Experiment
#3 - Identification of a Liquid Unknown Using IR and
NMR Spectroscopy
References:
Klein,
Chapters 15 (IR), 16 (NMR),
Mohrig, Chapters 21-22.3.
Overview:
You will
obtain 13C NMR, 1H NMR, and IR
spectra of a liquid unknown.
From these and the molecular weight, which will be
provided by the instructor, you will be able to deduce
the identity.
Safety
Precautions - Most of the unknowns are
classified as irritants, some are also toxic, and some
have disagreeable odors. As always it is best to
minimize exposure to possibly harmful substances by
wearing gloves while handling samples and carrying out
all transfers in a hood. CDCl3 has harmful
fumes. Avoid breathing it and dispense it in a fume
hood.
Procedure: Please
note that you will be expected to obtain IR and NMR
spectra many times in this course and I expect you to
eventually be able to obtain them unassisted.
Therefore, you should take note of the procedures you
use in this lab.
1. Obtain an unknown from the
instructor and record the unknown number in your lab
notebook. Carefully record any observations you can
make of the unknown.
2. Obtain the IR Spectrum
- We will use the procedure
outlined on page 286 and in figure 20.8) in Mohrig
to prepare a sample for IR spectroscopy. Working in
a fume hood and wearing gloves, a few drops of the
liquid are applied to one of the salt plates and the
second plate is placed over the liquid to form a
thin film of the liquid between the two plates. The
salt plates should be handled carefully (fragile!)
by their edges, as moisture from the skin will
dissolve some salt on the surface and lead to a
rough, non-transparent surface.
- The use of the IR
spectrophotometer is just as described on part 20.4
in Mohrig. The Lab Assistant will be available to
assist you. Print out your spectrum and label it
with unknown number, lab group and section.
- Rinse the salt plates off
with acetone into the waste solvent container in the
fume hood when done. Set them on a paper towel in
the hood to dry for the next lab group.
B. Obtaining the NMR Spectra
- NMR tubes and
solvents are very costly so please be very careful
with the tubes and do not waste the solvent. Also,
be very careful when capping and uncapping your NMR
tube. The tubes are fragile and the caps are tight
so it is easy to break a tube in the process of
capping or uncapping it.
- Add the unknown to a clean NMR
tube to a height of approx 0.5 cm. Make sure you use
a dry NMR tube. Add CDCl3 solvent to a
height of approx 4 cm. Cap the tube with a plastic
cap. Invert and/or agitate the tube in order to
dissolve the unknown in the solvent (warning - the
caps can leak). Label your tube with
a piece of labeling tape stuck to the cap.
- Operation of the NMR
spectrometer will be demonstrated by the instructor,
who will assist you in obtaining both the C-13
spectrum and the proton spectrum. NMR
sample solutions should be drained into the waste
solvent container when finished. Rinse the tube
several times with acetone into the waste solvent
container.
Using Delta
for Processing 1H NMR
- The procedure for processing and
printing 1H NMR is mostly
the same to that used for 13C
NMR. These procedures were outlined in the handout
for Expt 2.
- One additional task
that always needs to be done before printing the 1H NMR is
integration. The procedures for doing so will be
demonstrated by the instructor.
Report - For this experiment a
full report is not required. Include the following
sections only: Title Page, Attachments
(Labeled spectra printouts):
Label each spectrum
with your group name(s), course number, section number,
and experiment number. The structure of the compound and
its code number should be written prominently on each
spectrum.
- For the C-13 spectrum
label all sets of equivalent carbons a, b, c, etc. and
then label the peaks in the spectrum a, b, c, etc. to
show which carbons in the molecule are represented. Also label any extraneous
peaks such as the CDCl3 solvent peak in
the C-13 spectrum and impurity peaks.The peaks should also be labeled with
chemical shift (use Delta for this and make sure there
are no extraneous chemical shift labels for
"nonpeaks").
- For the proton NMR
label the Hs in the molecule and then label the
resonances accordingly. As
with the C-13, label any extraneous peaks and make
sure the peaks are labeled with their chemical shifts.
The proton NMR should also be labeled with integration
values that have been normalized to show the number of
Hs represented by each. (The instructor will
demonstrate how to do this in a lecture.)
- In the IR spectrum
label all important functional group peaks. For peaks in the fingerprint region
label the major ones with the corresponding wavenumber
value for the same peak in a literature spectrum. Two
good places to look for literature spectra are the
Fiveash Database and the NIST WebBook, both available
through the library at http://www.winona.edu/library/eref/chem.html#chemspec.
There is also a bound version of the Aldrich Library
of IR Spectra available in the IR lab.
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