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  1. #1
    Premium User
    Pre-Medical

    Join Date
    Oct 2013
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    1

    Default H NMR Question Heptane

    This question is from the quizzes (ID 1258). Why does a heptane not have 4 distinct signals, but only three? The solutions said that the 3 center carbon's hydrogens experience the same average environment, but aren't the 4th carbon's hydrogens unique? I can see why C1 and C7; C2 and C6; and C3 and C5 have similar hydrogens.

  2. #2
    Chad
    Join Date
    Jun 2007
    Posts
    1,884

    Default

    First of all I agree with you completely here. There are 4 distinct hydrogen environments and I looked back at the original file where I wrote this question and that is exactly how it was answered. I'm not sure when it was changed but I just changed it yet again. I discuss the problem with this question below.

    With most long chain alkanes all the methylene (CH2) groups are in similar enough environments that the hydrogens won't be resolvable and will appear as a giant singlet in the spectrum. But the methyl groups will show up as as distinct signal. So by this line of reasoning, although there are 4 hydrogen environments, you could argue that there will only be 2 signals (which isn't even what the correct answer you saw stated). Whereas I would prefer students give the correct answer as 4, I have seen professors who wanted students to answer 2 signals. The problem is this requires students to know a rather technical piece of info about NMR and even so this becomes a matter of whether there are only 2 signals or whether there are 4 signals but 3 of them happen to accidentally overlap.

    As I stated above I changed this question even from the original to remove all ambiguity. The question now asks for the number of signals in the 13C spectrum. Even though the hydrogens aren't resolvable, the carbons definitely are and there will indeed be 4 carbon signals in the 13C NMR spectrum.

    Sorry for the trouble but thanks for the heads up.

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