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    Hey.

    I'm looking at a 13C NMR spectrum for this structure:


    I used a simulator to see what it should look like, and got this:


    As you can see, the peak around 135 is identified as the C highlighted yellow on the structure.. But I'm confused as to why this is, since I thought C-Cl carbons had a chemical shift of 30-70 ppm?

    Any insight would be helpful. Thanks!
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    (Original post by StaShe)
    Hey.

    I'm looking at a 13C NMR spectrum for this structure:


    I used a simulator to see what it should look like, and got this:


    As you can see, the peak around 135 is identified as the C highlighted yellow on the structure.. But I'm confused as to why this is, since I thought C-Cl carbons had a chemical shift of 30-70 ppm?

    Any insight would be helpful. Thanks!
    It is not just a C-Cl carbon, it is also a carbon in a benzene ring environment.
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    (Original post by charco)
    It is not just a C-Cl carbon, it is also a carbon in a benzene ring environment.
    Right... Why does that have more influence than the C-Cl though?
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    (Original post by StaShe)
    Right... Why does that have more influence than the C-Cl though?
    Hmm... normally both show up, though the C-Cl peak is puny.

    http://www.unm.edu/~orgchem/304L%20p...0Aromatics.pdf
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    (Original post by StaShe)
    Right... Why does that have more influence than the C-Cl though?
    Chlorine is electronegative, so is inductively withdrawing in a C-Cl bond. That's why a C-Cl is fairly significantly deshielded for a non aromatic compound.

    The reason aromatic environments have such high shifts is because of how the delocalised π electrons behave - they generate a pretty strong magnetic field. It turns out that the chlorine doesn't really effect this delocalisation - it only withdraws through the σ bond - so the aromatic C-Cl carbon is only slightly more deshielded.
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    (Original post by Peroxidation)
    Hmm... normally both show up, though the C-Cl peak is puny.

    http://www.unm.edu/~orgchem/304L%20p...0Aromatics.pdf
    You can’t trust 13C NMR peak intensities, there’s all sorts of reasons why they might be less then you’d expect!
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    (Original post by KombatWombat)
    Chlorine is electronegative, so is inductively withdrawing in a C-Cl bond. That's why a C-Cl is fairly significantly deshielded for a non aromatic compound.

    The reason aromatic environments have such high shifts is because of how the delocalised π electrons behave - they generate a pretty strong magnetic field. It turns out that the chlorine doesn't really effect this delocalisation - it only withdraws through the σ bond - so the aromatic C-Cl carbon is only slightly more deshielded.
    Thanks! Makes sense.

    So the spectrum I showed a picture of is a pretty accurate prediction for the structure?
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    (Original post by KombatWombat)
    You can’t trust 13C NMR peak intensities, there’s all sorts of reasons why they might be less then you’d expect!
    That's true, though they can still tell you quite a bit about things like the positions of electron donating/withdrawing groups in aromatic molecules. Aromatic C-Cl peaks' intensity are greatly reduced by the chlorine atom 'donating' a lone pair to the delocalised ring (put very simply at least). This changes the bonding between the carbon and chlorine atoms. You end up with a much shorter and stronger bond because of it's increased bond order. I think the bond order of C-Cl in aryl chlorides is 1.5 though I haven't checked in a while, it's only 1 in alkyl chlorides though. While chlorine does draw the electrons closer to itself in alkyl chlorides, this effect is diminished by it's donation of a lone pair to the ring.

