Recrystallization:
Solvent selection and technique
(Mohrig 3rd Ed Chapter 15; Adapted from: Rodic/Clark, & M. Pauly)
Purpose:
To determine how to select an appropriate solvent for recrystallization.
To recrystallize Benzoic Acid from water.
Techniques & Principles:
Recrystallization
Vacuum Filtration
Safety Concerns:
Ethanol and acetone are flammable! No flames!
Discussion:
Recrystallization from solvents is a most useful process for purification of organic solids. In this process,
an impure sample is dissolved in solvent in which the solubility of the sample is different than the solubility
of its impurities and in which the solubility of the sample varies with temperature.
A solvent that dissolves the sample to be purified but does not dissolve the impurities is ideal as solid
impurities may be filtered away from the dissolved sample. If the solvent is sufficiently volatile it can then
be evaporated from the desired sample. For many purifications however, it is not quite so easy as most
impurities also dissolve in solvents.
Solvents should be chosen in which the sample is more soluble in hot solvent than in cold solvent but the
impurities will stay dissolved in the cold solvent. In this way samples and impurities can be dissolved in
hot solvent and then as the solvent cools the pure sample will crystallize out leaving the impurities
dissolved. Thus, pure solid sample can be filtered out leaving soluble impurities remaining in the filtrate.
Part 1: Selecting an Appropriate Solvent (Adapted from M. Pauly City College of San Franscico)
In this experiment, you will test the solubility of resorcinol (also called benzene-1,3-diol) and benzoic acid
in five different solvents: water, methanol, acetone, toluene, and petroleum ether. After observing the
solubility properties, you will be able to draw conclusions regarding the most appropriate recrystallization
solvents for these compounds. Resorcinol is used as an antiseptic and disinfectant and in the synthesis of
dyes and plasticizers. Benzoic acid, along with its salts, is used as food preservative.
O
HO
OH
OH
Resorcinol
Benzoic Acid
Part 2: Recrystallization of Benzoic Acid
In this experiment we will be practicing recrystallization techniques on a known sample of benzoic acid
using water as our recrystallization solvent.
The following information is available for benzoic acid:
Temperature
95 C
75 C
25 C
20 C
4o C
0 C
Solubility/100 mLs H2O
6.80 g
2.20 g
0.34 g
0.29 g
0.18 g
0.17 g
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CH241 Lab 2 Recrystallization (F14)
1
It is only necessary to weigh the sample for the first tube, then use
that as a visual guide for the other four.
Procedures:
Part 1: Selecting an Appropriate Solvent
1.
2.
3.
Obtain 10 clean, dry 10x75 mm test tubes. Label each
tube with a different solvent
 water, methanol, acetone, toluene, and petroleum
ether
2
You may find it helpful to use the following flow chart as a guide:
3.Add solvent to solid. Mix well.
To each of five test tubes, transfer about 15-20 mg of
the first compound to be tested. 1
Soluble at room temp?
No
Add 10 drops of each solvent to the tubes. Swirl to
mix well. Only if necessary, use a thin stirring rod to
break up any lumps. 2
Observe and record whether the solid is readily
soluble ("s") or insoluble ("i") at room temperature.
Soluble hot?
(Insoluble.)
For solids insoluble at room temperature:
5.
If the solid does not dissolve (“i”) in a given solvent at
room temperature, heat the test tube containing the mixture
in a hot (~90°C) water bath (in a beaker) heated on a hot
plate. 3
A. If the solid completely dissolves at the elevated
temperature, record that it is soluble in the hot solvent.
B. If some but not all dissolves, record that it is partially
soluble ("ps") and slowly add drops of hot solvent
until it completely dissolves.
C. Once the substance dissolves in hot solvent, allow the
solution to cool completely to room temperature, and
then place in an ice bath for about 10 minutes. If
necessary, induce crystallization by rubbing the walls
of the tube with a stirring rod. Describe the shape of
the crystals that form (needles, plates, or blocks, etc.).
D. If the solid remains undissolved, even at an elevated
temperature with additional drops of solvent added,
simply record it to be insoluble in the hot solvent.
6. Add H2O (except for
toluene or pet. ether).
5.Heat in water bath.
No
4.
Yes
Yes
Partially
Add more solvent
to dissolve
Cool to
crystallize
Soluble?
No
Cool to
crystallize
Yes
(Readily soluble.)
Cool to
crystallize
3
Acetone and petroleum ether will evaporate quickly when placed
in hot water (see boiling points). Cool down your water bath (by
adding ice) before heating a test tube that contains these solvents.
For solids soluble at room temperature:
6.
If the solid dissolves (“s”) in methanol or acetone at room
temperature, add a few drops of water (one drop at a time)
to the solution to see if the solid precipitates (turns
cloudy).4
A. If no precipitate forms, record that it is readily soluble
at room temperature in the solvent/water mixture.
B. If a precipitate forms, record that it is partially soluble
at room temperature in the solvent/water mixture.
