Delocalization, of negative charges

Compare electrostatic potential maps for the two dienes. Does the more stable isomer show greater delocalization of negative charge ... 

It is commonly supposed that the stabilization of carbanions by +R groups is due to delocalization of negative charge from the carbanionic carbon. In the case of NO2, resonance structures as in 24 are written184 ... 

The barriers to acetyl group rotation in 84a to d are at least as high as in the acetylacetonate anion (Table 18), indicating a considerable delocalization of negative charge into Ac—Q—Ac. As mentioned later (Table 19), the twist angle in 84a is 73°. ... 

The 2- and 4-haIopyrimidines are even more reactive, and snbstitnte at room temperature. This is becanse of the improved delocalization of negative charge in the addition anion. 5-Halopyrimidines are... 

Delocalization of negative charge into carbonyl group is not possible in peroxyacetate ion. 

C, these p values are 1-2 and 1-6 respectively . Our interpretation of these nearequal p-values is that there is considerable (>50%) delocalization of negative charge into the carbonyl centre in the activated complexes of reaction (159). 

The same factors (structural rearrangement on protonation and delocalization of negative charge) that produce the low proton basicity of polynuclear anions are probably also responsible for the slow rate at which protonation equilibria are established with polynuclear systems (see Table 2). If [HOs(CO) Os(CO)J is compared with the mononuclear Os anions in the previous two subsections, it is clear that the second 05(00)4 unit has decreased the rate of protonation as well as the proton basicity. 

Fig. 2 Delocalization of negative charge in the phenate ion where (a) methyl and (b) nitro functional groups are present.

The Marcus intrinsic barriers for deprotonation of carbon acids to form enolates that are stabilized by resonance delocalization of negative charge from carbon to oxygen are larger than for deprotonation of carbon acids to form carbanions where the charge is localized mainly at carbon. 

The increase in acid strength with increase in the number of O atoms attached to atom E is generally attributed to the greater possibility in the conjugate base of delocalization of negative charge onto the O atoms. 

Better delocalization of negative charge makes this less basic and less nucleophilic. 

Figure 11.37 Delocalization of negative charges (via mobile hydrogen ions) over different phenol groups of p-cresol tetramers of novolac, following deprotonation of one of the phenolic groups.

Fig. 13.4.5. Bimolecular delocalization of negative charge, illustrated for TTFB (12).

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