Expt 2. Acid/Base Extraction, Recrystallization, Sublimation, and C-13 NMR.
Part 2 Recrystallization, Sublimation, and C-13 NMR

Reading Assignment - Mohrig Chapter 7.1, 15, 16.1-16.2

Background

Two techniques used to purify organic solids are recrystallization (Mohrig Chap 15) and sublimation (Mohrig 16.1-16.2). In recrystallization, the solid is dissolved in an appropriate solvent and induced to recrystallize back out leaving impurity molecules behind in the solvent. In sublimation, the solid is heated (often under reduced pressure) so as to induce it to sublime with the vapors allowed to deposit on a cold surface. Impurity molecules do not sublime along with the solid and are left behind in the heated flask. Sublimation only works for solids with relatively high vapor pressures and so is less generally useful than is recrystallization, which can be used for almost any sold.

Nuclear magnetic resonance (NMR) is the most important characterization tool available to organic chemists. In this lab you will obtain the C-13 NMR spectrum in order to provide evidence that it is the expected compound.

Overview

You will use recrystallization to purify the crude ibuprofen obtained in part 1 and sublimation to purify the crude caffeine. The purified solids will be tested for purity using mp determinations. The DEET obtained in part 1 will not be purified but instead will be analyzed by C-13 NMR.

Pre-Lab

In your prelab include a prediction of the number of peaks expected in the spectrum of DEET.

Also make sure to install the JEOL Delta software on your laptop according to the directions at http://course1.winona.edu/tnalli/f16/NMR.html

Experimental Procedures

Recrystallization of Ibuprofen (make sure to see Mohrig for further information about how to carry out each of these steps)

1. Dissolve the crude ibuprofen in a minimum amount of boiling methanol. Use a reflux setup (Mohrig Chap 7.1) so as to accomplish this without having to worry about the solvent boiling away.
2. Allow the solution to cool slowly to room temperature. Crystals should eventually come out of solution.
3. Cool on an ice bath to maximize crystal growth and then carry out vacuum filtration on a Buchner or Hirsch funnel (Mohrig Chap 10.4).
4. Wash the crystals on the Buchner funnel with a small amount of ice cold methanol, then leave them on the filter with the vacuum on for at least 20 min so to remove final traces of methanol.
5. Weigh the obtained crystals and determine the mp.

Sublimation of Caffeine

1. Use a 25-mL filter flask and centrifuge tube to sublime the crude caffeine from part 1.  See Chap 16.2 and Fig 16.1a(a) of Mohrig
2. Weigh the obtained solid and determine the mp.

C-13 NMR of DEET

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 that need to be carried out 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 2"x2" ziploc bags. (See the lab report guidelines.) Dispose of the left-over DEET in the waste solvents container.

NMR Processing. General Instructions for Using Delta   

• The NMR is set to save data files directly to our class-storage readonly folder T:\20173000149M\ReadOnly. (See http://course1.winona.edu/tnalli/f16/NMR.html for information about how to access your NMR data files.
• Once you find your data file copy it to your desktop (or other preferred folder on your computer). Then double click on it to open it in Delta.
  
• To annotate the spectrum with peak chemical shifts use the "Auto Peak Pick" icon.
  
• Use the zoom tool (looks like a magnifying glass) to zoom in as necessary by simply 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 "hyphen" to revert to the previous (unzoomed) view. 
• Print the spectrum by simply clicking on the icon that looks like a printer and selecting the desired printer from the scroll list.
• The instructor will provide more detailed instructions in lecture.