Benzene and proton in 1H NMR

Question

The chemical shift in benzene is around $\pu{7.27 ppm}$, which is lower than, for example, aldehydes. However, the magnetic field due to the pi electrons are in line with the applied magnetic field $B_0$, resulting in $B_{eff}>B_0$. Whereas in aldehydes, the hydrogen is simply "less shielded". So shouldn't the $\ce H$ atoms in benzene (or alkynes etc.) have even greater chemical shifts than species that are deshielded "normally" through electron withdrawing substituents?

Also, somewhat of a side question: What kind of chemical shift would we see for a proton ($\ce{H+}$) in a proton NMR, since it is completely deshielded? Could this theoretically be measured?

(Following the previous logic, shouldn't benzene have a higher chemical shift than a naked proton?)

Answer 1

The aldehyde is both deshielded by reduction in electron density about the carbon it is bonded to through electron withdrawal and by the magnetic anisotropy of the double bond. Although a benzylic hydrogen is slightly more deshielded than the hydrogen attached to a two electron pi bond like a carbonyl or an alkene, it does not also have electron density withdrawn from its adjacent carbon. Generally electron withdrawal through resonance is more important than inductive electron withdrawal, so the difference in chemical shift between an aldehyde and an alkene will be greater than the difference in chemical shift between the proton proton geminal to a halogen in a halogen-substituted alkene and the proton in its unsubstituted analogue. All that is to say the additive effects of resonance electron withdrawal and magnetic anisotropy make an aldehyde proton more shifted than a benzylic proton.