Electronic effects, inductive conjugation

There are a number of structural features in R—H that promote the removal of H by bases through making it more acidic, and also features that serve to stabilise the resultant carbanion, Re in some cases both effects are promoted by the same feature. The main features that serve to stabilise carbanions are (cf. factors that serve to stabilise carboca-tions, p. 104) (a) increase in s character at the carbanion carbon, (b) electron-withdrawing inductive effects, (c) conjugation of the carbanion lone pair with a polarised multiple bond, and (d) aromat-isation. 

The p Ta of cyclohexylamine (10.6) is typical of an aliphatic amine (methylamine also has p/fa 10.6). Aniline is a much weaker base (the smaller the p Ta, the weaker the base) than cyclohexylamine. This is a combination of an inductive effect and a resonance effect, which reinforce each other. The phenyl ring provides an electron-withdrawing inductive effect that destabilizes the conjugate acid. More important, though, is the resonance effect that stabilizes the uncharged amine. 

The electron-attracting inductive effect of oxygen is much greater than that of sulfur on the other hand, the conjugative effect is much smaller in furan than in thiophene, as shown by the larger degree of bond fixation observed in the former. Nevertheless, furan is much... 

The cyano group of jp-cyanoaniline exerts an electron-withdrawing inductive effect and can delocalize the electron pair of the base by resonance. On the basis of reasoning identical to that presented for jp-nitrophenol, the conjugate acid of / -cyanoaniline should be a stronger acid than the conjugate acid of aniline. Indeed, this is the case the pKais 1.74. 

The halogens deactivate the ring towards electrophilic attack but direct ortho and para. The only way this makes sense is if there are two opposing effects—electron donation by conjugation and electron withdrawal by induction. The halogen has three lone pairs, one of which may conjugate with the ring... 

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... 

Frenking and co-workers proposed 5 to represent a true trivalent silylium ion in condensed phases. [92] For this cation, the effect of inductive electron withdrawal and conjugative backdonation by substituents Y in Y3Si+ is nicely illustrated by calculated charges (see Table 3) The total charge at the Si atom is... 

Alkyl groups are substituents with a positive induction effect which increases electron density in a series ortho- > meta > para-positions. The secondary effect of conjugation imparts an additional negative charge in the ortho- and para-positions. The resulting action of both of these effects is similar to that of a positive mesomeric effect. 

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