Charge-delocalized anions

The transition states are composed of loose ion pairs in so far as they involve a charge-delocalized anion, thereby enhancing polarity compared with the ground states (in which the ion pairs are tighter), because of an increase in anionic dissociation as the more bulky product anion is formed. As a consequence, specific micro-wave effects, directly connected to polarity enhancement, should depend on the structure of reactive ion pairs in the GS ... 

Anionic SN2 Reactions Involving Charge-delocalized Anions... 

Pyridine is attacked by alkyllithium reagents at its most electropositive atom (C2) to give the charge delocalized anion 87112). In a similar manner the heavier heterobenzenes are attacked at their most electropositive atoms (the heteroatoms) to give heterocyclohexadienides 88 113). 

Methylpyrazine in the presence of excess potassium amide at — 40° undergoes deprotonation to give the charge delocalized anion (10) (665). 

X 10 ]. The occurrence of this absorption band is connected with the presence of a charge-delocalized anion, which is the actual active centre for polymerization, viz. 

The fluorenyl salts seem to be particularly susceptible to the formation of solvent separated pairs. Their formation is favoured by the presence of a large charge-delocalized anion plus a small compact cation which can be strongly solvated (or presumably vice-versa). The solvating agent should be highly polar, preferably small in size, or with the ability to offer multiple coordination as with the polyglycol-dimethylethers. With polystyryl salts, the formation of solvent-separated ion-pairs is less extensive. The absorption spectra are not particularly conclusive because the absorption... 

Compare electrostatic potential maps for planar and pyramidal forms of 2-methyl-2-propyl anion. For which is the negative charge more delocalized Is this the lower-energy structure For this case, does charge delocalization lead to stabilization Explain. 

Phenol has different chemical properties from those of typical alcohols. Display the electrostatic potential map for phenol. Does this suggest that phenol is likely to be a stronger or weaker acid than any of the compounds discussed above Compare the electrostatic potential map for 4-nitrophenol to that for phenol. What effect does substitution by nitro have on acid strength Explain your result by considering charge delocalization in the conjugate base. Draw all reasonable Lewis structures for phenoxide anion and for 4-nitrophenoxide anion. Which is more delocalized Is this consistent with experimental pKa s ... 

According to Branchini et al. (2004), luciferase modulates the emission color by controlling the resonance-based charge delocalization of the anionic keto-form of oxyluciferin in the excited state. They proposed the structure C5 as the yellow-green light emitter, and the structure C6 as the red light emitter. 

More recently considered candidates are large molecular anions with delocalized anionic charge, which offer low lattice energies, relatively small ion-ion interaction, and hence sufficient solubility and relatively large conductivity. Delocalization of the charge is achieved by electron-with drawing substituents such as -F or - CF3. Furthermore, these anions show a good electrochemical stability to oxidation. In contrast to Lewis acid-based salts they are chemically more stable with various solvents and often also show excellent thermal stability. 

Noncoordination anions with extensive charge delocalization have been very suc-cesful in enhancing the performance of dry polymer electrolytes. Examples of these are given in Table 1. 

The most stabilized anion is PO3 which has three equivalent charge delocalizing oxygens. The other three anions have only two such oxygens. The stability differences for these three anions are due to the electron-withdrawing effect of the OH group. The stability increases as the number of OH group increases from zero to two. 

The TS for anionic SN2 reactions involves loose ion pairs as in a charge delocalized (soft) anion. On the another hand, the GS could involve a neutral electrophile and either tight or loose ion pairs depending on the anion structure (hard or soft) (Eq. 32). 

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