EXPERIMENT 4B ■ Distribution of a Solute between Two Immiscible Solvents
35
sulfate, rinse the sodium sulfate with about 0.5 mL of fresh methylene chloride. Stir
this with a dry spatula and then transfer this solution to the same preweighed flask.)
Evaporate the methylene chloride by heating the flask in a hot water bath at about
45°C. This should be done in a hood and can be accomplished more rapidly if a stream
of dry air or nitrogen gas is directed at the surface of the liquid (see Technique 7,
Section 7.10). When the solvent is evaporated, remove the flask from the bath and dry
the outside of the flask. When the flask has cooled to room temperature, weigh it to
determine the amount of caffeine that was in the methylene chloride solution. Compare this weight with the amount of caffeine calculated in the Pre-Lab Calculation.
4B E X P E R I M E N T 4 B
Distribution of a Solute between
Two Immiscible Solvents
In this experiment, you will investigate how several different organic solids distribute
themselves between water and methylene chloride. A solid compound is mixed with
the two solvents until equilibrium is reached. The organic layer is removed, dried over
anhydrous sodium sulfate, and transferred to a tared container. After evaporating the
methylene chloride, the weight of the organic solid that was in the organic layer is deter
determined. By finding the difference, the amount of solute in the aqueous layer can also be
determined. The distribution coefficient of the solid between the two layers can then be
calculated and related to the polarity of the solid and the polarities of the two liquids.
Three different compounds will be used: benzoic acid, succinic acid, and sodium benzoate. Their structures are given below. You should perform this experiment on one of the solids and share your data with two other students who
worked with the other two solids. Alternatively, data from the entire class may be
pooled and averaged. Before performing this experiment, it would be helpful if
you predict the relative solubilities of the three compounds in the two solvents.
O
C
O
HO
Benzoic acid
O –Na+
O
OH
C
C
O
CH2
CH2
Succinic acid
C
OH
Sodium benzoate
PROCEDURE
NOTE: To obtain good results, you should make all weighings as accurately as possible, preferably on a balance that is accurate to within 0.001g.
Place 0.050 g of one of the solids (benzoic acid, succinic acid, or sodium benzoate) into
a 5-mL conical vial. Add 2.0 mL of methylene chloride and 2.0 mL of water to the vial.
Cap the vial and shake it as described in Experiment 4A for about 1 minute. Check for
undissolved solid. Continue shaking the vial until all the solid is dissolved. After the layers have separated, transfer the bottom organic layer to another vial or a small test tube.
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A Microscale Approach to Organic Laboratory Techniques 6/e ■ Pavia, Lampman, Kriz, Engel
Using the same procedure just described in Experiment 4A (see the section on “Drying
the Organic Layers”), dry this organic layer over granular anhydrous sodium sulfate.
Transfer the dried methylene chloride solution with a clean, dry Pasteur pipette
to a dry, preweighed test tube, leaving the drying agent behind. Evaporate the methylene chloride by heating the tube in a hot water bath while directing a stream of
dry air or nitrogen gas at the surface of the liquid. When the solvent is evaporated,
remove the tube from the bath and dry the outside of the tube. When the tube has
cooled to room temperature, weigh the tube to determine the amount of solid solute
that was in the methylene chloride layer. Determine by difference the amount of the
solid that was dissolved in the aqueous layer. Calculate the distribution coefficient for
the solid between methylene chloride and water. Because the volume of methylene
chloride and water was the same, the distribution coefficient can be calculated by
dividing the weight of solute in methylene chloride by the weight of solute in water.
Optional Exercise
Repeat the preceding procedure using 0.050 g of caffeine, 2.0 mL of methylene
chloride, and 2.0 mL of water. Determine the distribution coefficient for caffeine
between methylene chloride and water. Compare this to the literature value of 4.6.
4C E X P E R I M E N T 4 C
How Do You Determine Which One Is the
Organic Layer?
A common problem that you might encounter during an extraction procedure is not
knowing for sure which layer is organic and which is the aqueous one. Although
the procedures in this textbook often indicate the expected relative positions of the
two layers, not all procedures will give this information, and you should be prepared for surprises. Sometimes knowing the densities of the two solvents is not
sufficient, because dissolved substances can significantly increase the density of a
solution. It is very important to know the location of the two layers because usually
one layer contains the desired product and the other layer is discarded. A mistake
at this point in an experiment would be disastrous!
