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

Part 2 - Recrystallization and C-13 NMR

Reading Assignment - Mohrig Chapter 7.1, 15

Background

A standard technique that has long been used to purify organic solids is recrystallization (Mohrig Chap 15). In a recrystallization procedure, the solid is dissolved in an appropriate solvent and then induced to recrystallize back out (usually by cooling the solution) so that it can be collected by filtration. Impurity molecules typically do not fit into the crystal lattice structure and/or have greater solubility than the desired compound and are, thus, left behind in the solvent.

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 of each compound from part 1 in order to provide evidence that it is the expected compound.

Overview

You will use recrystallization to purify the crude trans-cinnamic acid obtained in part 1. You will also carry out mp determinations of the obtained solids. C-13 NMR spectra of all three separated compounds will also be obtained by the class as a class project.

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.

Also make sure to install the JEOL Delta software on your laptop according to the directions at http://course1.winona.edu/tnalli/f17/NMR.html

Experimental Procedures

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

1. Carry out small scale tests (Mohrig p 188) to determine an appropriate solvent to use for your recrystallization.
   
2. Dissolve the crude cinnamic acid in a minimum amount of the selected solvent. Use a reflux setup (Mohrig Chap 7.1) so as to accomplish this without having to worry about the solvent boiling away.
3. Allow the solution to cool slowly to room temperature. Crystals should eventually come out of solution.
4. 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).
5. Wash the crystals on the Buchner funnel with a small amount of ice cold recrystallization solvent, then leave them on the filter with the vacuum on for at least 20 min so to remove final traces of solvent.
6. Weigh the obtained crystals and calculate the percent recovery of the recrystallization.
   

C-13 NMR Spectroscopy

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.) 

NMR Processing. General Instructions for Using Delta   

• The NMR is set to save data files directly to our class-storage readonly folder T:\20183001212M\ReadOnly. (See http://course1.winona.edu/tnalli/f17/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.