Expt 2. Separation of the Compounds in a Mixture

Part 2 - Melting Point, Mass Spectrometry and C-13 NMR

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.

Mass spectrometry (MS) is another very important method to help prove the structure of a compound. In this lab you will use a relatively new type of mass spectrometry, ASAP-MS,  to verify the molecular weights of your isolated compounds from part 1. 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.

Much more background information about both of the above spectroscopic techniques (NMR and MS) can be found in the Mohrig and Brown textbooks and the instructor will fill in needed details during lecture and at the start of the lab. You will not find anything about ASAP ionization in your textbooks so here is a reference that explains how ASAP works. https://www.chem.pitt.edu/facilities/mass-spectrometry/mass-spectrometry-introduction

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

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.

Mass Spectrometry

The mass spectrometer is in SLC 335. Simply bring your compound to this room when given the go ahead and the instructor or TA will assist you in obtaining the atmospheric solids analysis probe (ASAP) mass spectrum.

Product Submission - Submit your solid "products" in fully labeled 2"x2" ziploc bags. (See the lab report guidelines.)