Cation ion pairs

The order of enolate reactivity also depends on the metal cation which is present. The general order is BrMg < Li < Na < K. This order, too, is in the order of greater dissociation of the enolate-cation ion pairs and ion aggregates. Carbon-13 chemical shift data provide an indication of electron density at the nucleophilic caibon in enolates. These shifts have been found to be both cation-dependent and solvent-dependent. Apparent electron density increases in the order > Na > Li and THF/HMPA > DME > THF >ether. There is a good correlation with observed reactivity under the corresponding conditions. 

Common-ion rate depressions and the involvement of vinyl cation ion pairs similar to those observed by Miller and Kaufman (131) were observed by Rappoport and Gal (136) in the solvolysis of 150 in acetic acid. Further interesting evidence for ion-pair involvement in the solvolytic generation of some vinyl cations comes from the investigation of cis- and trans-1,2-dianisyl-2-phenylvinyl halides, 151 (1937). In 80% ethanol, solvolysis... 

In contrast to the lack of reactivity of ketones with PPN [HCr(CO)d, Brunet et al. reported different reactivity with K+ rather than PPN+ as the counterion. They found that K HGr(CO)5 reacts with cyclohexanone in the absence of acid [35]. Hydrolysis with acid led to a 50% yield of cyclohexanol. These results suggest assistance from the K+ cation ion-pairing in metal anions has been studied in detail by Darensbourg [36]. 

Keywords Alkene radical cations Ion pairs Kinetics Stereochemical memory effects Tandem reactions... 

The acidity constants for H4[Fe(C e] are pA i = 2.54, pA 2= 108, pAT3 = 2.65, pACj=4.19. Association constants have been published for alkali metal cations ion pairing with hexacyano-ferrate(II). ... 

Assay of ascorbic acid by chromatography with a cationic ion-pairing reagent and electrochemical detection... 

Thus the growing anionic chain can assume at least two identities the free anion and the anion-cation ion pair (several types of solvated ion-pairs can also be considered). Furthermore, the kinetics of these propagation reactions, which generally show a fractional dependency on chain-end concentration ranging from one-half to unity, can best be explained by assuming that the monomer can react with both the free anion and the ion-pair (4, 5, 60, but at different rates. Thus, for example, in the polymerization of styrene by organosodium, the rate of polymerization (Rp) can be expressed as... 

Li NMR spectra show several peaks as a result of several different CIP modes arising from coordination to different sites of the anion, and (temperature-dependent) exchange between CIP and SSIP. Solvent separated lithium cation ion-pairs appear at ca +2 ppm relative to the NMR standard (LiBr in THF, 0.0 ppm)13. In the CIP solvation state the line shows a sharp deviation from the standard. Proximity to the anion shows lines at ca... 

In the presence of an electron rich donor molecule an alternative to direct fission or reaction via an excimer is the formation of an exciplex or radical anion/radical cation ion pair (Eq. 2). The radical anion has been viewed as the key intermediate which undergoes fission to aryl radical and halide ion (Eq. 3). With polyhaloarenes there is an additional option. A polychloroarene radical anion, for example, has two possible modes for bond fission (a) fission to produce aryl radical and chloride ion or (b) fission to form an aryl carbanion and chlorine atom (Scheme 6). The options for fragmentation of a haloarene radical anion... 

In nonpolar and weakly polar aprotic solvents, the salts of Nu are present as ion-pairs (or ion-clusters) in which the nearby cations diminish the reactivity of the anion. Since, with a given cation, ion-pairing is strongest with the smallest ion, F , and weakest with the largest ion, I", the reactivity of X- decreases as the size of the anion decreases. In polar protic solvents, hydrogen-bonding, which also lessens the reactivity of X, is weakest with the largest ion,... 

Fig. 38 Sizes (in A) of anions and cations (ion pairs) that interact with the NDI nanotubes [25]...

Hexacyanoferrate(II). K4[Fe(CN)6] has been used to photosensitize Ti02. The acidity constants forH4[Fe(CN)6] are pKi = 2.54, p 2 = T08, pA 3 = 2.65, p 4 = 4.19. Association constants have been pubhshed for alkali metal cations ion pairing with hexacyanoferrate(II). Substitution at hexacyanoferrates(II) is very difficult, though it can be catalyzed by metal ions such as Hg +. Such catalysis can be augmented by surfactants such as sodium dodecyl sulfate (SDS), and indeed SDS-catalysis of Hg +-catalyzed replacement of cyanides in [Fe(CN)6]" ... 

In the field of surface-active cations, ion-pair chromatography is predominantly applied to the analysis of quaternary ammonium compounds, pyridine, pyrrolidine, and piperidine quatemisates, and for sulfonium, phosphonium, ammonium, and hydrazin-ium salts. 

MPIC Cations Ion-pair formation neutral HC1 Hexane-sulfonic acid Octane-sulfonic acid In addition to the cations listed under HPIC alkylamines, alkanolamines, quaternary ammonium compounds, cationic surfactants, sulfonium compounds, phosphonium compounds... 

Outer-sphere complexes, also described as ion pairs, involve the association of a hydrated cation and an anion, held by long-range electrostatic forces. The association is transient and not strong enough for the anion to displace any of the water molecules in immediate contact with the cation. Ion pairs are most often formed between major (>10" m) monovalent and divalent metal... 

The stability constants of ion pairs (their log /Cassoc values) have been shown to be proportional to the electrostatic function ZMzJd, where z Z/. are the charge of metal cation and ligand, and d rM + ri, the sum of their crystal radii (cf. Fig. 3.5). Mathematical models for predicting ion pair stabilities generally assume this proportionality and include the simple electrostatic model, the Bjerrum model, and the Fuoss model (cf. Langmuir 1979). Such models can predict stabilities in fair agreement with empirical data for monovalent and divalent cation ion pairs. 

Table 10.2 probably adsorb as the undissociated neutral molecules, or even as organic anion-metal cation ion pairs on humus. As the solution pH is raised, the neutral molecules dissociate to a greater extent, and adsorption is further suppressed by electrostatic repulsion (and the fact that the charged form of the molecule is more soluble than the uncharged form). Thus, p>Ka values of organic acids are correlated positively with tendency to adsorb and negatively with solubility in water. 

Phase-Transfer Catalysis. Since the efficiency of the reaction requires that bisphenoi A be used in the form of water-soluble phenolate anions, while phosgene must be dissolved in a chemically inert and hence water-insoluble solvent, the desired reaction would have to depend on the diffusion rate of the two reactants to the interface between the immiscible solvents. The area of the interface can be increased by vigorous stirring of the two-phase reaction mixture, but a more efficient way to accelerate the process is to induce one of the reactants to migrate into a phase that is not particularly receptive to it. In this example, the sodium phenolate ions are in equilibrium with a phase-transfer catalyst such as tetra-n-butylammonium chloride, and while one of the products of the equilibrium (sodium chloride) remains in the aqueous phase, the other products of the equilibrium (the BPA anion-tetra-rc-butylammonium cation ion pairs) are of sufficiently covalent character to migrate into the nonaqueous phase where they encounter phosgene and the reaction takes place. 

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