The purpose of this experiment is to give you some practice in determining which layer is aqueous and which layer is organic (see Technique 12,
Section 12.8). As described in Section 12.8, one effective technique is to add a few
drops of water to each layer after the layers have been separated. If the layer is water,
then the drops of added water will dissolve in the aqueous layer and increase its volume. If the added water forms droplets or a new layer, then it is the organic layer.
PROCEDURE
Obtain three test tubes, each containing two layers.4 For each tube, you will be told the
identity of the two layers, but you will not be told their relative positions. Determine
experimentally which layer is organic and which layer is aqueous. Dispose of all these
4 The three mixtures will likely be (1) water and n-butyl chloride, (2) water and n-butyl bromide,
and (3) n-butyl bromide and saturated aqueous sodium bromide.
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EXPERIMENT 4D ■ Use of Extraction to Isolate a Neutral Compound from a Mixture Containing an Acid or Base Impurity
37
mixtures into the waste container designated for halogenated organic wastes. After
determining the layers experimentally, look up the densities of the various liquids in
a handbook to see if there is a correlation between the densities and your results.
4D E X P E R I M E N T 4 D
Use of Extraction to Isolate a Neutral Compound from a
Mixture Containing an Acid or Base Impurity
In this experiment you will be given a solid sample containing an unknown neutral
compound and an acid or base impurity. The goal is to remove the acid or base by
extraction and isolate the neutral compound. By taking the melting point of the
neutral compound, you will identify it from a list of possible compounds. There are
many organic reactions in which the desired product, a neutral compound, is contaminated by an acid or base impurity. This experiment illustrates how extraction
is used to isolate the product in this situation.
In Technique 10, “Solubility,” you learned that organic acids and bases can become ions in acid–base reactions (see Section 10.2B, “Solutions in Which the Solute
Ionizes and Dissociates”). Before reading on, review this material if necessary. Using this principle, it is possible to separate an acid or base impurity from a neutral
compound. The following scheme, which shows how both an acid and a base impurity are removed from the desired product, illustrates how this is accomplished:
O
R
C
O
R´
R
Neutral
compound
C
OH
R
NH2
N
Base
impurity
Acid
impurity
(Dissolved in ether)
Add NaOH(aq)
Ether layer
Aqueous layer
O
R
C
O
R´ R
R
NH2
N
C
O – Na +
Add HCl(aq)
Ether layer
Aqueous layer
O
R
C
R´
R
+
NH3 Cl
N
–
Flow chart showing how acid and base impurities are
removed from the desired product.
The neutral compound can now be isolated by removing the water dissolved
in the ether and evaporating the ether. Because ether dissolves a relatively large
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38
A Microscale Approach to Organic Laboratory Techniques 6/e ■ Pavia, Lampman, Kriz, Engel
quantity of water (1.5%), the water must be removed in two steps: In the first step,
the ether solution is mixed with a saturated aqueous NaCl solution. Most of the
water in the ether layer will be transferred to the aqueous layer in this step (see
Technique 12, Section 12.9). Finally, the remainder of the water is removed by drying the ether layer over anhydrous sodium sulfate. The neutral compound can then
be isolated by evaporating the ether. In most organic experiments that use a separation scheme such as this, it would be necessary to perform a crystallization step to
purify the neutral compound. However, in this experiment the neutral compound
should be sufficiently pure at this point to identify it by melting point.
The organic solvent used in this experiment is ether. Recall that the full name
for ether is diethyl ether. Because ether is less dense than water, this experiment
will give you practice in performing extractions where the nonpolar solvent is less
dense than water.
The following procedure details the removal of an acid impurity from a neutral
compound and isolating the neutral compound. It contains an additional step that
is not normally part of this kind of separation scheme: The aqueous layers from
each extraction are segregated and acidified with aqueous HCl. The purpose of this
step is to verify that the acid impurity has been removed completely from the ether
layer. In the Optional Exercise, the sample contains a neutral compound with a
base impurity; however, a detailed procedure is not given. If you are assigned this
exercise, you must create a procedure by using the principles discussed in this introduction and by studying the following procedure for isolating the neutral compound from an acid impurity.
