Benzene attached protons, chemical shift values

Table 6 lists proton chemical shifts for a selection of monosubstituted furans including some with a metal or other less common heteroatom attached to the ring, but perhaps it should be noted that values are not closely comparable unless obtained in closely comparable conditions. Solvents can have a substantial effect upon chemical shifts (66CR(B)(263)1227>, especially benzene (72JA8854). Table 7 lists some ring interproton coupling constants and Table 8 some shift values for disubstituted furans. Polysubstituted furans are covered in the reviews and compilations mentioned above. 

Fig. 3. Plot of chemical shifts (in p.p.m. from benzene), of the ring carbon atoms of heterocyclics, against the chemical shift of the attached proton (in t values). (From Page et...

Figure 3.20 clearly shows that there are some types of protons whose chemical shifts are not easily explained by simple considerations of the electronegativity of the attached groups. For instance, consider the protons of benzene and other aromatic systems. Aryl protons generally have a chemical shift as large as that of the proton of chloroform Alkenes, alkynes, and aldehydes also have protons whose resonance values are not in line with the expected magnitudes of any electron-withdrawing or hybridization effects. In each of these cases, the anomalous shift is due to the presence of an unsaturated system (one with r electrons) in the vicinity of the proton in question. 

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