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H Nmr

  1. Jul 26, 2004 #1
    Okay I am lost. My Professor has given me the formula ( C4 H8 Br2) and the signals 3.8(s, 2H) and 1.8(s, 6H). How the hell am I suppose to compose a molecule from that?!?! Any ideas? :biggrin:
     
  2. jcsd
  3. Jul 26, 2004 #2

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    Six equivalent hydrogens? Two equivalent hydrogens? A four carbon skeleton? Two bromines? It doesn't get any simpler --- try reading the book.
     
  4. Jul 27, 2004 #3
    Dissect all the information that is given to you. The first thing you should always do when looking at an H NMR is to look at how many signals appear. You have 2, which means the molecule has only 2 distinct hydrogen environments. Now look at each individual signal. both of them have s's meaning singlets. Using the n+1 rule you know that the h's that produce those signals have a carbon neighbor with 0 hydrogens attactched. Thus you know there are 2 carbons that have no hydrogens. The singal at 1.8ppm says there are 6H's so that is hinting at a C?(CH3)2 group. The ? means you have to figure out if there is going to be a H attatched. so how many atoms are left? All that is left is 2Br's 2 H's and a Carbon. The carbon must be attatched to the C?(CH3)2 part so now you have (CH3)2C---C. There are only two signals so the other 2 hydrogens must be attatched to one of the red C's or both. Obviously the C on the left can not have an H because if it did the 1 H left would go on the right C, but that would give you 3 signals because you have 3 hydrogen environments. Thus the only thing that would make sense would be a Br. From there the problem falls apart everything else must be attatched to the C on the right. You should finally figure out that the molecule is (CH3)2CBr-------CH2BR. I hope the structure is clear the way i wrote it, because that is the best way i could "draw" it.
     
  5. Jul 31, 2004 #4
    Oh my Professor forgot to teach us how to do this crap before assigning it. Plus the book never showed questions or presented information in this format. Thanks for the help though.
     
  6. Aug 2, 2004 #5
    I've always hated NMR questions like this because in real life, if you're not sure of the structure after collecting a protron spectrum, you could always run a DEPT-135 spectrum. You don't have to be all that clever sometimes... Although in this case, it is a rather easy problem.
     
  7. Aug 2, 2004 #6
    I am sorry I posted such an easy question. I was not taught how do to it and obviously a little lost. I was just looking for a little help. If it make you feel any better I did get a B on my test (no thanks to either some people's help). Again I am sorry I offended you guys.
     
  8. Aug 3, 2004 #7
    noxflos... Actually, it's good that you asked, and for my part I'm sorry that I sounded condescending in answering it. There's only one way to learn to interpret NMR spectra and that's practice, practice, and ... more practice.
     
  9. Aug 3, 2004 #8
    Ostwald- thanks for your advice. You are correct it does take lots of practice and it is not so bad once you learn a few rules. :)
     
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