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Homework Help: Identify an organic unknown with only spectroscopic data

  1. May 30, 2010 #1
    1. The problem statement, all variables and given/known data

    I've been given the IR spectrum, 1H NMR, and Mass Spec data for an unknown and must identify it correctly and assign all significant spectroscopy/spectrometry signals to the molecule accordingly. I am probably 95% sure that my compound is ethyl phenylacetate, and I've assigned all peaks in the IR and 1H NMR, but I'm having some trouble with a couple signals in the mass spec print out. Here is a link to a pdf of all the data printouts, but the most relevant is the mass spec data (unless someone figures out that I'm actually wrong in identifying the compound as ethyl phenylacetate):

    http://dl.dropbox.com/u/1406355/all%20data.pdf [Broken]

    2. Relevant equations

    (I don't think anything goes in this section for this type of problem.)

    3. The attempt at a solution

    Namely, I cannot figure out the m/z ratios: 39, 51, 65, and 105. I've assigned the rest of the m/z ratios: 15=methyl cation, 29=ethyl cation, 91=benzyl cation, 119=benzyl acylium cation, and 164=molecular ion. Any chance someone might know where to help me start with the rest? I thought maybe it was some sort of McLafferty rearrangement but I don't think so because the m/z wouldn't match up to any of the above.

    Thanks very much for your help!
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. May 31, 2010 #2
    I hope that the reason nobody hasn't posted anything on this yet isn't because of some mis-formatting on my part. Please let me know if I wasn't clear enough!
  4. Jun 1, 2010 #3
    The proton NMR ratios are correct for ethylphenyl acetate. The IR is basically good for telling you that there is an ester group present in the compound from the strong peak in the 1700 range. The fragments you listed are the major ones and coincide with ethylphenyl acetate. The benzyl cation will be the most stable of the ones you listed due to resonance, therefore we expect it to have the greatest peak in comparison to other fragmentation patterns. Finally, the molecular ion peak is correct for ethylphenyl acetate.
    Last edited: Jun 1, 2010
  5. Jun 1, 2010 #4
    Thanks very much for the response, I am pretty confident with the conclusions I have come to already from the IR, NMR, and MS; what I cannot figure out and is being asked of me is to actually assign all peaks in the MS. That's essentially my problem:

    Once again, thanks for your insight; if you have any idea about what fragments these signals represent, I'd appreciate it.
  6. Jun 1, 2010 #5


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    91 is not benzyl cation. Its tropylium and 65 is the result of the further rearrangement of that ion. I can't tell you any more without working it out for you.
  7. Jun 2, 2010 #6
    Oh! Thank you, I hadn't considered that. So please let me know if this is wrong: to form the tropylium cation, the benzyl group is cleaved and it rearranges to incorporate the extra carbon into a 7-membered aromatic ring, which is the large signal at 91. Then the minor signal at 65 is due to the loss of neutral acetylene, forming a five membered aromatic cation.

    This still leaves the signals at 39, 51, and 105 unaccounted for. The only thing I can think of for the signal at 105 is that somehow CH2 is lost from the benzyl acylium cation, but I'm not sure what rearrangement would yield that.

    Also, the 51 signal looks like it has lost CH2 from the structure at the 65 signal. Then the 39 signal appears to have just lost C from the 51 signal. Are these just further breakdowns of that tropylium ion? Could you give me some hint as to how to imagine these rearrangments?

    Thank you!
  8. Jun 3, 2010 #7
    I found a site that will help you visualize the ion formations. http://www.chemistry.ccsu.edu/glagovich/teaching/316/ms%20(old)/aromatic.html" [Broken]
    Last edited by a moderator: May 4, 2017
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