Extraction, Purification and
Identification of an
Unknown Mixture
John Stephenson (410-1306)
Performed on Oct 18, Oct 25, Nov 1, 2000
Report Prepared on November 3, 2000
Chem288 - Section H
TA: Allyson Campbell
Abstract
The purpose of this experiment was 1) to separately extract any amines, phenols,
carboxylic acids, salts/sugars and/or neutrals, present in a supplied mixture. 2) to purify
two of the components by recrystallisation 3) to identify the two purified compounds by
melting point, and mixed melting point tests. The unknown amine was identified as
benzocaine, and the unknown carboxylic acid was identified as benzoic acid.
References
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Merck Index 12th Edition (on cd-rom)
Chemfinder (http://www.chemfinder.com)
Organic Chemistry Laboratory Manual:
 Separatory funnel (3.1.3)
 Drying a liquid (3.4.3.1)
 Weighing (3.4.3)
 Melting point (2.4.3, 4.1.1-4.1.1.2)
 Recrystallising (3.5-3.5.1)
 Suction filtration (3.2.3)
 Experimental procedures outlined in experiments (5.2.1, 5.3.2, 5.5.1)
“Separation of a mixture by extraction” hand-out
Procedure / Observations
Note to TA: Since the procedures were changed significantly from those detailed
in the references and since this is written as a combined report for the three
laboratories, the changes will be noted during the recount of the procedure.
Observations have been included with the procedures to give a better “flow” to
this report. This reporting style isn’t anything new, it is similar to that found in
chemical abstracts. I hope this agrees with you.
Laboratory 1: Separation by solvent extraction
2.996g of sample 7A was weighed out into a 125ml flask. 50ml of dichloromethane was
added.
All of the mixture dissolved, indicating that the mixture did not contain any salts or
sugars.
2x20ml HCl washes using a 250ml separatory funnel were done on the mother liquid.
The aqueous washes (top layer) were combined and the mother liquid saved. The pH was
raised with 3M NaOH(aq). Crystals formed, and would dissolve again, until the pH was
raised high enough to turn red litmus paper indicator blue. The white crystals stayed in
solution and settled to the bottom. They were suction filtered off. Some amine crystals
were lost due to a faulty vacuum source. The crystals were allowed to air dry. After
drying, 0.821g of unknown amine crystals were recovered.
3x20ml saturated NaHCO3(aq) washes were done on the mother liquid. The aqueous
washes (top layer) were combined and the mother liquid saved. The pH was lowered with
3M HCl(aq) in an ice bath. Upon addition of HCl(aq) the solution effervesced. The
solution was acidified until the solution stopped effervescing and congo red indicator
turned blue. White fluffy crystals formed, and were suction filtered off from the aqueous
solution. After drying, 1.654g of unknown carboxylic acid crystals was recovered.
2x20ml 1.5M NaOH(aq) washes were done on the mother liquid. The aqueous washes
(top layer) were combined and the mother liquid saved. The pH was lowered with 3M
HCl(aq) in an ice bath until congo red indicator turned blue. No crystals were formed. No
phenol compound was present.
The mother liquid was dried with sodium sulfate, and decanted from it into a flask. A
boiling stick was added and the sample was evaporated to dryness on a steam bath.
0.170g of neutrals were recovered and discarded.
Impure carboxylic acid and amine samples were placed into separate labeled vials and
stored until laboratory 2.
Yieldamine = (massamine) / (mass7A sample)
= (0.821g/2.996g) (100%)
= 27.4%
Yieldcarboxylic acid = 55.2%
Yieldneutral = 5.74%
Yieldtotal = (massamine + masscarboxylic acid + massneutral) / (mass7A sample)
= 88.2%
Laboratory 2: Purification through recrystallisation
A small sample of each compound was tested in a small quantity (1-2ml) of water,
methanol, ethanol, acetone, and hexane to choose one suitable for recrystallisation.
