Expt 1.
Separation of the Compounds in a Mixture
Part 2 - Melting Point
Determination and C-13 NMR Spectroscopy
Relevant textbook
reading - Mohrig - Chapter 14, 22.1-22.3,
23.1-23.2 Hart, et.al., Chapter 12.1-12.3
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.
You will also carry out
mp determinations of the obtained solids from part 1
both as a way to confirm their identities and to assess
their purity.
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
Recoveries. Now that the obtained
solids have had a week to air dry they should be
reweighed and final % recoveries calculated.
Melting Point
Determination
Use a Mel-Temp
apparatus to determine 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
detailed in Mohrig, Chapter 14. Also be careful to
record your observations during the melting process and
to document the rate of temperature increase (by
recording temperature and time) 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 1 cm. (For H-NMR we will use much less.)
- Add the NMR solvent,
CDCl3, to a height of
approx 5 cm in the tube. Cap and then invert
and/or agitate the tube in order to dissolve the
liquid 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
(name, course, compound name or structure) glass vials.
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