Experiment 1 Recrystallization of Benzoic Acid
Experiment 1 Recrystallization of Benzoic Acid
Experiment 1 Recrystallization of Benzoic Acid
Quezon City:
Office of Research and Publications, Ateneo de Manila University; 2005.
Experiment 1:
After food containing benzoic acid or sodium benzoate is digested, the two are rapidly
absorbed from the gastrointestinal tract and metabolized and metabolized in the liver by
conjugation with the amino acid glycine. Benzoic acid is first converted to benzoyl CoA,
which then conjugates with glycine to hippuric acid. Hippuric acid is rapidly excreted
through the urine. Owing to rapid metabolism and excretion, an accumulation of the
benzoates or their metabolites in the body is not to be expected.
In this experiment, crude benzoic acid will be treated with decolorizing charcoal and
recrystallized.
Prelab Questions
1. Why should the crystallization solution be cooled first before adding decolorizing
charcoal?
2. How is premature crystallization prevented when filtering the insoluble impurities
from the hot crystallization solution?
3. Why shouldn’t the hot crystallization solution be immediately placed in an ice-water
bath?
4. What is the importance of covering the crystallization solution covered with a watch
glass or beaker during cooling?
5. Why should the crystals be washed with cold water during suction filtration?
Equipment
4.5 × 4.5 in filter papers
2.0 × 2.0 in filter papers
melting point apparatus
boiling chips
sample vials
Second Edition Draft For use only within the Ateneo de Manila University.
Guidote AM, del Rosario DR, Abuzo AL. Experiencing Organic Chemistry. Quezon City:
Office of Research and Publications, Ateneo de Manila University; 2005.
Chemicals
crude benzoic acid
decolorizing charcoal
Safety Notes
Be careful with adding charcoal and heating charcoal-treated solutions. Avoid getting
charcoal on your skin and clothing.
Procedures
TIP: Place an Erlenmeyer flask with funnel in an oven for use later in gravity filtration.
Weigh 2.0 g of contaminated benzoic acid into another Erlenmeyer flask. Add 2-3 boiling chips and
60 mL of distilled water, and then heat to boiling until the compound dissolves. Remove the flask
from the burner and add into it 15 mL of cold water. Add a spatula-point, or less, of decolorizing
charcoal.
TIP: If the filtrate still appears blue, treat the solution again with decolorizing charcoal.
Once the filtrate runs clear, let the solution cool, add 2-3 boiling chips into it and boil off excess
solvent until the volume is 50-60 mL. Allow the solution to cool down to room temperature and
crystallize. Chill it afterwards in an ice-water bath and then harvest the crystals using suction
filtration. Air-dry the crystals overnight. Determine the percent recovery and determine melting
point using a melting point apparatus.
TIP: Do not move the container while waiting for the compound to crystallize.
Compare the appearance of the crystals with crude benzoic acid. Place the recovered crystals in
separate vials, label, and hand them to your instructor.
Postlab Questions
1. Explain the difference between crystals and precipitates. Was the acetanilide
obtained in the experiment crystals or precipitates?
2. How would the amount of charcoal used affect the recovery of the experiment?
3. How would the temperature of the water used to wash the crystals during suction
filtration affect the recovery of the experiment?
4. What factors could affect the melting point of the benzoic acid obtained in the
experiment?
5. The solubility of benzoic acid at 100 °C is 6.8 g/100 mL while its solubility at 0 °C is
0.17 g/100 mL. What would be the maximum theoretical %recovery from the
recrystallization of 2.0 g benzoic acid in 60 mL water?
Second Edition Draft For use only within the Ateneo de Manila University.
Guidote AM, del Rosario DR, Abuzo AL. Experiencing Organic Chemistry. Quezon City:
Office of Research and Publications, Ateneo de Manila University; 2005.
References:
Most CF. Experimental Organic Chemistry. USA: Wiley & Sons; 1988.
Second Edition Draft For use only within the Ateneo de Manila University.