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The Classification of an Unknown Substance Using Wet Tests
and NMR, IR, and Mass Spectrometry
Abstract
In this laboratory experiment a chemical of unknown identity was tested
for physical/chemical properties pertaining to specific functional groups. After
several tests, the functional group present in the molecule was determined, and
the configuration of the molecule was deduced using H-NMR, C-NMR, IR
spectroscopy, and mass spectrometry. The unknown was a white solid at room
temperature with a melting point of 76.0-77.0 Celsius. There was almost no odor
emitted from the compound. Important IR peaks were located at approximately
3200-3600 cm-1, 1720 cm-1, and 1400-1500 cm-1. C-NMR showed peaks at
178ppm, 43ppm, and multiple peaks in the 125-130ppm region. H-NMR had only
3 peaks: one singlet located at 9.7ppm, a multiplet at 7.3ppm, and a singlet at
3.6ppm. Perhaps most importantly, the mass spectrum’s parent peak was located
at 136m/z, and the base peak was located at 91m/z.
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Introduction
The purpose of the experiment was to determine the composition of an
unknown substance with limited quantity. This analytical approach of
determining the structure of a compound resembles what one would do in a
research environment.
Certain preliminary classifications are necessary and very helpful in
classifying a compound. Those include: the color, odor, state at room
temperature, density, and pH. After these classifications, the first test that should
be done to a solid compound should be to determine what it is soluble in. Figure
1 shows the general solubility rules for compounds containing various
substituents. Essentially, the IR spectrum can initially be used to show what
substituents you may have, then the wet tests and the other spectra can support
the initial hypothesis. One of the most helpful wet tests is the solubility test, as it
can quickly rule out possible functional groups and narrow it down to only one or
two. After the solubility test is completed, chemical classification tests can be
conducted based on what functional group is present (shown from the solubility
test). Once the tests are completed, and the functional group identified, the
remaining spectra can be used for the configuration of the entire molecule.
One of the most important spectrums to analyze when determining an
unknown is the mass spectrum. With it, the weight of the molecule and the
chemical formula can be found. In addition to the mass spec., Nuclear Magnetic
Resonance (NMR) can be used to determine the amount of Hydrogen (H) and
Carbon (C) environments. If any electronegative substituent groups are located
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on the molecule, we should expect to see peaks shifted downfield in the C-NMR
and H-NMR. This is because the electronegative groups will de-shield any carbon
and hydrogen environments that are near it.
Insert picture of solubility of compound
Figure 1. Summary of solubility of various functional groups. In bold represents route of
unknown.
Procedure
Preliminary classification tests were completed and recorded (i.e. odor,
color, etc.). Next, the solubility of the compound was tested in water, sodium
hydroxide, and hydrochloric acid. Using the information gathered from the
solubility tests, and referring to Figure 1, we determined what substituent was
most likely in our compound, and conducted chemical classification tests for said
substituent. Finally, we used the four spectrums (MS, C-NMR, H-NMR, IR) to
determine the structure of our molecule.
Results


RCOOH(S) + NaHCO3 (l) RCOO- Na+(aq) + H2CO3
H2CO3 CO2(g) + H2O(aq)
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Figure 2: Structure of unknown molecule.
Wet Tests
Ignition Test
Ph Test
Melting Point
Halogen Test
Water
Sodium
Hydroxide
Sodium
Bicarbonate
Positive
Approximately
3-4
76.7-78.1
Negative
Not Soluble
Soluble
Soluble+gas
Table 1: Wet test results
Please see attachment for spectrums.
Discussion and Conclusion
The parent peak (M) located at 136m/z shows that our molecule has a
mass of 136amu’s. The base peak (M-45) located at 91m/z refers to the
formation of a tropylium ion, after the loss of the carboxylic acid. Turning to the
IR, a broad peak located around 3600-3200 cm-1 and a sharp peak at 1720 cm-1
are a good indication of a carboxylic acid. In the H-NMR a singlet located at
9.7ppm is yet another indicator of a carboxylic acid. This, along with the other
evidence from the wet test and the C-NMR peak at 178ppm, proves there is a
carboxylic acid.
Aromatic C-H bending absorbance patterns are located in the IR spectrum
at 1035 cm-1 and 1080 cm-1. In addition C-C stretches in the aromatic ring are
located at around 1450 cm-1-1500 cm-1. No further evidence in the IR shows
aromatic absorption patterns, because they are covered up by the carboxylic acid
peaks. There were multiple peaks located at 125-130ppm in the C-NMR, further
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suggesting an aromatic ring. In addition, the positive result from the ignition test
proved that there was some degree of unsaturation in the structure. Finally, the
multiplet located at 7.3ppm in the H-NMR is proof of an aromatic α-carbon,
therefore, all of the evidence suggests there must be an aromatic ring with a
carboxylic acid substituent.
Considering the mass spectrum and the formation of the tropylium cation
after the loss of the carboxylic acid, it was determined that there must be a
methylene substituent located in between the carboxylic acid and the benzene
ring. The evidence from the H-NMR supports this, as a peak at 3.6ppm with an
integration value of 2 was present. The C-NMR also shows a peak at 41ppm
indicating a βcarbon region not on the benzene ring or corresponding to the
carbon in the carboxylic acid.
Combining all of the information gathered from the wet tests and the mass
spectrum, IR, and NMR, we deduced that our unknown was phenylacetic acid.
The experimental melting point and the theoretical melting point of phenylacetic
acid were identical, and the molecular weight of phenylacetic acid is, in fact,
136amu’s, as our mass spec showed. Therefore, the unknown substance is
phenylacetic acid.
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