    I haven't looked at the physics behind NMR in quite a while so please bear with me here. I imagine the C-Cl peak's intensity is so low because it's not really a C-Cl bond at all. Like I said above, it's essentially a hybrid (just to clarify, I mean a cross between the two here not anything to do with orbital hybridisation) of the sigma and pi bonds giving a bond order of about 1.5. The precessions of nuclei in molecules are affected by the electric fields between the nuclei and electrons as well as by the magnetic field applied, which is one of the reasons why altering the bonding gives different shift values. You've also got things like coupling constants and all that jazz going on too. Consequently, the weird half sigma-half pi bond will give slightly different shift values and with different intensities to the normal alkyl chloride bonds. You tend to find that the C-Cl peaks are still there but with much lower intensities than normal in aryl chloride C13 NMR spectra. It would make sense if the aromatic peaks had higher intensities too due to the increased number of delocalised electrons. I guess the simulation just cut out the puny C-Cl peak.
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    (Original post by Peroxidation)
    That's true, though they can still tell you quite a bit about things like the positions of electron donating/withdrawing groups in aromatic molecules. Aromatic C-Cl peaks' intensity are greatly reduced by the chlorine atom 'donating' a lone pair to the delocalised ring (put very simply at least).
    Not really, no. It’s to do with relaxation - which is how after you’ve excited the nuclei they get back to their resting state. This is a pretty decent introduction. For the C-Cl bond, it’ll be mainly through scalar relaxation because chlorine is a quadrupolar nucleus (it has a spin greater than 1/2) and dipolar relaxation through hydrogen, where available.

    This changes the bonding between the carbon and chlorine atoms. You end up with a much shorter and stronger bond because of it's increased bond order. I think the bond order of C-Cl in aryl chlorides is 1.5 though I haven't checked in a while, it's only 1 in alkyl chlorides though. While chlorine does draw the electrons closer to itself in alkyl chlorides, this effect is diminished by it's donation of a lone pair to the ring.
    Yes, I agree with this bit! You’d see the difference in the IR stretches and bond enthalpies for example. I’ve never heard the bond order being called 1.5 though it wouldn’t surprise me if someones done some sums and tacked that number on. Inductive σ withdrawal and π donation happen at the same time though, as the π system and σ system are orthogonal.

    I haven't looked at the physics behind NMR in quite a while so please bear with me here. I imagine the C-Cl peak's intensity is so low because it's not really a C-Cl bond at all. Like I said above, it's essentially a hybrid (just to clarify, I mean a cross between the two here not anything to do with orbital hybridisation) of the sigma and pi bonds giving a bond order of about 1.5. The precessions of nuclei in molecules are affected by the electric fields between the nuclei and electrons as well as by the magnetic field applied, which is one of the reasons why altering the bonding gives different shift values. You've also got things like coupling constants and all that jazz going on too. Consequently, the weird half sigma-half pi bond will give slightly different shift values and with different intensities to the normal alkyl chloride bonds. You tend to find that the C-Cl peaks are still there but with much lower intensities than normal in aryl chloride C13 NMR spectra. It would make sense if the aromatic peaks had higher intensities too due to the increased number of delocalised electrons. I guess the simulation just cut out the puny C-Cl peak.
    You’re getting intensities/integration ratios and shift/sheilding mixed up, I think. A bit of a generalisation but they can usually be thought of separately. If you’re interested, NMR by Peter Hore is a really good intro to NMR!
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    (Original post by KombatWombat)
    Not really, no. It’s to do with relaxation - which is how after you’ve excited the nuclei they get back to their resting state. This is a pretty decent introduction. For the C-Cl bond, it’ll be mainly through scalar relaxation because chlorine is a quadrupolar nucleus (it has a spin greater than 1/2) and dipolar relaxation through hydrogen, where available.

    You’re getting intensities/integration ratios and shift/sheilding mixed up, I think. A bit of a generalisation but they can usually be thought of separately. If you’re interested, NMR by Peter Hore is a really good intro to NMR!
    Ah okay, so it's due to changes in relaxation times and the routes taken then... cool! Like you said I'm probably getting muddled with the terms too. I haven't looked at this in a while but I'm surprised at how much I've forgotten. I'll definitely go back over that, I'm looking forward to it actually, it's a really interesting topic!

    Thanks for the book recommendation too, I'll check that one out.
 
 
 
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