Proton NMR of Xylene

[sam26880]

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How many peaks will you see in Xylene below? can someone explain by drawing out the equivalent and non-equivalent protons and explain all the splitting? thanks

 

[Decan]

My best guess, 4

1. There is an internal plane of symmetry so the methyl groups will show a singlet (n+1 rule)

2. The proton attached to the carbon 2 (lets say that the top carbon is carbon 1) is in a different chemical environment so it will show a singlet (n+1 rule)

3. Protons attached to carbon 4 and 6 are equivalent, and will each show a doublet (n+1 rule)

4. Carbon 5 is next to two protons, so the final signal will be a triplet (n+1 rule)

Hope that made sense...someone please correct me if I'm wrong.

 

[Dentalicous1234]

how do you get singlets if the carbon on the ring has a proton, shouldn't that be a doublet because N+1 so 1+1=2...hence doublet?

I guess what exactly will give you singlets? A carbon that does not have a carbon next to it and...?

like for example topscore test 1, ochem #73 it says which compound shows only 2 singlets?
answer:


H3CO---benzene---OCH3

so the two methyls on the end have no neighboring carbons and thus are 0+1 =1 so they show a singlet since they're also equivalent BUT how do the carbons on the ring make a singlet?

 

[Decan]

In this case, you get singlets from the two methyl groups. You look at the adjacent carbon. If it has hydrogens, you use the n+1 rule. In this case, the carbon adjacent to the methyl group does not have any protons so there will not be any splitting (the three hydrogens are equivalent) so you will get a singlet.

Does that make sense?

 

[Dentalicous1234]

In this case, you get singlets from the two methyl groups. You look at the adjacent carbon. If it has hydrogens, you use the n+1 rule. In this case, the carbon adjacent to the methyl group does not have any protons so there will not be any splitting (the three hydrogens are equivalent) so you will get a singlet.

Does that make sense?

Sorry I am still a little lost. We're talking about my example right? In it the methyl groups don't have any carbons attached to them, they have oxygen so that's why it's just a singlet cuz 0+1=1...0 from there being no adjacent carbons, right?

but I don't get how they got another singlet because shouldn't the protons on the benzene ring be triplets? since for carbon on the ring has 2 carbons next to it each having a proton so wait thats a triplet? cuz 2+1=3? I think I'm messing this up more, please help 😕

 

[Decan]

Sorry I am still a little lost. We're talking about my example right? In it the methyl groups don't have any carbons attached to them, they have oxygen so that's why it's just a singlet cuz 0+1=1...0 from there being no adjacent carbons, right?

but I don't get how they got another singlet because shouldn't the protons on the benzene ring be triplets? since for carbon on the ring has 2 carbons next to it each having a proton so wait thats a triplet? cuz 2+1=3? I think I'm messing this up more, please help 😕

I'm sorry! I was still talking about xylene. In your example, you should see two singlets. The two methyl groups produce a singlet but since they are equivalent, you will only see one signal. The benzene hydrogens are also equivalent so they will not split the signal. Only non-equivalent Hs split the NMR singal. For this reason, you will only see two singlets. Hope that helps!

 

[Dentalicous1234]

okay so only use n+1 for nonequivalent protons.

to clairfy on equivalent ones, is it just methyls that don't have any protons next to them like here due to symmetry and ones in a benzene ring...any other good examples?