PROCEDURE
Isolating a Neutral Compound from a Mixture Containing an Acid Impurity.
Add 0.150 g of an unknown mixture5 to a screw-cap centrifuge tube. Add 4.0 mL of
ether to the tube and cap it. Shake the tube until all the solid dissolves completely.
Add 2.0 mL of 1.0 M NaOH to the tube and shake for 30 seconds. Let the layers
separate. Remove the bottom (aqueous) layer, and place this in a test tube labeled
“1st NaOH extract.” Add another 2.0-mL portion of 1.0 M NaOH to the centrifuge
tube and shake for 30 seconds. When the layers have separated, remove the aqueous
layer and put this in a test tube labeled “2nd NaOH extract.”
With stirring, add 6 M HCl dropwise to each of the two test tubes containing
the NaOH extracts until the mixture is acidic. Test the mixture with litmus or pH
paper to determine when it is acidic. Observe the amount of precipitate that forms.
What is the precipitate? Does the amount of precipitate in each tube indicate that all
the acid impurity has been removed from the ether layer containing the unknown
neutral compound?
The drying procedure for an ether layer requires the following additional step
compared to the procedure for drying a methylene chloride layer (see Technique 12,
Section 12.9, “Saturated Salt Solution”). To the ether layer in the centrifuge tube,
add 2.0 mL of saturated aqueous sodium chloride. Shake for 30 seconds and let the
layers separate. Remove and discard the aqueous layer. With a clean, dry Pasteur
pipette, transfer the ether layer (without any water) to a clean, dry test tube. Now
dry the ether layer over granular anhydrous sodium sulfate (see Technique 12,
5 The mixture contains 0.100 g of one of the neutral compounds given in the list in the following
table and 0.050 g of benzoic acid, the acid impurity.
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EXPERIMENT 4E ■ Critical Thinking Application
39
Section 12.9, “Drying Procedure with Anhydrous Sodium Sulfate”). Complete
Steps 1–3 in the “Microscale Drying Procedure.” Step 4 is described in the next
paragraph.
Transfer the dried ether solution with a clean, dry Pasteur pipette to a dry,
preweighed test tube, leaving the drying agent behind. Evaporate the ether by heating
the tube in a hot water bath. This should be done in a hood and can be accomplished
more rapidly if a stream of dry air or nitrogen gas is directed at the surface of the liquid (see Technique 7, Section 7.10). When the solvent has evaporated, remove the test
tube from the bath and dry the outside of the tube. Once the tube has cooled to room
temperature, weigh it to determine the amount of solid solute that was in the ether
layer. Obtain the melting point of the solid and identify it from the following list:
Melting Point
Fluorenone
82–85°C
Fluorene
116–117°C
1,2,4,5-Tetrachlorobenzene
139–142°C
Triphenylmethanol
162–164°C
Optional Exercise: Isolating a Neutral Compound from a Mixture Containing a Base
Impurity. Obtain 0.150 g of an unknown mixture containing a neutral compound and
a base impurity.6 Develop a procedure for isolating the neutral compound, using the
preceding procedure as a model. After isolating the neutral compound, obtain the melt
melting point and identify it from the list of compounds given above.
4E E X P E R I M E N T 4 E
Critical Thinking Application
PROCEDURE
1. Add 4 mL of water and 2 mL of methylene chloride to a screw-capped centrifuge tube.
2. Add 4 drops of solution A to the centrifuge tube. Solution A is a dilute aqueous solution of sodium hydroxide containing an organic compound.7 Shake the
mixture for about 30 seconds, using a rapid rocking motion. Describe the color
of each layer (see the following table).
3. Add 2 drops of 1 M HCl. Let the solution sit for 1 minute and note the color
change. Then shake for about 1 minute, using a rapid rocking motion. Describe
the color of each layer.
6 The mixture contains 0.100 g of one of the neutral compounds given in the list above and 0.050 g
of ethyl 4-aminobenzoate, a base impurity.
7 Solution A: Mix 25 mg of 2,6-dichloroindophenol (sodium salt) with 50 mL of water and 1 mL of
1 M NaOH. This solution should be prepared the same day it is used.
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