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proton NMR

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.
(edited 11 years ago)
Reply 1
Avatar for Xenilbohl
Xenilbohl
OP
can someone please help me with this?
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 ..
Reply 3
Avatar for Xenilbohl
Xenilbohl
OP
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?
Reply 4
Avatar for Kyri
Kyri
15
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.
Reply 5
Avatar for Big-Daddy
Big-Daddy
13
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.
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.
Reply 7
Avatar for Kyri
Kyri
15
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.
Reply 8
Avatar for Xenilbohl
Xenilbohl
OP
Thank you all for your help :wink:
Reply 9
Avatar for Big-Daddy
Big-Daddy
13
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.
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!
(edited 11 years ago)
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) :tongue: 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!
Original post by Big-Daddy
No you're right, I wrote the wrong thing (it was 2 AM) :tongue: 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.
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. :tongue: Yeah you're right then, 3 it is - adding up to 5 peaks in total for 1H.

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