H-NMR splitting

  • #1
210
0
Hello.
I remember a problem I had with drawing a splitting tree for nitrotoluene. The coupling constants varied, you know, para, meta, ortho, they were all different. My teacher told us to assume certain values for each. What I don't understand is how a proton in nitrotoluene (or even toluene, whatever) is split by these substituents that are so far away. I read that protons can only be split by a neighbouring carbon, or, (if there's a double bond) the neighbouring carbon's neighbour, mr. carbon 3! (mr. carbon 1 is the carbon whose proton is being split). I hope you're not too confused. And so what's the point of having a coupling constant for something that's ortho, something that's more than 3 bonds away (2 sigma bonds for pi and a sigma bond)? There's something obviously wrong with the way I'm looking at this, someone PLEASE enlighten me.
 

Answers and Replies

  • #2
You can have coupling occur at a distance of 4 or five bonds. In an aliphatic system the coupling constant further than 3 bonds (>J3) is often small and unless you're using a very powerful NMR they won't be seen. In unsaturated systems, however, the coupling constant is greater and can easily be seen on most modern NMRs.
 
  • #3
210
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Thanks, that was helpful.
 

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