# NMR spectroscopy ques

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#1

I really dislike this topic because I never know how to answer te questions

I approached it like this
1) 5 carbon environments
2) mr of 106
Then I got stuck

Can someone guide me through this question
Thanks

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4 years ago
#2
(Original post by Lilly1234567890)

I really dislike this topic because I never know how to answer te questions

I approached it like this
1) 5 carbon environments
2) mr of 106
Then I got stuck

Can someone guide me through this question
Thanks

Posted from TSR Mobile
Use the mass spectrum to get the molecular ion and hence the relative mass = 106

You know that its aromatic, so it must have at least 6 carbon atoms = 72 mass units. Hence there are 106 - 72 = 34 mass units left to find. There will be a few hydrogen atoms floating around so that leaves only 2 carbon atoms.
Mass of carbon atoms = 8 x 12 = 96
So there are 10 hydrogen atoms.

There are only two possibilities:

dimethylbenzene
ethylbenzene

The dimethylbenzene could be:
1,2-dimethylbenzene
1,3-dimethylbenzene
1,4-dimethylbenzene

That means in total there are four possible structures.

Now we consider how many different carbon environments each structure would have.

ethylbenzene = 6 different environments
1,2-dimethylbenzene = 4 different environments
1,3-dimethylbenzene = 5 different environments
1,4-dimethylbenzene = 3 different environments

so you look at the number of signals in the 13C NMR and count them!

There are five.

1,3-dimethylbenzene (otherwise known as meta-xylene)
0
#3
(Original post by charco)
Use the mass spectrum to get the molecular ion and hence the relative mass = 106

You know that its aromatic, so it must have at least 6 carbon atoms = 72 mass units. Hence there are 106 - 72 = 34 mass units left to find. There will be a few hydrogen atoms floating around so that leaves only 2 carbon atoms.
Mass of carbon atoms = 8 x 12 = 96
So there are 10 hydrogen atoms.

There are only two possibilities:

dimethylbenzene
ethylbenzene

The dimethylbenzene could be:
1,2-dimethylbenzene
1,3-dimethylbenzene
1,4-dimethylbenzene

That means in total there are four possible structures.

Now we consider how many different carbon environments each structure would have.

ethylbenzene = 6 different environments
1,2-dimethylbenzene = 4 different environments
1,3-dimethylbenzene = 5 different environments
1,4-dimethylbenzene = 3 different environments

so you look at the number of signals in the 13C NMR and count them!

There are five.

1,3-dimethylbenzene (otherwise known as meta-xylene)
Oh waw thanks !

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0
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