Help with Isomer Identification Using NMR of Xylenes

[gravenewworld]

I was wondering if someone could please help me with my lab. We are synthesizing NMR shift reagents in order to determine what isomers are present in a mixture of xylenes. I have the NMR and I can't figure out how to determine which isomers are present. If I have the NMR of o,m,p-xylene by themselves I can tell the difference between each one, but I can't figure out how to determine which of the isomers are present in an unknown mixture of them. How can I use the NMR to determine which isomers are present? What should I be looking for?

 

[movies]

The most obvious difference should be in the splitting patterns of the aromatic C-H in the 1H NMR. For example, m-xylene will have a C-H singlet for the aromatic C-H between the two methyl groups, neither of the other isomers will have a singlet in the aromatic region. You should be able to predict what the splitting patterns will be for the ortho and para isomers.

 

[gravenewworld]

Will there be a possibility of signal overlap with the two differnt isomers present for say a p and o xylene mixture?

 

[gravenewworld]

Also will I be able to distinguish the m-xylene proton single you described and the solvent signal since CDCl3 was used? Doesn't CDCl3 have a singlet in the aromatic region?

 

[movies]

Yes, there is a possibility of overlap, but you will probably be able to see the splitting patterns anyway. The singlet from CHCl3 will be at 7.27 ppm. You should be able to recognize it just based on its relative size; that is, the integration won't work out. I don't think that the singlet for m-xylene will be quite that far downfield, probably more like 6.9 ppm.