Turn on thread page Beta

Carbon NMR- equivalent environments-question help. watch

    • Thread Starter
    Offline

    17
    ReputationRep:
    Hi,

    There are 5 peaks in the carbon NMR spectrum for this molecule, indicating that there are 5 carbon environments. However, I'm really struggling to determine which environments are equivalent.

    Any help would be really appreciated.

    Thanks
    Attached Files
  1. File Type: docx Carbon NMR.docx (394.7 KB, 22 views)
    Offline

    20
    ReputationRep:
    (Original post by VioletPhillippo)
    Hi,

    There are 5 peaks in the carbon NMR spectrum for this molecule, indicating that there are 5 carbon environments. However, I'm really struggling to determine which environments are equivalent.

    Any help would be really appreciated.

    Thanks
    Are you sure the structure's correct? The nitrogen has four bonds to it, which cannot be possible if the nitrogen is neutral. And the boron has 4 bonds to it, which isn't possible either with neutral boron. In any case, the bottom portion of the molecule isn't relevant for the carbon spectrum. In a symmetrical molecule, the corresponding carbons on either side of the plane of symmetry will be equivalent.

    • Thread Starter
    Offline

    17
    ReputationRep:
    (Original post by Plantagenet Crown)
    Are you sure the structure's correct? The nitrogen has four bonds to it, which cannot be possible if the nitrogen is neutral. And the boron has 4 bonds to it, which isn't possible either with neutral boron. In any case, the bottom portion of the molecule isn't relevant for the carbon spectrum. In a symmetrical molecule, the corresponding carbons on either side of the plane of symmetry will be equivalent.

    Thanks, it's a bit of a strange structure but I copied it from my lab report so I think it's correct. If I had to put the environments in increasing order of chemical shift would it be a, b, c, e and then d? I'm finding it really hard to distinguish between the peaks.
    Offline

    20
    ReputationRep:
    (Original post by VioletPhillippo)
    Thanks, it's a bit of a strange structure but I copied it from my lab report so I think it's correct. If I had to put the environments in increasing order of chemical shift would it be a, b, c, e and then d? I'm finding it really hard to distinguish between the peaks.
    If I were to make a rough prediction I'd expect d to be the most downfield, then b, then c, then e, then a. But with very similar carbons like that it's often not useful to try and predict which will be more downfield or upfield as the actual spectrum can often be different to what you thought intuitive. With a molecule like that I would personally run HMBC and HSQC spectra to really get a better idea of which carbon is which.
    • Community Assistant
    • Study Helper
    Offline

    17
    ReputationRep:
    Community Assistant
    Study Helper
    There is nothing wrong with the structure. The nitrogen has donated its lone pair into the electron deficient boron, making a dative covalent bond.

    Boron is more electropositive and will appear to induce charge towards the ring affecting the closest carbon atoms the most.
    Offline

    11
    ReputationRep:
    (Original post by VioletPhillippo)
    Hi,

    There are 5 peaks in the carbon NMR spectrum for this molecule, indicating that there are 5 carbon environments. However, I'm really struggling to determine which environments are equivalent.

    Any help would be really appreciated.

    Thanks
    In terms of the structure, Nitrogen ought to have a formal positive charge and Boron ought to have a formal negative charge
    Offline

    20
    ReputationRep:
    (Original post by MexicanKeith)
    In terms of the structure, Nitrogen ought to have a formal positive charge and Boron ought to have a formal negative charge
    Indeed, thats what I alluded to by questioning the structure. As far as I’m aware the only time a dative covalent bond can be represented without including formal charges is when the bond is drawn as an arrow. But when it’s a solid bond, as the structure in the document, formal charges should be included.
 
 
 

University open days

  • Sheffield Hallam University
    City Campus Postgraduate
    Wed, 17 Oct '18
  • Staffordshire University
    Nursing and Midwifery Undergraduate
    Wed, 17 Oct '18
  • Teesside University
    Undergraduate open day Undergraduate
    Wed, 17 Oct '18
Poll
If a uni gives me an unconditional offer they....

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.