CHEM 351 - Principles
of Organic Chemistry II .........................Prof. T
Nalli, Summer 2014, Winona State University
Experiment #1 -
Identification of a Liquid Unknown Using IR and NMR
Spectroscopy
References:
Smith, Chapters 13 (IR), 14
(NMR), Mohrig, Chapters 21-22.3.
Overview:
The class will work together
to obtain 13C NMR, 1H NMR, and
IR spectra of several liquid unknowns. (Each lab
team will be assigned a particular unknown for which to
obtain spectra.) From the spectra and the molecular
weights, which will be provided by the instructor, we will
determine the identities of these unknowns. (All groups
are responsible for determining all of the unknowns not
just the one that was assigned to them.)
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 quickly 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.
3. 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.
- Class data will be pooled for the purposes of the
lab report. NMR spectra for the entire class will be
available in the class storage folder. Each file will
be clearly named with the unknown number and type of
spectrum. Copy these files to your computer and use
Delta to process and print them.
4. Using Delta for Processing NMR Data Files
1. Copy the desired file from class storage to your
computer. Double clicking on it should then open it in
Delta's "Process 1D" window. What you see initially is a
plot of the NMR signal as a function of time. This is
called the "Free-Induction Decay" or FID.
2. To convert the data to a useable form
you need to Fourier Transform it , phase correct it , and set the x-axis units to
ppm . (Or with newer versions of
Delta you can just select the 1H or 13C
icon as appropriate.)
3. To annotate the spectrum with peak
chemical shifts use the "Auto Peak Pick" icon, .
4. Zooming in is accomplished by clicking
and dragging on the spectrum. Clicking and dragging in
the gray area below the x-axis just zooms in along the
x-axis with the y-axis remaining at full scale. Use the
keyboard key: "BackSpace" to revert to the previous
(unzoomed) view.
5. Print the spectrum by simply printing
on the icon that looks like a printer and selecting the
desired printer from the scroll list.
6. One additional task that always needs
to be done before printing the 1H
NMR is integration. (C-13 NMR spectra are generally
not integrated.) 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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