Benzene is a beautiful molecule. It really is. In terms of bonding structure, it’s especially beautiful – it’s aromatic. Each carbon in the benzene molecule is sp2 bonded to another carbon and a hydrogen (or a heteroatom, if one so pleases, but we will discuss only hydrogen for reasons to be seen later). This forms the sigma bond network in the benzene ring. One remaining p orbital remains open, and it turns out that all the remaining p orbitals bond form a separate pi bond network. The remaining pi electrons are delocalized through this pi molecular orbital that circulates around the benzene ring. What I just said is illustrated in the following diagram:
Not only does this delocalization afford special stability, it means any and all pi electrons can circulate through the pi molecular orbital. They can move about back and forth – kinda like electrons in a closed circuit. Now it gets interesting. Physics 7B time!
Suppose you apply an external magnetic field to this closed aromatic circuit. Because this ring is closed, there’s a non-negligible magnetic flux. As the external B field increases, the flux increases, which induces a voltage (potential difference) across the aromatic ring, producing a current across the ring. The direction of the current can be given by Lenz’s Law, which states that the induced current will produce an induced magnetic flux opposing the source flux. For instance, if the external B field points up, the induced current will flow CW to produce a magnetic field that points down in the ring.
Why does any of this matter? Because NMR spectroscopy works by introducing an external magnetic field across a molecule! In the presence of an applied magnetic field and electromagnetic radiation, nuclei will resonate at a certain frequency. This frequency is determined by the exposure of the nuclei to the magnetic field. When electron donating functional groups are located near the nuclei, the extra electron density “shields” the nuclei from the field; with electron withdrawing groups the nuclei are deshielded and exposed to the field. These are quantified in chemical shifts – the larger the shift, the greater exposure to the magnetic field.
In benzene however, the mechanism works differently. By the right hand rule, an induced B field that opposes the external field will come around and point in the direction of the external field outside the ring, where the hydrogens lie. Since we talk about 1H NMR in organic chemistry, this means that the induced current increases the hydrogen nuclei’s exposure to magnetic fields – entailing a larger chemical shift! And that is why benzene has that weird 6.0-8.5ppm shift, greater than alkenes and almost comparable to aldehyde proton shifts.
This blew my mind when I first read it (never was taught this in my organic chemistry class)… so I thought I’d share for the 0.1 readers who visit my blog. 😀