CHEM 350 - Principles
of Organic Chemistry I
..................................Prof. T Nalli, Fall
2012, Winona State University
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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