exam4-2007

exam4-2007 - Name Exam 4 2007 CHEM/BCMB 4190/6190/8189 1...

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Unformatted text preview: Name _________________________ Exam 4, 2007: CHEM/BCMB 4190/6190/8189 1 Exam 4: CHEM/BCMB 4190/6190/8189 (113 points) Thursday, 13 December, 2007 1 ). Shown on pages 3-4 are DEPT-135, 1 H- 1 H COSY, and 1 H- 13 C HETCOR NMR spectra for the compound shown (geraniol, right). An expansion of one region of the COSY spectrum is also shown. The carbon centers, and associated hydrogens, are numbered 1-10 (ignore the –OH group). The 13 C chemical shifts observable in these spectra are (high to low field): 16, 18, 25, 26, 39, 59, 124, and 124.5 ppm. The observable 1 H chemical shifts are: 1.6, 1.7, 1.72, 2.1, 2.2, 4.1, 5.1, and 5.4 ppm. At each of the methylene positions, the two hydrogens are magnetically equivalent. In the COSY and HETCOR spectra, the peak at 0.0 ppm is from TMS. Fill out the table (right) with the correct chemical shifts for each group. You must justify your answers by providing your rationale for the assignments. If you make assignments based on shielding arguments, you must provide additional, independent verification. You will not receive credit for entries that are not properly justified. The following page (page 2) is blank, so you can use that space for your answers if you like. ( 20 points ) First, there are 10 centers, but only 8 H and 13 C shifts. This is consistent with the structure, as there are no hydrgens on the quaternary carbons 3 and 7 and carbons 3 and 7 will not produce signals in DEPT or HETCOR spectra. These are marked “NA” to indicate this fact. In the DEPT-135 spectrum, signals from methylene groups will appear as negative peaks. Thus, the 13 C chemical shifts of the three methylene carbons are 26, 39, and 59 ppm. Remaining are the three methyl groups and two ethylenic groups. Methyl signals are always upfield, so the 13 C chemical shifts for these three are clearly 16, 18, and 25 ppm. The ethylenic signals, which will be shifted downfield, are therefore the remaining two with 13 C chemical shifts of 124 and 124.5 ppm. The HETCOR spectrum allows all of the 1 H chemical shifts to be correlated to the respective 13 C chemical shifts. From the HETCOR spectrum, the following chemical shift correlations ( 1 H / 13 C) can be made: Methylene: 2.2 / 26; 2.1 / 39; 4.1 / 59 Methyl: 1.6 / 18; 1.7 / 16; 1.72 / 25 Ethylenic: 5.4 / 124; 5.1 / 124.5 There are are two instances where a methylene group is next to an ethylenic group: 1 / 2 and 5 / 6. Of the three methylene groups, 1 most likely will be the most deshielded. We suspect, therefore, that the chemical shifts of 1 are 4.1 and 59 ppm (4.1 / 59). There should be a large COSY crosspeak connecting 1 and 2. There is a large crosspeak at 1 H (ppm) 13 C (ppm) 1 4.1 59 2 5.4 124 3 NA NA 4 2.1 39 5 2.2 26 6 5.1 124.5 7 NA NA 8 1.6 or 1.72 18 or 25 9 1.6 or 1.72 18 or 25 10 1.7 16 Name _________________________ Exam 4, 2007: CHEM/BCMB 4190/6190/8189 2 (you can continue your answers for question 1 on this page) 4.1/5.4, suggesting that 2 is the ethylenic group with chemical shifts 5.4 / 124. 4....
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exam4-2007 - Name Exam 4 2007 CHEM/BCMB 4190/6190/8189 1...

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