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    Hi,

    I was just wondering whether anyone has any tips on answering the longer n.m.r questions as I'm struggling a bit.

    I've just been doing one where you have to find the 2 substances that match the 2 graphs given. I started off by trying to identify some of the main groups present in the substances and matching them up on the graphs but I've got a bit stuck.

    I'm also a confused about how to identify how many peaks the nmr graph will have by identifying the number of environments e.g. in the question attached.

    Any help would be really appreciated, thank you
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    For the last picture there are 6 peaks because:
    1. For the C of CH3
    2. For the C in C=O
    3. For the C in the ring joined to N
    4. For the two C adjacent to the above C. This is because both C are the same as they are symmetrical
    5. For the next two C as they too are identical to each other
    6. For the last C of the ring

    Ive sent you resources that I have made on NMR of carbon and also hydrogen based on the spec for you to look at
    Attached Files
  1. File Type: docxNMR of carbon.docx (25.9 KB, 30 views)
  2. File Type: docxNMR of hydrogen.docx (40.3 KB, 28 views)
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    (Original post by Hajra Momoniat)
    For the last picture there are 6 peaks because:
    1. For the C of CH3
    2. For the C in C=O
    3. For the C in the ring joined to N
    4. For the two C adjacent to the above C. This is because both C are the same as they are symmetrical
    5. For the next two C as they too are identical to each other
    6. For the last C of the ring

    Ive sent you resources that I have made on NMR of carbon and also hydrogen based on the spec for you to look at
    Thank you
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    (Original post by VioletPhillippo)
    Thank you
    Np. Was a bit worried if the reasoning of 6 peaks did not make sense but all is good
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    (Original post by VioletPhillippo)
    Hi,

    I was just wondering whether anyone has any tips on answering the longer n.m.r questions as I'm struggling a bit.

    I've just been doing one where you have to find the 2 substances that match the 2 graphs given. I started off by trying to identify some of the main groups present in the substances and matching them up on the graphs but I've got a bit stuck.

    I'm also a confused about how to identify how many peaks the nmr graph will have by identifying the number of environments e.g. in the question attached.

    Any help would be really appreciated, thank you
    For the matching up question I would advise playing a bit of an elimination game.

    Carbonyl carbons invariably have shifts at 200+ppm, neither spectrum has a peak that high, so products P and Q can be discounted immediately!

    The next thing to know is that an electronegative atom like oxygen deshields a carbon just next door, so the carbon in a 13C-O bond will have a shift around 60-70ppm (you should be able to see these sorts of numbers on whatever data sheet you have).

    figure 1 has one carbon in this environment so could be R or S

    figure 2 has 2 carbons like this, so must be T!

    working out whether figure 1 is R or S is easy, you have already identified that it must have C=C and so it must be compound R.

    The lower shift peaks correspond to the other carbons in each compound and are harder to assign because they appear close together!


    Hope that makes sense, just let me know if you have any questions!
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    (Original post by MexicanKeith)
    For the matching up question I would advise playing a bit of an elimination game.

    Carbonyl carbons invariably have shifts at 200+ppm, neither spectrum has a peak that high, so products P and Q can be discounted immediately!

    The next thing to know is that an electronegative atom like oxygen deshields a carbon just next door, so the carbon in a 13C-O bond will have a shift around 60-70ppm (you should be able to see these sorts of numbers on whatever data sheet you have).

    figure 1 has one carbon in this environment so could be R or S

    figure 2 has 2 carbons like this, so must be T!

    working out whether figure 1 is R or S is easy, you have already identified that it must have C=C and so it must be compound R.

    The lower shift peaks correspond to the other carbons in each compound and are harder to assign because they appear close together!


    Hope that makes sense, just let me know if you have any questions!
    Hi,

    Thanks so much for the explanation, it's made me feel much more confident. The only thing that I'm a bit unsure about is, if 2 peaks are close together such as the 2 furthest to the left on figure 1 but there are also 2 groups that cover that range, is there a way to work out which group it is. E.g. the carbon double bond and the carbon on a benzene ring both cover the range but is there a way of knowing which one the peak represents? Thanks
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    (Original post by VioletPhillippo)
    Hi,

    Thanks so much for the explanation, it's made me feel much more confident. The only thing that I'm a bit unsure about is, if 2 peaks are close together such as the 2 furthest to the left on figure 1 but there are also 2 groups that cover that range, is there a way to work out which group it is. E.g. the carbon double bond and the carbon on a benzene ring both cover the range but is there a way of knowing which one the peak represents? Thanks
    In this case you could tell because none of the options had any benzene rings in!

    In general you can't tell apart 2 peaks that appear very close to one another, although often a bit of common sense will help.

    For example, if you were looking at a peak in the alkene/benzene region, you know that benzene would have 6 carbons, so you would expect several peaks in that region, so if there's only one or two, you're probably looking at an alkene.

    In proton nmr you can often use the peak splitting to help you, for example
    Name:  nmr1.jpg
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    if you ever see 4 peaks like the ones around 7.5 ppm in this spectrum, that's a tell tale sign of a 1,4 disubstituted benzene ring.

    Other times its more difficult, but if you're doing A level they're unlikely to be too mean when setting questions!
 
 
 
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