overview of the process of recrystallization is the following:
The crude mixture is placed in the minimum volume of the solvent of choice that would dissolve
the entire solid at (or near) its boiling point.* This mixture is then heated and stirred to reach
boiling. The objective at this point is to obtain a saturated solution at boiling. After reaching
the boiling point, with the entire solid dissolved, the solution is left to cool slowly. As the
temperature is slowly dropped, the compound of interest gradually starts to precipitate out
(crystallize), while the impurities stay dissolved. Normally, the mixture is cooled in an ice bath
in order to maximize the difference in the solubility behaviors of the compound of interest with
that of the impurities, and to also maximize the formation of the crystals of the compound of
interest.
The pure crystals of the cooled mixture can then be separated through a filtration. The filtrate
(pure crystals) should be rinsed with a small amount of cold solvent in order to insure a higher
purity.
Relevant Calculations: Calculation of % Recovery for the recrystallization process measures
the efficiency of the purification and isolation combined.
Final Mass of Pure Crystals
% Recovery = __________________________________________ x 100
Initial Mass of Crude Crystals
* The minimum amount of solvent needed to achieve a saturated solution at boiling is not easy
(or sometimes possible) to predict. The experimenter is therefore advised to take a trial-and-
error approach. Start with a small volume, heat and stir to boil. If there still are solid particles,
add another small portion of the solvent. This will cool down the entire mixture momentarily,
and more solid will precipitate out of the solution. However, upon continued heating and
reaching the boiling point, the added solvent will provide more space to dissolve the remaining
solids.
Procedure: Mass 2.0 grams of crude acetanilide into a 100 mL Erlenmeyer flask and add a
small volume of water and a magnetic stir bar. Place the flask on a hot plate/stirrer and bring it to
a gentle boil with the entire solid dissolved (i.e., a saturated solution). Gently take the flask off
the hot plate/stirrer and set it aside, with the minimum agitation. Let the flask cool slowly and
without any agitation, in order to obtain high-purity crystals of acetanilide. As the contents of the
flask are being cooled, prepare an ice bath using a crystallizing dish or a large beaker. By the
