Electron withdrawing groups, effect aromatic chemical shifts

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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