exam2-2010 - Name _ Exam 2: CHEM/BCMB 4190/6190/8189 (135...

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Name _________________________ Exam 2, 2010: CHEM/BCMB 4190/6190/8189 1 Exam 2: CHEM/BCMB 4190/6190/8189 (135 points) Thursday, 7 October, 2010 1 ). 13 C NMR spectra for one of the configurational isomers (“A” or “B”) of the alkene (below) were recorded. Two signals from the spectra (one at ~164.3 ppm, the other at ~117.8 ppm) are shown (right). The top spectrum (“decoupled”) was recorded with selective decoupling of the hydrogens on the -CH 3 group. The bottom spectrum (“no decoupling”) was recorded without decoupling. a. Explain in detail how selective 1 H- 13 C decoupling of the hydrogens of the -CH 3 in this molecule is performed. ( 4 points ) In the 1 H NMR spectrum, the signal from the methyl hydrogens will be a singlet. This signal/peak is from hydrogens attached to 12 C. The signal corresponding to the hydrogens bound to 13 C nuclei in the methyl group will appear as a doublet, with peaks equidistant (one upfield, one downfield) from the singlet resulting from ( 1 H bound to 12 C). The total integral of these two peaks of the doublet will be about 1% of the integral of the singlet from 1 H bound to 12 C, because the natural abundance of 13 C is about 1%. In order to perform the selective 1 H- 13 C decoupling, during the 13 C experiment, both peaks of the doublet (from 1 H bound to 13 C) must be selectively irradiated with very long, low power pulses in order to decouple the 1 H from the 13 C. decoupled not decoupled A B
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Name _________________________ Exam 2, 2010: CHEM/BCMB 4190/6190/8189 2 b. One of the signals shown (above) from the 13 C spectrum is from the -CN group, and the other is from the -CO 2 - group. Based on the results of the selective decoupling experiment and any other information available in the spectra, identify which nucleus gives rise to each signal. You will have to provide clear and complete justification/explanation of your answer in order to receive credit. ( 6 points ) It is expected that the downfield signal (~164.3 ppm) is from the -CO 2 - group because of the two electronegative oxygen atoms bonded directly to the carbon (carbon will be highly deshielded ). The other signal (~117.8 ppm) then must be from the -CN group. This is supported by the fact that nitrogen is less electronegative than oxygen, so - 13 CN signal would be expected to be more upfield (more shielded ). 1 H- 13 C coupling between the 13 C of the -CN group and the 1 H nuclei of the -CH 3 group would be a 5-bond coupling (NC-C-C-O-C-H 3 ) and it is unlikely that it would be observed. When the methyl group hydrogens are selectively decoupled, we see no change in the signal at ~117.8 ppm, confirming our suspicion that this signal is from - 13 CN. However, the 1 H- 13 C coupling between the 13 C of the -CO 2 - group and the 1 H nuclei of the -CH 3 group would be a 3- bond coupling (C-O-C-H 3 ) which most likely would be observed. When the methyl group hydrogens are selectively decoupled, we see a small splitting removed from the signal at ~164.3 ppm, confirming that this signal is from - 13 CO 2 .
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exam2-2010 - Name _ Exam 2: CHEM/BCMB 4190/6190/8189 (135...

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