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Enol-keto interconversion

  1. Nov 11, 2004 #1
    Hi can someone help me on how to determine the enol and keto content from a H-NMR graph ? I know we have to use the integration of the peaks to find the ratio, but I just don't quite get it(like which peaks I should use). It would be great if someone can provide a detailed example.

    Thanks
     
  2. jcsd
  3. Nov 11, 2004 #2

    chem_tr

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    I think when a tautomerism occurs, the relevant peak intensities diminish, since it is shared between two interchangable structures.

    I strongly recommend that you use deuterium-exchange method to find out which peak shows this phenomena.

    I have no example on hand, but if you find the 1H-NMR spectrum of acetylacetone, maybe we can talk through this one, since this may be a good example of enol-keto tautomerism.
     
  4. Nov 11, 2004 #3

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    I have some experience with these systems. The chemical shift of the O-H versus the C-H should be very different. The integration of these two distinct peaks should total to what you would expect for the number of H's in your compound. If it's a beta-keto ester then this difference in chemical shift will be huge and it should be very easy to assign the peaks.
     
  5. Nov 11, 2004 #4
    Here is an example I found on the internet with the result. I don't know how they get this result. The product I will be getting to analyze is
    2-acetylcyclohexanone, I want to understand the concept before I go ahead and do it.

    thank you

    file is zipped with a *.jpeg file, no virus.
     

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  6. Nov 11, 2004 #5

    chem_tr

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    I presume the logic behind the interpretation of this NMR spectrum is as follows:

    • Start off with keto tautomer first. Assign the peaks; two CH3 groups are identical, so expect a singlet within methyl group's boundary.
    • The CH2 group is an activated one, so expect a singlet again, but at the extreme border of CH2 boundary.
    • So just two peaks will be present for keto tautomer; now let's look at enol derivative.
    • In enol form, methyl groups are identical again, so expect another singlet near keto group's signal.
    • =CH-OH group has characteristic alcohol and methylidene signals, assigning these two is fairly easy, since there are no additional peaks within their boundaries.
    • Now, use integration ratios to find out what the isomer ratio is. The crucial point lies on the ratio between methylidene (=CH-) and methylene (CH2) peak integrations. As you know which signal belongs to which group, devising a ratio is not hard. Should you encounter difficulties, it is a wise choice to study especially 13C-NMR, especially APT-type, in addition.

    About your molecule, I have included this attachment. Your molecule has (at least) three tautomers, in which the ordinary diketone has a characteristic triplet with 1H and a dublet of quartet with 2H. The integration of these peaks will be useful in determining tautomer ratios. About the other two, proton nmr will not provide a satisfactory solution, I think. Carbon-13 NMr will be a bit more useful.
     

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    Last edited: Nov 11, 2004
  7. Nov 14, 2004 #6

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    Chem_tr has given a nice explanation is his attachment. Note that in a cyclic case (like 2-acetyl cyclohexanone) there won't be a CH peak for the enol form, only for the keto form. The two diagnositic peaks will be the triplet at about 3.5 ppm for the keto CH proton and the singlet somewhere between 10 and 17 ppm for the OH of the enol form. In the cyclic case, the integration of these two peaks should sum to 1. Figuring the percent composition should be pretty simple from there!
     
  8. Dec 3, 2009 #7
    THANK YOU!!
    Now I got it
    I've spend 3 hours looking all over internet to find out how to calculate enol content from NMR and its so freaking simple!
     
  9. Mar 29, 2011 #8
    Thank you for posting this! Very helpful
     
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