I was expecting to see a question about determining an unknown compound from interpreting it's mass spectrum and IR spectrum on the exam. The example on page 484 of chapter 13 is a good representation of what I was expecting. The problem and solution is as follows:
Q: What information is obtained from the mass pectrum and IR spectrum of an unknown compound X? Assume X contains the elements C, H, and O.
Step 1: Use the molecular ion to determine possible molecular formulas. In this case, the molecular ion is 88. Use an exact mass to determine a molecular formula. Divide the mass by 12 (1 C atom) to determine the maximum amount of carbons. Replace 1 C by 12 H's for another possible molecular formula. Substitute 1 O for CH4 then repeat for a possible molecular formula with 2 O atoms. There will be three possible formulas: C5H12O, C4H8O2, and C3H4O3. The possible hydrocarbons are discounted because the compound X contains an O atom. If the molecular ion had an exact mass of 88.0580, the molecular formula of X is C4H8O2.
Step 2: Calculate the degrees of unsaturation. For a compound of molecular formula C4H8O2, the maximum number of H's = 2n + 2 = 2(4) + 2 = 10. Because the compound contains only 8 H's, it has 2 fewer H's than the maximum number. Since each degree of unsaturation removes 2 H's, X has one degree of unsaturation. This means that X has one ring or one pi bond.
Step 3: Determine what functional group is present from the IR spectrum. The two major absorptions in the IR spectrum above 1500 are due to sp3 hybridized C-H bonds (3000-2850) and a C-O double bond (1740). Thus, the one degree of unsaturation in X is due to the presence of the C-O double bond.
Mass spectrometry and IR spectrometry give valuable information when determining an unknown compound and that is why I expected a problem like this to be on the test. If the structure was wanted then NMR would be needed.
After taking the test I completely agree with you, there were a lot of questions I thought would be on the exam that wasn't. Especially questions on the sapling like problems 15 and 16, where it ask you to tell how many unique HMNR and CNMR signals exist for each compound, but instead there hardly wasn't anything about signals on the test. I also expected there to be a problem like number 17 on sapling, where it asked you to give the chemical shifts in parts per million (ppm) and it gave you the HMNR signal in Hz and the instrument in MHz. Overall, I really liked your question and would’ve loved to have seen that on the test. Also Dr. Mullins gave us a website: http://www.wfu.edu/~ylwong/nmr/c13/ if you need help with Carbon Nuclear Magnetic Resonance. I hope you did well on your test and I'm sure we will all be more prepared next test!
ReplyDeleteI thought the same thing as you. I thought there would be determining an unknown compound on test. Since we discussed so much in class mass spectrometry and IR spectroscopy, I figured there would be a lot of that on the test. Especially determining unknowns and interpreting the graphs.
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