CHEM 340 Organic Chemistry Survey, Prof. T Nalli, Fall 2003 Winona State University

Experiment #2 - Introduction to Nuclear Magnetic Resonance Spectroscopy. 13C NMR Spectra of Some Simple Alkanes

References:

McMurry, Chapter 13.5-13.7, 13.12; Lehman,  pp 247-248  

Procedure:

Each lab team will be assigned a compound from among the following list of simple alkanes: hexane, heptane, octadecane, cyclohexane, cycloheptane, 2,2-dimethylbutane, 2,3-dimethylbutane, 2-methylpentane, 3-methylpentane.

Safety Precautions - CDCl3 has harmful fumes. Avoid breathing it and dispense it in a fume hood.

Overview - First each group will prepare a sample for NMR analysis. Then the instructor will demonstrate the operation of the NMR spectrometer and print out a spectrum for each alkane analyzed. Each spectrum will require about a half hour to obtain so there will be down time during the lab. All of the spectra obtained in the two labs will be provided to each group.

Preparing the Sample. 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 it. Weigh the NMR tube and cap then use a Pasteur pipet or small spatula to add the alkane to the NMR tube to a height of approximately 2-3 mm. Cap the tube and reweigh. Now add the CDCl3 solvent carefully to a height  of 5 cm in the tube. Invert and/or agitate the tube in order to dissolve the assigned compound in the solvent (warning - the caps can leak). Label your tube by writing on the side of the cap with a permanent felt tip marker.

The instructor will demonstrate the operation of the NMR spectrometer and assist in obtaining the spectrum of your compound. Label the spectrum with your name, date, and compound name, then turn it in to the instructor who will copy it for  the rest of the class.

Report:

You will be provided with copies of the NMR spectra obtained throughout the week. (Print these out off of the course web site.) Write the structure of the compound prominently on each spectrum and label all sets of equivalent carbons a, b, c, etc. Now label the peaks in the spectrum a, b, c, etc. to show which carbons in the molecule are represented. Label any extraneous peaks including the TMS peak and CDCl3 solvent peak.

Start your report with a table that summarizes the NMR results for all of the compounds. The table should list compound names and structures, chemical shifts, and peak assignments. Do not include the extraneous peaks in the table.

Questions  

1. What is the range of chemical shifts of primary carbons (methyl carbons) in the alkanes analyzed? Answer the same question for the secondary, tertiary, and quaternary carbons. What is the effect of number of hydrogens attached on carbon chemical shift?

2. Give the structure of the unknown you identified in Experiment #1 and predict how many resonances would be observed in its 13C NMR spectrum.

(more questions may be assigned later)