Cool the mixture in ice bath for about 10 minutes.
Describe the shape of any crystals that form.
7.
Pour all mixtures from your test tubes into the Organic
Waste jar in the hood. Rinse the tubes sparingly with
acetone and wash them at the sink.
8.
Record all your results in a table in your notebook.5
9.
Make conclusions about which solvent(s) would be
most appropriate for recrystallizing resorcinol and
benzoic acid. Justify your reasoning.
4
Do not add water to either toluene or pet. ether since it is not
miscible with these solvents.
5
Include the following information for each compound in
each solvent:
 the solubility at room temperature,
 the solubility at the elevated temperature (if
applicable),
 the solubility at room temperature-with water added
(if applicable), and
 the appearance of the reformed crystals (if
applicable).
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CH241 Lab 2 Recrystallization (F14)
Part 2: Recrystallization of Benzoic Acid
1.
2.
Prepare a flask of recrystallization solvent by heating
deionized water (our solvent of choice) to a near boil6 on a
hot plate.
6
Place a bottle of deionized water in an ice bath for use in
rinsing later.
7
3.
Place approximately 1.0 grams7 of benzoic acid and a
boiling chip in a 125 mL Erlenmeyer flask .
4.
To the flask of benzoic acid add 10 mL of the hot water
solvent and a boiling stone and heat the mixture to near
boiling on a hot plate.
5.
Keeping track of the amount of water used continue to add
near boiling water in small increments until all of the
benzoic acid is dissolved.8
6.
Record the total amount of water used to dissolve the
sample.9
7.
Remove the flask from the heat and allow it to cool to
room temperature. If crystals do not appear at room
temperature, induce their formation by scratching the
inside of the flask with a glass rod or spatula.
8.
Continue cooling the recrystallization flask in an ice bath
until confident that crystallization is complete.
9.
Collect the recrystallized benzoic acid by vacuum
filtration in a Buchner funnel and wash/rinse the product
with a little ice cold deionized water.
10. Collect the mother liquor from the 1st filtration and cool it
in an ice bath to try and recover more crystals. If crystals
form then isolate them with vacuum filtration and
combine them with the original crystals.
The solvent should be nearly at boiling temperature but not to
a full rolling boil as it will evaporate.
Acurately measure the mass of your sample and report to the
precision of your balance.
8
Be patient; organic compounds often do not dissolve
instantaneously, especially in water.
9
This will be approximate as some of the water will have
evaporated in the heating process.
10
Allowing the sample to sit on the Buchner funnel with air
continuing through the vacuum before transferring to a watch
glass can hasten drying time.
11
The literature melting point of benzoic acid is 122.4 oC
although it begins to sublime at about 100 oC.
11. Allow the sample crystals to air dry10 on a preweighed
watch glass and determine the mass recovered.
12. Determine the theoretical yield from the amount of
solvent you used and the percent recovery of
recrystallized product.
13. Characterize your product by melting point if instructed.11
Post lab questions:
1. Organic student, Crystal, found that the percent recovery for her experiment was only 45%. Suggest three
reasons that might explain the low recovery.
2. The solubility of a compound is 59 g per 100 mL in boiling methanol and 30 g per 100 mL in cold methanol,
whereas its solubility in water is 7.2 g per 100 mL at 95oC and 0.22 g per 100 mL at 2oC. Which solvent would
be better for recrystallization of the compound? Explain.
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CH241 Lab 2 Recrystallization (F14)
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CH241 Lab 2 Recrystallization (F14)
CH241 Lab 2: Recrystallization
Pre lab Assignment:
Name ________________
1. At what temperature will the following solvents boil?
Solvent
Boiling Point
Water
Methanol
Acetone
Solvent
Petroleum Ether
Toluene
Boiling Point
2.
Describe the characteristics of a good recrystallization
solvent.
3.
Draw a typical set up for filtration using a Buchner
funnel with vacuum filtration.
4.
Why is cold (rather than room temperature) solvent used
for washing crystals already collected in the filter?
5.
Where will the impurities be after your recrystallization
is complete?
6.
What happens when an attempt at recrystallization
results in a compound “oiling out”? How can
formation of oils be avoided?
7.
Assuming the solvent chosen is an appropriate one,
what are some ways one can initiate crystallization in a
cooled solution that has not yet started to form crystals.
8.
Why are boiling chips (boiling stones) used in the recrystallization process?
9.
Using the information given in the lab for benzoic acid, determine the minimum amount of hot water needed to
completely dissolve 1.00 g of Benzoic Acid. (Show calculations)
10. Using the information given in the lab for benzoic acid determine the maximum amount of benzoic acid (theoretical
yield) that could be recovered if 1.00 g benzoic acid was dissolved in 30 mLs of hot water and then cooled to 0 oC.
(Show calculations)
Ethanol from Molasses:
1.
During the fermentation reaction of molasses with yeast it is important to prevent air from entering the reaction
chamber. Why?
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CH241 Lab 2 Recrystallization (F14)