CHEM 350 -  Principles of Organic Chemistry I
Prof. T Nalli, Fall 2006, Winona State University

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

References:

Smith, Chapter 15.1-15.3, 15.11; Pavia, Technique 27. Also see Section 26.1. 

Procedure:

You will join together with another team of three to form a supergroup of six for this lab. (This is for the in-lab procedures only. Each team should continue to keep its own in-lab journal and prepare its own pre-lab plan and report.) Each supergroup will be assigned a compound from among the following list of alkanes and alkyl halides: hexane, heptane, dodecane, cyclohexane, cycloheptane, 2,2-dimethylbutane, 2,3-dimethylbutane, 2-methylpentane, 3-methylpentane, 1-bromobutane, 2-bromobutane, 1,2-dichloroethane, 1-chlorobutane.

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

Agenda  - First, the instructor will present a brief lecture on the theory and interpretation of 13C NMR and the use of the MestRe-C software for data processing. Then, each supergroup will prepare a sample for NMR analysis and bring it to the NMR lab, where the instructor will demonstrate the operation of the NMR spectrometer and obtain the spectrum. Each spectrum will require about a half hour to obtain so there will be down time during the lab. Class data will be pooled for the purposes of the lab report.

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 assigned compound 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.  

Waste Disposal - 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.

Report:

NMR spectra for the entire class will be available in the class storage folder. Each file will be clearly named with the name of the compound. Copy these files to your computer and use MestRe-C to process and print them. The instructor will demonstrate the use of this software during the lab and can assist you at the end of the lab period or during office hours as needed.

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 data for all of the compounds. The table should list compound names and structures as well as chemical shifts and assignments for each peak. Use the NIMCR's ISDBS for Organic Compounds to verify your peak assignments and include the literature values for the chemical shifts in the table. Do not include the extraneous peaks (see above paragraph) in the table.

Questions  

1. How accurate are the chemical shift measurements? Does MestRe-C automatically print them using a reasonable of significant figures?

2. Consider first the data for the alkanes only. What is the observed range of chemical shifts for primary carbons (methyl carbons)? Answer the same question for the secondary, tertiary, and quaternary carbons. What is the general effect of number of hydrogens attached to a carbon on its C-13 chemical shift?

3. Now compare, contrast, and rationalize the chemical shifts observed in the alkyl halides. Use the data to compare the effect of a halogen on the chemical shift of the directly bonded carbon versus its effect on carbons further down the chain. Generalize as to the effect of chlorine vs that of bromine. Use the data to predict the spectrum (including chemical shift) of 1,2-dibromoethane.