Practice midterm - CHM 3122 ‐ practice exam – 2010 Note that this sample exam is longer than the midterm of October 20 will be 1 Short

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Unformatted text preview: CHM 3122 ‐ practice exam – 2010 [ Note that this sample exam is longer than the midterm of October 20 will be] 1. Short answers : (a) Determine the degree of unsaturation of a compound with molecular formula C2H3Cl3O2. (b) Protons have a Larmor frequency of 400 MHz in a magnetic field of 9.4 T. What is the Larmor frequency of protons in a magnetic field of 18.8 T? (c) Why is proton (1H) NMR more sensitive (better signal‐to‐noise) than 13C NMR ? (d) Does a cross peak in a NOESY spectrum indicate that there is a J coupling between the two protons which are correlated? Explain. No, the cross peaks tell us which protons are close in space COSY relies on j-coupling for spin spin (e) If you want to improve the signal‐to‐noise ratio of your 13C NMR spectrum, is it better to (i) double the concentration of the sample or (ii) record twice the number of FID scans? (f) Explain why the intensities of the peaks in a triplet due to coupling of a proton with two equivalent spin‐1/2 nuclei differ from those of the triplet resulting from the coupling of a proton with one 14N spin. (g) Proton NMR spectra often show complicated multiplets (“higher‐order” coupling). In 13C NMR spectra of organic compounds, why do we never observe higher‐order coupling patterns? the probability of 2 Carbon-13 atoms being in the same molecule are small. Carbon 13 to Hydrogen splitting is not seen due to broadband decoupling which suppresses the splitting. (h) Why do we observe peaks in the 1H NMR spectra of deuterated solvents? (Deuteration implies that the 1H are replaced with 2H). Predict the number of peaks in the 13C NMR spectrum of 13CD3Cl. 2. Chemical and magnetic equivalence. (a) What is the Pople notation for the four methylene protons in the molecule shown below? Which are chemically equivalent? Which are magnetically equivalent? Explain. O O H H H H (b) What is the Pople notation for the four methylene protons in the molecule shown below? Which are chemically equivalent? Which are magnetically equivalent? F‐CH2‐CH2‐NO2 3. Shown on the following pages are the infrared and 1H NMR spectra for a compound with the molecular formula C9H11NO2. The results of a 13C DEPT NMR experiment are also given. (a) 1H chemical shifts around 7 ppm suggests what in general? (b) What does the quartet‐triplet pattern ( labelled « c » and « a ») suggest? (c) Determine the structure of the compound using all available data. Draw your final answer in the box below: 1 H NMR spectrum Zoomed region of the 1H NMR spectrum. 13 C NMR spectrum DEPT‐135 positive negative positive positive positive positive no peak no peak no peak DEPT‐90 no peak no peak positive positive positive positive no peak no peak no peak 14 ppm 61 ppm 116 ppm 119 ppm 120 ppm 129 ppm 131 ppm 147 ppm 167 ppm (d) Draw the molecule again, and assign the peaks in the 1H NMR spectrum to the protons in the molecule. Explain the appearance of peak “b”. Explain the origin of the multiplet observed for the “e” protons. (e) ( 2 points ) Show that the molecule you’ve drawn is consistent with the degree of unsaturation which you can calculate from the molecular formula. (g) If you have drawn an aromatic molecule, showhow the proton NMR spectrum is consistent with the substitution pattern of the ring. 6. Shown on the following pages are NMR spectra for an unknown compound (mass 166 Da). Determine the molecular structure and assign the peaks in the spectra. Hints: the 1H NMR spectrum shows several higher‐order multiplets. Don’t rely too much on the multiplicities of the peaks. The INADEQUATE spectrum will be very useful. 1 H NMR 13 C DEPT NMR spectrum Zoomed region is shown on the following page. Zoomed region of the INADEQUATE spectrum. Draw your final answer in the box : ...
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This note was uploaded on 01/16/2011 for the course CHM 3122 taught by Professor Bryce during the Fall '09 term at University of Ottawa.

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