CHEM 350 -  Principles of Organic Chemistry I ..................................Prof. T Nalli, Fall 2010, Winona State University

Experiment #3 - Identification of a Liquid Unknown Using IR and NMR Spectroscopy

References:

Smith, Chapter 13-14, Mohrig, Chapters 18 and 19.

Pre-Lab Assignments:

The pre-lab plan part of the lab notebook is not necessary for this experiment.

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 at the top of page 237 and in figure 18.8) 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 18.4 (steps 1-5) 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. 

General Instructions for Using Delta

1. Choose "Process 1D" under the Processors menu and open the file by using the file menu, clicking on the icon, or hitting ctrl<o>. 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 .

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.

Report:

A full report is not required for this lab. Instead I expect you turn in fully-labeled printouts of your three spectra.

Label each spectrum with your group name(s), course number, section number, and experiment number.

The structure of the molecule 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.