Electron density withdrawal, from electronegativity

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. 

The electronegative O atoms of the carboxylic acid group withdraw electrons from the whole ring, thereby reducing its overall electron density. Moreover, resonance preferentially removes electrons from the ortho and para positions. To focus on the essentials, only the lone pairs of electrons involved in resonance are shown ... 

The strength of a Lewis acid is a measure of its ability to attract a pair of electrons on a molecule that is behaving as a Lewis base. Fluorine is more electronegative than chlorine, so it appears that three fluorine atoms should withdraw electron density from the boron atom, leaving it more positive. This would also happen to some extent when the peripheral atoms are chlorine, but chlorine is less electronegative than fluorine. On this basis, we would expect BF3 to be a stronger Lewis acid. However, in the BF3 molecule, the boron atom uses sp2 hybrid orbitals, which leaves one empty 2p orbital that is perpendicular to the plane of the molecule. The fluorine atoms have filled 2p orbitals that can overlap with the empty 2p orbital on the boron atom to give some double bond character to the B-F bonds. 

The chemical shift is determined by the extent to which a proton is deshielded by the groups to which it is attached. The more a proton is shielded by the electron density around it, the lower its 3 value. If a proton is attached to a system that withdraws electrons from its environment such as an electronegative group or to a group which affects its environment by creating a field opposing the applied... 

As with pyridine, not only does the electronegative nitrogen atom withdraw electron density from the ring, but under the acidic conditions of many electrophilic reactions the azole nitrogen is protonated. The azolium cation is relatively inert to further attack by a positively charged electrophile. 

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