CHEM 350
- Principles of Organic Chemistry I Experiment #2 - Introduction to Nuclear Magnetic Resonance Spectroscopy. 13C NMR Spectra of some Alkanes and Alkenes References: Klein
Chapter 16, Mohrig Chapter 21.1-21.3, 22.1-22.3 Pre-Lab Assignments: 1. Prepare a
pre-lab plan in your lab notebook that is specific for your group's assigned
compound.
No need to include data for the compounds that other
groups will be examining. 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 and alkenes, hexane, heptane, dodecane, octadecane, cyclohexane, cycloheptane, 2,2-dimethylbutane, 2,3-dimethylbutane, 2-methylpentane, 3-methylpentane, cyclohexene, 1-heptene, and 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 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. 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. 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. Class data will be pooled for the purposes of the lab 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. Your results table should summarize the NMR data for all of the compounds. Take care in designing it for optimal conciseness and ease of use. Included should be 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. Here are some things to definitely include in your Summary section: 1. Comment on the accuracy of the chemical shift measurements. Be quantitative. 2. Comment on the general effect of the number of hydrogens attached to a carbon on its C-13 chemical shift? Use the data to illustrate. Concisely provide a thoretical basis for 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? Use the data to illustrate.
Again, concisley state the thoretical basis for this
effect. 4. Can the number of peaks observed in the C-13 spectrum be accurately predicted from the structure? Use the data to illustrate. |