A level chemistry benzene ring NMR help Watch

soph99jk
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I understand that benzene on its own has 1 proton environment because they're all identical. But why for example, in benzoic acid or a molecule where a benzene ring is attached, does benzene only have 1 proton environment??
because wouldn't the carbon in the benzene ring that is attached to for example the C=O group have a different proton environment than the other carbons in the benzene ring that are attached to the adjacent carbons in the ring? Because there would be adjacent protons on the adjacent carbons in the benzene ring, but for the carbons in the benzene ring attached to the C=O bond, they would be bonded to a carbon with no protons, and a carbon adjacent to it in the benzene ring?

so for this example i would expect 3 different proton environments in the benzene ring of benzoic acid but why in markschemes and places are there only one peak caused by the benzene in a complex molecule??

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Ash8991
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(Original post by soph99jk)
I understand that benzene on its own has 1 proton environment because they're all identical. But why for example, in benzoic acid or a molecule where a benzene ring is attached, does benzene only have 1 proton environment??
because wouldn't the carbon in the benzene ring that is attached to for example the C=O group have a different proton environment than the other carbons in the benzene ring that are attached to the adjacent carbons in the ring? Because there would be adjacent protons on the adjacent carbons in the benzene ring, but for the carbons in the benzene ring attached to the C=O bond, they would be bonded to a carbon with no protons, and a carbon adjacent to it in the benzene ring?

so for this example i would expect 3 different proton environments in the benzene ring of benzoic acid but why in markschemes and places are there only one peak caused by the benzene in a complex molecule??

thanks
Forgive me if I'm entirely wrong but would the carbon in the benzene ring attached to the c=o group, not have a proton environment? I don't think it has a H bonded to it
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Pigster
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The simplification that you will use at A level is that all the H atoms attached to the benzene ring will have similar shift values, so are counted as all in one environment. The problem we are simplifying is the issue of the rather splitting patterns produced by phenyl H's, which are considered too hard to interpret at A level (and rightly so).

Ash8991 is correct, the phenyl carbon with the COOH attached does not have an H atom attached.

There is also a chance that you are confusing H-NMR and C13-NMR.
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soph99jk
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(Original post by Pigster)
The simplification that you will use at A level is that all the H atoms attached to the benzene ring will have similar shift values, so are counted as all in one environment. The problem we are simplifying is the issue of the rather splitting patterns produced by phenyl H's, which are considered too hard to interpret at A level (and rightly so).

Ash8991 is correct, the phenyl carbon with the COOH attached does not have an H atom attached.

There is also a chance that you are confusing H-NMR and C13-NMR.
But if we say the carbon attached to the C=O group is carbon 1 in the ring, which wouldn't have any hydrogens as it is substitued by the C=O. wouldnt carbons 2 and 6 be in the same proton environment and carbons 3 and 5 in another environment and the carbon 4 in its own proton environment?
because carbons 2 and 6 are each bonded to a carbon with no hydrogens (carbon1) and then an adjacent carbon in the benzene ring which has a hydrogen. therefore it would have a different proton environment to carbons 3 and 5, which are bonded to a hydrogen either side (from carbons in the benzene ring).

I understand this may not be covered in detail for a level but I've come across past paper questions where sometimes the benzene ring in a structure is just shown as one peak. whereas I've also seen a question where a disubstituted benzene ring was shown to have 6 proton environments (there were 4 proton environments outside the ring) so I'm presuming they included the 2 inside the ring? i understand they have similar chemical shift values but just wanted to clarify that there would be more than one proton environment in the ring? because they are attached to different groups/different number of adjacent hydrogens.
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Pigster
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(Original post by soph99jk)
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There are multiple environments and these will be split in a way that will not look like splitting you are used to. It will probably appear as very many peaks of varying heights, which may look like a whole bunch of singlets/multiplets, giving you the impression that there are LOADS of environments.

At A level it is much easier to just assume that you have a number of H atoms in a benzene environment and then use the integration values and/or the empirical/molecule formula/e to work out how many H atoms you have and therefore how many H atoms in a benzene environment and therefore how many benzene rings you have.

The best thing to do is have a look at a bunch of NMR spectra so you get to know what you should expect to see. Familiarisation is key. These two will help:

http://sdbs.db.aist.go.jp/sdbs/cgi-bin/cre_index.cgi
http://www.nmrdb.org/new_predictor/i...html?v=v2.81.3
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