Expt 2. Acid/Base Extraction and C-13 NMR
Part 2 - C-13 NMR and mp determination
Relevant textbook readings – Karty Chap 17.1-17.3,
17.12 (Chapter references below are from this source.)
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 your group with this. 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.
Post-Lab Report - NMR 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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