Anion destabilization

In the cases of cyclopropyl anions, destabilization by the d-d pairing and stabilization in the open forms for the most reactive substrates, and the opposite scenario for the least reactive substances are evident. 

The addition reactions of thiolates to NP are generally followed by the decomposition of the red adducts. A survey on the persistence of the red colors showed that the adduct with thiosuccinic acid was extremely stable over days (118). Some rationalization on the different adduct stabilities has been advanced thus, the presence of electron-rich or electron-withdrawing groups on the thiolate anion destabilize or stabilize the adducts, respectively. The great stability of the thiosuccinic acid adduct with NP has shown to be valuable for the quantitative determination of the nitrosyl complex (34,35). 

Dinitromethide ions add to methyl acrylate in the Michael fashion. When the remaining hydrogen of the methide ions is replaced by fluorine, the Michael addition proceeds with considerable rate enhancement (61). The phenomenon has been attributed to anion destabilization through C-F bond weakening by the more electronegative trigonal carbon atom and the repulsion of the delocalized p electrons by the fluorine lone-pair electrons. The effect can simply be viewed as a consequence of the inharmonious union of the hard fluorine with the soft, delocalized carbanion center. 

One characteristic of these sols is that they can be dried and repeptized. Since the alumina-coated sols provide no soluble silica in solution and no silanol groups are exposed at the surface of particles, no siloxane bonds are formed between particles when the sol is dried to a powder. The counter chloride ions remains on the surface and the powder can be redispersed to a sol in water at pH 3-S (430). The stability of these positive sols has been examined by Katsanis and Matijevic (431). At low pH certain anions show a destabilizing effect, as might be expected from polyvalent anions. Destabilization by leaching of Al + from the surface was observed at very low pH. 

The regioselectivity (a vs. y) is controlledby many factors, such as the nature of the heteroatom, the base, the substituents bonded to the heteroatom, the electrophile, the solvent, and the reaction conditions. It has been proposed, as a rule of thumb, that for a given counter ion, e.g., lithium, anion-destabilizing substituents (e.g., OR, NR2) favor the attack by alkyl halides and protons at the y-position, while carbonyl compounds undergo reaction preferentially at the a-position. The complementary regioselectivity is encountered when anion-stabilizing substituents (e.g., SR, BR2) are bonded to the allyl moiety. A list of n-butyllithium/potassium ferf-butoxide metalated heterosubstituted alkenes is shown in Table 3. 

In contrast, non-stabilized ylides possess an R substituent (Figure 1.1) that is anion-destabilizing/electron-releasing (e.g. alkyl groups). Reactions of these ylides with an aldehyde or ketone are generally under kinetic control and as a consequence give the Z-alkene. A number of factors influence the E/Z ratio of alkenes in reactions of non-stabilized ylides, and these are listed in Table 1.1. 

Two-step HW reaction (isolation and separation of predominantly erythro /S-hydroxy phosphine oxide, then decomposition with base). Best if R group of Ph2POR is not anion-destabilizing. 

AH latex compounds should contain at least 5 phr of zinc oxide. This is needed to absorb evolved hydrochloric acid either in the compound or finished part. A larger amount should be considered if the part contains or is in contact with acid-sensitive materials such as cotton cloth. Magnesium oxide may destabilize anionic soap systems, and is avoided for that reason. The compound should also contain two parts of an antioxidant, and consideration should be given to the need for a uv screen in light-colored products. 

AH detergent proteases are destabilized by linear alkylbenzenesulfonate (LAS), the most common type of anionic surfactant in detergents. The higher the LAS concentration and wash temperature, the greater the inactivation of the enzyme. The presence of nonionic surfactants, however, counteracts the negative effect of LAS. Almost aH detergents contain some nonionic surfactant therefore, the stabHity of proteases in a washing context is not problematic. 

This is opposite from the order in solution as revealed by the pK data in water and DMSO shown in Table 4.14. These changes in relative acidity can again be traced to solvation effects. In the gas phase, any substituent effect can be analyzed directly in terms of its stabilizing or destabilizing effect on the anion. Replacement of hydrogen by alkyl substituents normally increases electron density at the site of substitution, but this effect cannot be the dominant one, because it would lead to an ordering of gas-phase acidity opposite to that observed. The dominant effect is believed to be polarizability. The methyl... 

The relative stability of the anions derived from cyclopropene and cyclopentadiene by deprotonation is just the reverse of the situation for the cations. Cyclopentadiene is one of the most acidic hydrocarbons known, with a of 16.0. The plCs of triphenylcyclo-propene and trimethylcyclopropene have been estimated as 50 and 62, respectively, from electrochemical cycles. The unsubstituted compound would be expected to fall somewhere in between and thus must be about 40 powers of 10 less acidic than cyclopentadiene. MP2/6-31(d,p) and B3LYP calculations indicate a small destabilization, relative to the cyclopropyl anion. Thus, the six-7c-electron cyclopentadienide ion is enormously stabilized relative to the four-7c-electron cyclopropenide ion, in agreement with the Hixckel rule. 

The Hiickel rule predicts aromaticity for the six-7c-electron cation derived from cycloheptatriene by hydride abstraction and antiaromaticity for the planar eight-rc-electron anion that would be formed by deprotonation. The cation is indeed very stable, with a P Cr+ of -1-4.7. ° Salts containing the cation can be isolated as a product of a variety of preparative procedures. On the other hand, the pK of cycloheptatriene has been estimated at 36. ° This value is similar to those of normal 1,4-dienes and does not indicate strong destabilization. Thus, the seven-membered eight-rc-electron anion is probably nonplanar. This would be similar to the situation in the nonplanar eight-rc-electron hydrocarbon, cyclooctatetraene. 

The situation with phosphoric anhydrides is similar. The phosphorus atoms of the pyrophosphate anion are electron-withdrawing and destabilize PPj with respect to its hydrolysis products. Furthermore, the reverse reaction, reformation of the anhydride bond from the two anionic products, requires that the electrostatic repulsion between these anions be overcome (see following). 

The electron-withdrawing nitro group stabilizes an aikoxide ion, but the electron-donating methoxyl group destabilizes the anion. 

Cations are by no means the only species where the effects of hyperconjugative delocalization reveal themselves in such a striking manner. Similar effects exist in neutral systems or in anions. For instance, the normal propyl anion should tend to be eclipsed (E) since in this manner the molecule would optimize the 4-electron interactions between the ethyl group t orbital and the p orbital which carries the electron pair. In the bisected conformation, where ttchs and ttchs have both been raised in energy, the four-electron, destabilizing (see Section 1.7, rule 2) p ->7r interaction is stronger than in the eclipsed conformation. At the same time the two-electron, stabilizing p ->ir interaction is weaker than in the eclipsed conformation. Both effects favor the eclipsed conformation. 

Damage to epicuticular waxes Altered photosynthesis Increased water loss Accumulation of acidic anions Leaching of ions, sugars, etc. Mineral imbalances Altered metabolism Increased susceptibility to winter freezing injury Death of fine roots Destabilization of trees Reduced water/mineral uptake Reduced water uptake Cations leached below roots Accumulation of acidic anions Altered structure/texture Altered microflora Reduced litter decomposition Altered N transformations Solubilization of metal ions... 

Enolate anions (4e) that have been heated by infiared multiple photon absorption for which torsional motion about the H2C-C bond, which destabilizes the 7t orbital containing the extra electron, is the mode contributing most to vibration-to-electronic energy transfer and thus to ejection. 

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