Hey there! Sign in to join this conversationNew here? Join for free
x Turn on thread page Beta
    • Thread Starter
    Offline

    0
    ReputationRep:
    The question is:

    Predict the number of peaks, the relative peak areas and the chemical shift in a proton NMR spectrum.

    The compound in question is C6H5CH2CH3

    From my knowledge I know there is a benzene ring there with a alkyl chain, I thought there will be 5 carbon environments, one on the CH3 and CH2 side chain and then 3 more on the rest of the benzene but in the book there are 3 peaks for the compound, can someone please explain this?

    Thank you.
    • Thread Starter
    Offline

    0
    ReputationRep:
    can someone please help me with this?
    • Community Assistant
    • Study Helper
    Offline

    17
    ReputationRep:
    Community Assistant
    Study Helper
    (Original post by Xenilbohl)
    The question is:

    Predict the number of peaks, the relative peak areas and the chemical shift in a proton NMR spectrum.

    The compound in question is C6H5CH2CH3

    From my knowledge I know there is a benzene ring there with a alkyl chain, I thought there will be 5 carbon environments, one on the CH3 and CH2 side chain and then 3 more on the rest of the benzene but in the book there are 3 peaks for the compound, can someone please explain this?

    Thank you.
    They are probably approximating the aromatic environments into one ..
    • Thread Starter
    Offline

    0
    ReputationRep:
    (Original post by charco)
    They are probably approximating the aromatic environments into one ..
    By that do you mean that the carbon environments are slightly different therefore they a lumping them together - but they don't do this with carbon-13 NMR spectroscopy, so why with proton NMR spectroscopy?
    Offline

    12
    ReputationRep:
    My guess would be because carbon-13 NMR is usually proton decoupled. This means the instrument is set to remove the peak splitting you'd get between carbons and protons (you'd see too much splitting for it to be useful) so you can see all the separate peaks clearly. For proton NMR the 3 aromatic environments are very similar so with the splitting you get a mess of peaks with very similar chemical shifts. So it's easier to count them as one peak with a relative integration of 5.
    Offline

    12
    ReputationRep:
    When I imagine the structure I imagine there should be 6 peaks - 4 associated with the C atoms in the benzene ring (integration ratio of 1 each) and one each for the two in the alkyl chain (integration ratio of 2 for CH2 and 3 for CH3).

    Although Kyri's argument about the peaks being almost too close together to tell them apart would explain your answer of 3 where the benzene peaks get a ratio of 5. BUT this kind of approximate thinking is generally not great, for instance it won't work with 13C NMR instead of 1H.
    Offline

    18
    As above really, although there are five distinct proton environments this may look like three due to the similarity of the aromatic signals in the spectrum.
    Offline

    12
    ReputationRep:
    (Original post by Big-Daddy)
    When I imagine the structure I imagine there should be 6 peaks - 4 associated with the C atoms in the benzene ring (integration ratio of 1 each) and one each for the two in the alkyl chain (integration ratio of 2 for CH2 and 3 for CH3).

    Although Kyri's argument about the peaks being almost too close together to tell them apart would explain your answer of 3 where the benzene peaks get a ratio of 5. BUT this kind of approximate thinking is generally not great, for instance it won't work with 13C NMR instead of 1H.
    I think you've misread the structure. It's ethyl benzene so there are 5 protons in the benzene ring so 3 peaks. do agree with you though that this sort of approximation isn't great. I don't agree with questions relying on this sort of thing being in A level exams which should only be testing your understanding of the theory but it's often what you have to do in real life as that's how they'd appear on an actual spectrum.
    • Thread Starter
    Offline

    0
    ReputationRep:
    Thank you all for your help
    Offline

    12
    ReputationRep:
    (Original post by Kyri)
    I think you've misread the structure. It's ethyl benzene so there are 5 protons in the benzene ring so 3 peaks. do agree with you though that this sort of approximation isn't great. I don't agree with questions relying on this sort of thing being in A level exams which should only be testing your understanding of the theory but it's often what you have to do in real life as that's how they'd appear on an actual spectrum.
    I was making the point that the 5 benzene H peaks aren't equivalent. They're only even close in 1H spectra.
    Offline

    16
    ReputationRep:
    (Original post by Big-Daddy)
    I was making the point that the 5 benzene H peaks aren't equivalent. They're only even close in 1H spectra.
    I think you probably just wrote the wrong thing by accident... but there would not be 5 peaks. A phenyl group in a simple molecule such as this, with no chiral centres or fixed stereochemistry (eg alkene) will only ever give you 3 peaks at best. However, in real life, it is not at all uncommon for aromatic environements to overlap and all you can conclude is that you have a multiplet of integral 5, which is what I presume is the case for this molecule.

    Once you start adding multiple aromatic groups the whole thing becomes a mess. As someone who's spent the last few days analysing spectra from my project, I can tell you it's a real ball ache!
    Offline

    12
    ReputationRep:
    (Original post by illusionz)
    I think you probably just wrote the wrong thing by accident... but there would not be 5 peaks. A phenyl group in a simple molecule such as this, with no chiral centres or fixed stereochemistry (eg alkene) will only ever give you 3 peaks at best. However, in real life, it is not at all uncommon for aromatic environements to overlap and all you can conclude is that you have a multiplet of integral 5, which is what I presume is the case for this molecule.
    No you're right, I wrote the wrong thing (it was 2 AM) Now I look back on it, though, it looks like 4 benzene peaks (6 total peaks)?

    (Original post by illusionz)
    Once you start adding multiple aromatic groups the whole thing becomes a mess. As someone who's spent the last few days analysing spectra from my project, I can tell you it's a real ball ache!
    I can imagine!
    Offline

    16
    ReputationRep:
    (Original post by Big-Daddy)
    No you're right, I wrote the wrong thing (it was 2 AM) Now I look back on it, though, it looks like 4 benzene peaks (6 total peaks)?
    4 carbon environments but one of these doesn't have any hydrogens, so only 3 peaks in proton NMR. There would be 4 in a carbon NMR.
    Offline

    12
    ReputationRep:
    (Original post by illusionz)
    4 carbon environments but one of these doesn't have any hydrogens, so only 3 peaks in proton NMR. There would be 4 in a carbon NMR.
    OHH I see, that was stupid. For some reason I was seeing the phenyl as a cyclohexyl in my head, completely forgot about the double bonds. Yeah you're right then, 3 it is - adding up to 5 peaks in total for 1H.
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: April 14, 2013
Poll
Do I go to The Streets tomorrow night?

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.