Spectroscopic ID - Handout
SPECTROSCOPIC IDENTIFICATION OF AN
UNKNOWN COMPOUND:
Aldehyde, Ketone, Carboxylic Acid, Alkene, Alcohol,
and/or Aromatic Compounds
Discussion
In this experiment, you will be given an unknown compound that you are to identify. To do
this, you will gather information about the compound through two methods:
A. Measurement of physical properties
B. Interpretation of spectral data (IR and Mass Spec)
The unknown compound will be either aliphatic or aromatic, and can be either an alkene,
alkyne, alcohol, ether, aromatic hydrocarbon or aryl halide. Tables of possible unknown
compounds can be found at the end of this handout. The lists are compiled in the order of
increasing boiling point of for each compound class.
This lab is a bit different from many of the experiments you have performed. Thus, for this
report, we will skip a formal prelab. However, you are responsible for having read all of the
instructions prior to coming to class so you have a definite plan of attack in mind and are
prepared to obtain information about your unknown from all the sources described herein.
Procedure
You will need to perform the following methods, though not necessarily in the order listed. It
is important to work efficiently, using the equipment and instrumentation such as the
infrared spectrophotometer, the boiling point apparatus, and the Abbey refractometer,
whenever they become available for your use.
A. Measurement of Physical Properties
1. Note any characteristic properties, such as:
a. Odor (Don't ignore even the little things. If your compound smells like bananas,
and banana oil is on the list, then that should tell you something.)
b. Solubility (Is the compound water soluble, for example?)
2. Some properties you will need to measure include:
a. Micro boilingpoint determination (see below)
b. Index of refraction
c. Density (Halogenated compounds are relatively dense.. hint, hint.).
Add 1mL of your liquid to a tared test tube and weigh.
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Spectroscopic ID - Handout
B. Spectroscopic Data (Padías, pp. 64 – 72 and pp. 98 – 105; also consult your lecture text)
Infrared Spectroscopy:
You need to prepare a sample, record an infrared spectrum, and obtain a printout of the
spectrum. This is not difficult, but requires some care in both the sample preparation and the
treatment of the Fourier transform infrared instrument. Please be efficient while working
with the instrument, so as not to occupy it for an extended period of time when other
students are waiting.
Gas Chromatogram / Mass Spectrometry:
You need to prepare a sample for GC/MS. Fill an autosampling vial with 1mL of
HPLC-grade dichloromethane and add a single drop of your unknown compound. Working
in groups, you will use the autosampler to inject your sample onto the GC column and take a
Mass Spec of your unknown compound. The method we will use will not take a Mass Spec
of the solvent (dichloromethane). The GC/MS is automated – you do not need to monitor the
system. However, you will need to instruct the GC/MS as to which peaks are your compound
and you will need to obtain a printout of the GC/MS of your unknown.
Suggested Basic Strategy
1.
Focus on methods that will help you to determine the structure of your compoundthe functional group(s) present, the boiling point of your liquid, and the mass of your
compound.
2.
Some functional group(s) can be established using infrared spectroscopy. Knowing
the functional group will substantially limit your list of possible compounds.
3.
The boiling point will also be an important piece of evidence for limiting the number
of compounds under active consideration. Please note, however, that boiling points
are somewhat difficult to determine accurately, so all compounds plus or minus 10
degrees of the experimentally determined value should be considered as possible
candidates. (The lists of possible compounds are arranged in order of increasing
boiling point to facilitate this process.)
4.
Using GC/MS, you can determine the molecular weight of your compound. In
addition, the fragmentation pattern could indicate which functional groups are
present and/or the geometry of your compound.
5.
Other data you will want for structural confirmation certainly includes an index of
refraction (corrected for temperature) and perhaps a density, as mentioned above.
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Spectroscopic ID - Handout
Specific Experimental Procedures
Solubility in Water: Compounds that contain four or fewer carbons and also contain an
oxygen or nitrogen are often soluble in water. Compounds having five or six carbons and an
oxygen or nitrogen are often insoluble in water or have borderline solubility, but bear in
mind that branching of the alkyl chain tends to lead to an increase in the water solubility.
Place approximately1 mL of water in a small test tube and add one drop of your unknown
liquid directly into the water. Gently tap the test tube with your finger to ensure mixing.
The disappearance of the separate drop of liquid indicates that the unknown has dissolved.
Add several drops more of the liquid to determine the extent of the compound’s solubility.
Micro Boiling Point Determination: (Padías, p. 52) A micro-bp determination is used
when you have too little sample to do a regular distillation (as is the case with your
unknown), or as a quick check to see where a compound boils before purifying it by
distillation. The micro-bp method uses a small test tube, filled with the sample to a height of
about 1 cm. A shortened capillary melting point tube (about 2 cm long and sealed at one
end) is placed with the open end down in the test tube of liquid, which is then heated in any
one of two or three oil baths set up in the laboratory for this purpose.
