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How many equivalent hydrogens are there in octane?

  1. Jun 20, 2011 #1
    The answer to my Kaplan test (studying for the DAT, just FYI) says 3, but I think it's 4?

    Am I wrong?
     
  2. jcsd
  3. Jun 20, 2011 #2

    SpectraCat

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    To which structural isomer of octane are you referring? And do you mean non-equivalent hydrogens?

    Assuming you mean n-octane, I think you are correct in principle that there are 4 kinds .. in other words, if you use the substitution test, you can create 4 distinct molecules by substituting a single H-atom with an F-atom. However, for the purpose of determining NMR shifts in alkanes (which I assume is what you are asking about), I am fairly sure that only the nearest-neighbor groups matter. Assuming that is true, can you now see why n-octane has only 3 non-equivalent types of hydrogens?
     
  4. Jun 20, 2011 #3
    I'd be inclined to disagree there, even for NMR, though for e.g. distinguishing between 3- and 4-halooctanes there may not be that much of a difference in shift, in principle this still exists.
     
  5. Jun 20, 2011 #4

    SpectraCat

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    I agree .. in practice it's all a question of resolution. The fact is the the OP is right, and strictly speaking there are 4 non-equivalent types of hydrogens in n-octane. However I am not sure if there is a strong enough magnet to distinguish the difference in chemical shifts of H-atoms on the 3rd and 4th carbons.

    However given the phrasing of your post, I think it's important to point out that we are not talking about halo-octanes at all, but just normal n-octane (at least I think so, the OP has still not confirmed that is the correct structural isomer). So, there is no heteroatom in the molecule we are considering .. the substitution test I mentioned is just a thought-experiment to determine chemical non-equivalence. I do agree that if there were a heteroatom in the molecule, the dependence of the chemical shifts of H-atoms on the distance from the substitution site would be more pronounced.
     
  6. Jun 21, 2011 #5
    Yes, I was talking about n-octane and yes, I meant non-equiv hydrogens.

    Thanks guys, I guess Kaplan is wrong here.
     
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