Expt 2. Acid/Base
Extraction and C-13 NMR
Part 2 - C-13 NMR and mp
determination
Background
Nuclear magnetic resonance (NMR) is the most important characterization tool
available to organic
chemists. In this lab you will obtain a
C-13 NMR spectrum of each compound
from part 1 in
order to provide evidence that it is the
expected compound.
Pre-Lab
In your prelab include
a prediction of the number of peaks expected in the C-13
NMR spectrum of all three components of the original
mixture.
Experimental
Procedures
Melting
Point Determination
Obtain
the mp range of each solid you obtained from part 1.
Make sure to follow all of the precautions necessary
for a sharp mp to be obtained as learned in expt 1.
Also be careful to record your observations during the
melting process and to document the rate of
temperature increase in your lab notes.
C-13 NMR
Spectroscopy
There will
only be enough time for you to obtain the spectrum
of one of your three isolated compounds. Each lab
group will be assigned a specific compound and we
will share data between groups so as to be able to
report spectra for all three compounds.
Please note
that you will be expected to obtain NMR spectra many
times in this course and eventually be able to
obtain them unassisted. Therefore, you should take
careful notes on the procedures you use in this lab.
Preparing the NMR Tube.
- 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 very fragile and the
caps are tight so it is easy to break a tube in the
process of capping or uncapping it.
- Use a Pasteur
pipet to add a small amount of your product to your
NMR tube. For C-13 NMR a good amount of compound is
needed to obtain a useful spectrum in the time
available to us, so make sure the height of the
liquid in the tube is at least 0.5 cm. (For H-NMR we
will use much less.)
- Add the NMR
solvent, CDCl3, to a height
of 4-5 cm in the tube. Make sure the height is at
least 4 cm! Cap and then invert the tube repeatedly
as needed to dissolve the product in the solvent
(warning - the caps can leak).
Obtaining the C-13 NMR Spectrum. The
instructor will assist you in obtaining the NMR. The
basic steps are;
Place tube
in spinner and adjust height.
Place tube
with spinner at top of magnet.
Then use the Delta software to...
Lower the
tube and spinner into the magnet.
Check that
tube is spinning and the correct solvent is selected.
Lock and
shim the sample.
Set up the
desired experiment (in this case C-13) with a file
name of your choosing.
Submit the
experiment.
Eject the
tube from the magnet.
Disposal and Clean Up.
- NMR sample
solutions should be drained into the waste solvent
container. Be very careful when removing the
plastic cap as it is easy to break the top of the
tube when doing so. Rinse the tube several
times with acetone into the waste solvent container.
Store the tube uncapped so the acetone will fully
evaporate by the time of its next use.
Product
Submission - Submit your solid "products" in
fully labeled glass vials. (See the lab report
guidelines.)
Post-Lab ReportNMR Reporting and
Interpretation
- The printouts of
the NMR spectra (the one your group obtained as well
as a spectrum for each of the other two compounds)
should be attached at the end of your lab report.
Write the structure of the compound on each NMR and
label the carbons on the structure and label the peaks
correspondingly - this process is referred to as
"assigning the peaks". Also label solvent peaks (CDCl3)
and impurity peaks. Literature spectra are available
at the SDBS website (link can be found on the Chem
350 Links page) and will help you in assigning
the peaks.
- A results table
should be constructed that lists the chemical shifts
observed for each compound, alongside the literature
shifts and the peak assignments. C-13 shifts should
always be rounded to the nearest 0.1 ppm (the only
exception is when two peaks are closer to each other
than that). See the Lab Report Guidelines for more
information on results tables.
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