CHEM 350 - Principles of Organic Chemistry I Experiment #2 - Introduction to Nuclear Magnetic Resonance Spectroscopy. 13C NMR Spectra of some Alkanes and Alkenes References: Smith, Chapter 14.1-15.3, 14.11, Mohrig Chapter 19.1-19.5, 19.13 Pre-Lab Assignments: 1. Prepare a pre-lab plan in your lab notebook that is specific for your group's assigned compound. 2. Install the JEOL Delta software on your laptop. Go to the NMR section of the Chem 350 website and follow the instructuons there for installing Delta. Overview: 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: pentane, hexane, heptane, dodecane, octadecane, cyclohexane, cycloheptane, 2,2-dimethylbutane, 2,3-dimethylbutane, 2-methylpentane, 3-methylpentane, cyclohexene, 1-dodecene. 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 JEOL Delta 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.
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 or uncapping 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. 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. Store the tube uncapped so the acetone will fully evaporate by the time of its next use. General Instructions for Using Delta 1. Choose "Process 1D" under the Processors menu and open the file by using the file menu, clicking on the 2. To convert the data to a useable form you need to Fourier Transform it 3. To annotate the spectrum with peak chemical shifts use the "Auto Peak Pick" icon, 4. Zooming in is accomplished by 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: "BackSpace" to revert to the previous (unzoomed) view. 5. Print the spectrum by simply printing on the icon that looks like a printer and selecting the desired printer from the scroll list. 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 Delta 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 (if present) 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 our chemical shift measurements? Be quantitative. 2. Look at the data for just the alkanes first. 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 changing the number of hydrogens attached to a carbon on the C-13 chemical shift? Explain the reason behind this effect. 3. Look at the data for the alkenes in comparison to the alkanes. What is the effect of the hybridization of a carbon atom on it chemical shift? Explain the reason behind this effect. |