As the sample is gradually heated in the oil bath, there will be slow escape of air bubbles
from the end of the capillary tube due to an expansion of the air trapped in the tube
(PV = nRT). When the temperature of the oil in the bath reaches (or somewhat exceeds) the
boiling point of the compound, a rapid and continuous stream of bubbles from the end of the
tube will be observed (voilá, boiling!). Reading the thermometer at this time yields a crude
boiling point, but can be far from accurate. A more accurate result can be obtained by
turning down the heat source. The rate at which the bubbles form will lessen and finally at
the instant bubbles cease to come out of the capillary, and just before the liquid is sucked up
into the tube, the temperature is read. This gives the most accurate boiling point
determination. You may wish/need to repeat the determination to be assured of maximum
accuracy. You will need to drop in a new capillary tube for each determination.
Please note that, if you significantly exceed the temperature of the boiling point of your
compound, all of the liquid may boil away, leaving you with an empty tube. Be observant!
Also note that the white oil in these baths should not be heated to temperatures above
210 °C, as it will very likely catch fire. We have therefore limited the list to unknowns that
boil at less than this temperature. Higher temperatures can be obtained, but require the use
of a silicon oil bath (suitable to about 280 °C).
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Spectroscopic ID - Handout
Final Report
This final report will be written up as a formal report. You must present a thorough argument
for the proof of the structure of your unknown compound. Instructions found in the
Introduction section of the lab manual still apply. Here are some suggestions for the structure
of the report.
You should include your name, date, introduction, references to instructions, etc. and
incorporate a statement of the goal of the experiment. This statement should include your
Unknown Number.
Methods: This section will be quite small for this report. However, you should dedicate at
least one sentence to each method used; briefly mention what information you hoped to
obtain with each method.
Data and Observations: This will be a bigger section in your report, and it should include all
data collected and any observations regarding your unknown. Use tables so that you can
clearly identify significant data. Tables also allow you to easily compare results from your
unknown and the proposed identity of your unknown. In addition to the tables of
information, you should briefly discuss your results.
Results and Conclusions:
1. Summarize the above data. Try to tabulate this information as much as possible.
List literature values as well (remember to cite each literature source!).
2. Both spectra should be fully interpreted. You should assign all relevant
absorptions in the IR (e.g. ν~ = 1640 cm-1, C=C stretch in alkenes). For the massspectum, identify the structure of at least 3 peaks (the base peak, the molecular ion
peak, and at least one fragment).
3. Provide final concluding remarks discussing how the data is consistent in pointing
to a particular compound. If there are inconsistencies between the data you have
obtained and that expected for the compound, these should be discussed. If you are
unsure of your final assignment, that should also be included in this section. Finally,
discuss what other methods you could use to make a conclusive identification.
Table 1: Alkenes
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Spectroscopic ID - Handout
Boiling Point (oC)
Compound
2,4-Hexadiene
82
1,3,5-Cycloheptatriene
116
2,5-Dimethyl-2,4-hexadiene
134 (m.p. 13)
1,3,5,7-Cyclooctatetraene
142
1-Nonene
147
3-Phenyl-1-propene (Allylbenzene)
157
1-Decene
171
1-Phenylpropene (β-Methylstyrene)
175
Limonene
176
Table 2. Ketones and Aldehydes
Compound
B.P. (°C)
Butanal (Butyraldehyde)
75
2-Butanone (Methyl ethyl ketone)
80
3-Methyl-2-butanone (Isopropyl methyl ketone)
94
Pentanal (Valeraldehyde)
102
3,3-Dimethyl-2-butanone (t-Butyl methyl ketone)
(Pinacolone)
106
2-Ethylbutanal (Diethylacetaldehyde)
117
4-Methyl-2-pentanone (Isobutyl methyl ketone)
117
2,4-Dimethyl-3-pentanone (Diisopropyl ketone)
124
Hexanal
130
Cyclopentanone
131
5-Methyl-2-hexanone
145
Heptanal
153
Cyclohexanone
156
Furaldehyde (Furfural)
162
2,6-Dimethyl-4-heptanone (Diisobutyl ketone)
168
2-Octanone
173
Benzaldehyde
179
2-Nonanone
192
Table 3. Carboxylic Acids
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Spectroscopic ID - Handout
Compound
B.P. (°C)
Propanoic acid (Propionic acid)
141
3-Methylbutanoic acid (Isovaleric acid)
176
2-Chloropropanoic acid
186
2-Methylpentanoic acid (2-Methylvaleric acid)
195
Table 4: Aromatic Hydrocarbons
Compound
B.P. (°C)
Toluene
111
Chlorobenzene
132
Phenylethyne (Phenylacetylene)
142
1,2-Dimethylbenzene (o-Xylene)
144
Isopropylbenzene (Cumene)
153
Bromobenzene
156
4-Chlorotoluene
162 (m.p. 9)
t-Butylbenzene
169
p-Isopropyltoluene (p-Cymene)
177
1,2-Dichlorobenzene
180
4-tert-Butyltoluene
190
Table 5: Alcohols
Compound
B.P. (°C))
78
2-Propanol (Isopropyl alcohol)
82
1-Propanol
97
2-Butanol
99
2-Methyl-2-butanol (t-Amyl alcohol)
102
1-Butanol
118
2-Methyl-1-butanol
129
Cyclopentanol
141
1-Hexanol
156
Furfuryl alcohol
170
2-Octanol
177
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Ethanol