13C NMR Question

Hey.

I'm looking at a 13C NMR spectrum for this structure:


I used a simulator to see what it should look like, and got this:


As you can see, the peak around 135 is identified as the C highlighted yellow on the structure.. But I'm confused as to why this is, since I thought C-Cl carbons had a chemical shift of 30-70 ppm?

Any insight would be helpful. Thanks!

It is not just a C-Cl carbon, it is also a carbon in a benzene ring environment.

Right... Why does that have more influence than the C-Cl though?

Chlorine is electronegative, so is inductively withdrawing in a C-Cl bond. That's why a C-Cl is fairly significantly deshielded for a non aromatic compound.

The reason aromatic environments have such high shifts is because of how the delocalised π electrons behave - they generate a pretty strong magnetic field. It turns out that the chlorine doesn't really effect this delocalisation - it only withdraws through the σ bond - so the aromatic C-Cl carbon is only slightly more deshielded.

This changes the bonding between the carbon and chlorine atoms. You end up with a much shorter and stronger bond because of it's increased bond order. I think the bond order of C-Cl in aryl chlorides is 1.5 though I haven't checked in a while, it's only 1 in alkyl chlorides though. While chlorine does draw the electrons closer to itself in alkyl chlorides, this effect is diminished by it's donation of a lone pair to the ring.

I haven't looked at the physics behind NMR in quite a while so please bear with me here. I imagine the C-Cl peak's intensity is so low because it's not really a C-Cl bond at all. Like I said above, it's essentially a hybrid (just to clarify, I mean a cross between the two here not anything to do with orbital hybridisation) of the sigma and pi bonds giving a bond order of about 1.5. The precessions of nuclei in molecules are affected by the electric fields between the nuclei and electrons as well as by the magnetic field applied, which is one of the reasons why altering the bonding gives different shift values. You've also got things like coupling constants and all that jazz going on too. Consequently, the weird half sigma-half pi bond will give slightly different shift values and with different intensities to the normal alkyl chloride bonds. You tend to find that the C-Cl peaks are still there but with much lower intensities than normal in aryl chloride C13 NMR spectra. It would make sense if the aromatic peaks had higher intensities too due to the increased number of delocalised electrons. I guess the simulation just cut out the puny C-Cl peak.