Solvent
Water
Methanol
Ethanol
Acetone
Hexane
Carboxylic Acid
Low Temp
High Temp
Solubility
Solubility
Insoluble
Soluble
Soluble
.
Soluble
.
Soluble
.
Insoluble
Soluble
Amine
Low Temp
Solubility
Insoluble
Soluble
Soluble
Insoluble
Insoluble
High Temp
Solubility
.
.
.
.
Mostly
Hexane recrystallisation was chosen to purify both carboxylic acid and the amine.
20ml of hexane was added to 1.037g of unknown carboxylic acid in a 125ml flask. A
boiling stick was inserted, and the flask placed on a steam bath. All of the acid dissolved
into the hexane once the mixture was brought to a boil.
The flask was cooled in an ice bath once it came to room temperature, long shards of
carboxylic acid crystallized out of the flask.
The mixture was suction filtered and the carboxylic acid was recovered. The filtrate was
poured back into the flask to help rinse out any crystals, and this solution was filtered
again. The vacuum was left running for a few minutes to help air drying of the crystals.
They were then air-dried on a weighed watchglass and the mass was taken. 0.346g of
carboxylic acid was recovered (33.3% yield).
A similar procedure was conducted for the amine, however 40ml of hexane was used.
The initial mass of the amine was 0.708g and the mass of recrystallised amine was
0.319g (45.1% yield).
Recrystallised carboxylic acid and amine samples were placed into separate labeled vials
and stored until laboratory 3.
Laboratory 3: Identification through melting point and mixed melting points
Melting points of the carboxylic acid and amine were taken, then compared to literature
values of amines and carboxylic acids. A small sample each similar known was taken,
then a mixed melting point was taken of each sample with the known sample.
Carboxylic Acid
Unknown carboxylic acid
Benzoic acid
Mixed unknown and benzoic acid
Amine
Unknown amine
Benzocaine (ethyl 4-amino-benzoate)
Mixed unknown and benzocaine
MP 114-118C
MP 116-118C (lit. value 121-122C)
MP 116-120C
MP 85-87C
MP 85-86C (lit. value 88-90C)
MP 86-88C
Discussion
The yield from extraction was rather low, mostly due to a faulty vacuum filtration setup.
(the aspirator that was used to suction filer the amine was broken)
The yields from recrystallisation were quite low, in retrospect a mixed recrystallisation
from acetone and ethanol of the amine might have afforded higher yields, and water
seems to be the solvent of choice for recrystallising benzoic acid.
Conclusion
The sample positively contained the amine, ethyl 4-amino-benzoate (Figure 1), and also
contained the carboxylic acid, benzoic acid (Figure 2).
Figure 1: Ethyl 4-amino-benzoate (benzocaine) (MP 88-90C):
Figure 2: Benzoic acid (MP 121-122C):
Extra Questions
1. 2,4-dinitrophenol and phenol could be separated from each by adding water to the
sample, then titrating the pH to just past the pKa of one of the phenols. Filter to
remove the phenol. Then titrate past the other pKa. Filter again to remove the other
phenol. The sodium salts of each phenol would be converted back into the phenol by
treating with hydrochloric acid.
2. Diethyl ether and hexane are less dense than water and the organic layer will float on
top (if there is less than 69 grams of ether per liter of water, the ether will dissolve
into the aqueous layer). Trichloromethane (chloroform) and dichloromethane are
heavier than water and will sink to the bottom. Acetone is miscible in water and will
not form any layers (unless it contains a lot of dissolved inorganic salts, in which case
the acetone will float on top).
3. I’m unsure of this question due to its wording. I’ll assume that the crystals are
forming in the filtrate contained in the filter flask because the organic solvent with a
low boiling is evaporating (this is especially likely since the filter flask will have a
lower than atmospheric pressure). The organic solvent is saturated, so if it evaporates
crystals will precipitate out. In which case, these crystals can be either less pure or
have the same purity. If no impurities crystallize out from the organic solvent they
will be of the same purity. However should the filtrate also contain a lots of
impurities that also crystallize out, the resulting crystals will be less pure.