Rudeboy999
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im having trouble suggesting suitable structures from nmr. i dont understand how their can be several c-c peaks but they represent ch3 or ch2. how is everyone revising spectroscopy.
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indifferencepersonified
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Are you talking about proton nmr? If so, a CH3 will split the neighbouring protons in a quartet on the spectrum, and a CH2 group will split any neighbouring protons into a triplet. So, if you have a section of a spectrum with 2 peaks, one peak is split into a quartet, and the other peak is split into a triplet; it is likely you will have an ethyl group in the compund (obviously, remember to look at the value of the integration trace as well!).
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Charzhino
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give us some examples so when can see clearly
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gingerbreadman85
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(Original post by Rudeboy999)
im having trouble suggesting suitable structures from nmr. i dont understand how their can be several c-c peaks but they represent ch3 or ch2. how is everyone revising spectroscopy.
You'll never get a C-C peak on an NMR specta. C13 is spin active but has a very low relative abundance... C12 is not spin active so the chance of having 2 adjacent spin active carbons is negligible, therefore it's not a signal that will be visible in your specta. For the same reason, H1 NMR doesnt pick up couplings between H and C.

The signals in your spectra represent the interaction of an excited H atom with any other spin active nucleus within 3 bonds of it. For A-level you only really have to worry about H-H coupling.

The rule is you will get a signal for each H in unique environments (i.e. not equivilant to each other), each signal split into n+1 peaks, where n is the number of nearby H.

Thus something like CH3CH2CH3 will have 2 signals, one for the H in the CH3 (which are equivilant to each other) and one for the CH2, the integral of these peaks will give a ratio 3:1. The CH3 peaks will be split by the 2 CH2 H atoms, giving a triplet (2+1=3) and the CH2 will be split by the 6 CH3 H atoms, giving a multiplet.


It gets more complicated when a H atom has 2 or more different H environments to couple with, however i dont think you need to know in that level of detail for A-level.

EDIT - Double post..... oops, not sure what happened there!
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Rudeboy999
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how do we know which of the peaks corresponds with the ch3 and one with the the ch2
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gingerbreadman85
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The integration and the multiplicity
(Original post by gingerbreadman85)
The rule is you will get a signal for each H in unique environments (i.e. not equivilant to each other), each signal split into n+1 peaks, where n is the number of nearby H.

Thus something like CH3CH2CH3 will have 2 signals, one for the H in the CH3 (which are equivilant to each other) and one for the CH2, the integral of these peaks will give a ratio 3:1. The CH3 peaks will be split by the 2 CH2 H atoms, giving a triplet (2+1=3) and the CH2 will be split by the 6 CH3 H atoms, giving a multiplet.
Integration should give you a peak ratio that corresponds to the ratio of H atoms in that environment.
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Vxckyy_
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In carbon NMR if there's 2 C-C peaks does that automatically mean there's 2Carbon bonds in the structure?
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charco
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(Original post by Vxckyy_)
In carbon NMR if there's 2 C-C peaks does that automatically mean there's 2Carbon bonds in the structure?
You are not detecting bonds you are detecting the presence of atoms in the molecule. If you have two peaks in carbon-13 NMR then you have two carbon atoms in different environments.
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Vxckyy_
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(Original post by charco)
You are not detecting bonds you are detecting the presence of atoms in the molecule. If you have two peaks in carbon-13 NMR then you have two carbon atoms in different environments.
I mean that is on the CNmr graph of there are 2 peaks that are accountable for the C-C bond does that mean there are 2 carbons attached?? I'm confused.
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charco
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(Original post by Vxckyy_)
I mean that is on the CNmr graph of there are 2 peaks that are accountable for the C-C bond does that mean there are 2 carbons attached?? I'm confused.
1 peak corresponds to 1 carbon atom in the structure.
2 peaks correspond to 2 carbon atoms in the structure (in different environments)
etc
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