Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Determining compound from IR and mass spec

  1. Jun 11, 2010 #1
    Using the IR full scan, I have to determine the formula and the structure of the compound. I have attached the scans. Looking at the IR scan, I see that I have an NH and a CH. So is my compound CH2N??

    I also have a mass spec output for an unknown organic compound. I have to determine the structure and formula. Using the CxHy rule, I have the following:

    42= C3H6
    50= C4H2
    67= C5H7
    81= C6H9
    93= C7H9
    107= C8H10
    138= C11H6

    I am not sure what to do after this...do I just write the formulas together and that is my answer??
     

    Attached Files:

    • c.jpg
      c.jpg
      File size:
      14.9 KB
      Views:
      209
    • m.jpg
      m.jpg
      File size:
      24.5 KB
      Views:
      192
    • i.jpg
      i.jpg
      File size:
      6.9 KB
      Views:
      179
  2. jcsd
  3. Jun 11, 2010 #2
    Start by looking at this, as it seems that you lack some fundmental knowledge about how mass spec works. http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/MassSpec/masspec1.htm"
     
    Last edited by a moderator: Apr 25, 2017
  4. Jun 12, 2010 #3
    ok so i went to the link and i still think i did everything right. what am i doing wrong?? :(
     
  5. Jun 13, 2010 #4
    Hi mimibajiduh, I looked at the spectra and they seem peculiar. Do they correspong to the same compound or to two different ones?
     
  6. Jun 13, 2010 #5
    It is definite that the mass spectrum gives clue that the compound contains bromine - it follows from the fact that in nature Br is represented by two isotopes Br-79 and Br-81 respectively in almost 50:50 abundance (thus its averaged molar mass given in Periodic Table is 79,9). So if a substance contains one Br atom in its mass spectrum some of the peaks will go in pairs of pretty the same intensity and separated by 2 m/z units - just what you have at 81, 93, 107 and 138 (these correspond to molecules containing Br-81). The mass of molecule without a Br atom would then be 138-81=57 wuala! It is just what as we have in the spectrum :).
     
  7. Jun 15, 2010 #6
    oh sorry for not mentioning it in my original post, they are two different ones.
     
  8. Jun 16, 2010 #7
    Well it is kind of impossible to guess the structures then - see yourself: IR spectrum only gives us information about functional groups in the compound 1 and we can say that it has either NH or OH group (both give a broad peak at 3500-3000 cm-1) and from mass spectrum of the compound 2 we can say that it contains one Br atom and that is all. I hardly believe that there are people in the world who could tell what are these compound basing on spectra you give, but good luck anyway ))
     
  9. Jun 16, 2010 #8
    i have attached 2 more scans of the rest of the info i have for the IR. i dont know if it will help or not.
     

    Attached Files:

    • h.jpg
      h.jpg
      File size:
      11 KB
      Views:
      154
    • e.jpg
      e.jpg
      File size:
      12.7 KB
      Views:
      145
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook