Electron density withdrawal, from

That leads ns to the following explanation Carbon is more electronegative than phosphorus, but less so than oxygen. In conseqnence, carbon snbstitution on oxygen increases the electron density at oxygen and resnlts in less electron density withdrawal from phosphorus in the case of 0=P(0R)3. That translates into an npfield shift with increased carbon substitution. For 0=PR3, the sitnation is different. Since carbon is more electronegative than phosphorus, increased carbon snbstitntion on phosphorus results in decreased electron density on phosphorus, and thns a downfield shift. 

Because carbon is sp hybridized m chlorobenzene it is more electronegative than the sp hybridized carbon of chlorocyclohexane Consequently the withdrawal of electron density away from carbon by chlorine is less pronounced m aryl halides than m alkyl halides and the molecular dipole moment is smaller... 

The basic reaction remains the same when substituents are present, as illustrated in Figure 4-16. In this example, the aldehyde is an electron-withdrawing group (the electronegative oxygen pulls electron density away from the double bond). The polarity arrow illustrates this electron shift. This shift of electron density speeds up the reaction (a lower temperature is necessary). 

The percentage of the ortho product increases from fluorobenzene to iodo-benzene. We might have expected the amount to decrease as the size of the halide increases because of increased steric hindrance at the ortho position but this is clearly not the case. The series can be explained by the greater inductive effect of the more electronegative atoms (F, Cl) withdrawing electron density mostly from the ortho positions The relative rates follow a U-shaped sequence fluorobenzene nitrates most quickly (but not as fast as benzene), followed by iodo-, then chloro-, and then bromo-benzenes. This is a result of two opposing effects electron donation by conjugation and electron withdrawal by inductive effect... 

Atoms more electronegative than carbon—including N, O, and X—pull electron density away from carbon and thus exhibit an electron-withdrawing inductive effect. 

Figure 4 compares the relative a- and 7c-donor ability of three distinct sulfur donors thiolate, sulfinate, and thioether 17, 23, 24, 26-28). Based on simple charge considerations, both the thiolate and sulfinate donors are anionic, while the thioether is neutral. In terms of a-donor ability, the thiolato sulfur should be stronger than the sulfinate since the oxygen atoms of the latter withdraw significant electron density away from sulfur. The neutral thioether is the poorest a-donor. In terms of 7c-donor ability, only the thiolate sulfur possess the... 

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