Ion pair species

The concept of ion pairs in nucleophilic substitution is now generally accepted. Presumably, the barriers separating the intimate, solvent-separated, and dissociated ion pairs are quite small. The potential energy diagram in Fig. 5.4 depicts the three ion-pair species as being roughly equivalent in energy and separated by small barriers. 

The ion pair mechanism initially suggested by Darwish and McLaren28 (equation 2) has received further support from related studies conducted by several other investigators38-42. For example, Fava and coworkers38 have reported that during isomerization in acetic acid, optically active benzhydryl p-toluenesulfmate loses optical activity at a rate which is about two and a half times faster than the rate of sulfone formation, thus indicating that return from an ion-pair species is occurring (equation 3). 

Since the ratio of the two sulfones 2 and 3 increases with the polarity of the solvent (from 1 4 in benzene to 16 1 in formamide) a possible concerted [2,3]sigmatropic rearrangement for the formation of sulfone 3 was first considered. However, other evidence such as the effects of solvent and added salts seem to support an ionization mechanism, with the formation of the two sulfones by recombination from two different ion-pair species . 

An important improvement in the oxy-Cope reaction was made when it was found that the reaction is strongly catalyzed by base.212 When the C(3) hydroxy group is converted to its alkoxide, the reaction is accelerated by a factor of 1010-1017. These base-catalyzed reactions are called anionic oxy-Cope rearrangements, and their rates depend on the degree of cation coordination at the oxy anion. The reactivity trend is K+ > Na+ > Li+. Catalytic amounts of tetra-rc-butylammonium salts lead to accelerated rates in some cases. This presumably results from the dissociation of less reactive ion pair species promoted by the tetra-rc-butylammonium ion.213... 

At first sight, these strong effects might not seem to be predictable, given that the ferrocene reactant is uncharged and thus the formation of the precursor complex should be unaffected by the charge of the other reactant. The reaction of the ion-paired species, however, is not a simple electron-transfer reaction, because transfer of the anion must also occur. A detailed understanding of the dynamics of the process remains to be developed. 

The present author has developed a novel method called ion-association method. This is also a simple and versatile method for the preparation of ion-based organic dye nanoparticles in pure aqueous solution by the ion association approach [23]. It utilizes the control of hydrophilicity/hydrophobicity of the ionic material itself via ion-pair formation for example, addition of a cationic target dye solution into aqueous solution containing a certain kind of hydrophobic anions forms an electrically neutral ion-pair because of the strong electrostatic attraction, followed by aggregation of ion-pair species originated from van der Waals attractive interactions between them to produce nuclei and the subsequent nanoparticles (Fig. 3). In this case, hydrophobic but water-soluble anions, such as tetraphenyl-borate (TPB) or its derivatives (tetrakis(4-fluorophenyl)borate (TFPB), tetrakis [3,5-... 

DASPE-TFPB), respectively. The obtained solid precipitates were brightly emissive whereas that of the native DASPE-I were almost nonemissive (Fig. 7a the photo is taken under normal illumination and UV-light irradiation). This indicates that, in the solid of the ion-pair species between DASPE+ and TPB (or TFPB ), concentration quenching is effectively suppressed, and more importantly, these ion-pair complexes can generate fluorescent... 

Benzylideneacetone reacts with lithiated phenylacetonitrile under kinetic control, in THF and media that favour association, to give 1,2- and 1,4-adducts in proportions which are directly related to concentrations of monomeric and dimeric ion pair species, respectively." An attempt has been made to explain the different regioselec-tivities towards a,-unsaturated carbonyl compounds, including cyclic a-enones and cinnamaldehyde, in terms of intermediate complex formation. 

C02(C0)s ZnO Adsorption from solution and vacuum treatment Ion-pairing species stable up to 160°C under CO/H2 high selectivity to C1-C3 alcohols in CO hydrogenation, performance in ethylene hydroformylation [139]... 

Basic supports induce a surface disproportionation reaction on the carbonyl, giving homo- or heteronuclear ion-pairing species with Co or a surface cation as counter-cations [139, 150] ... 

An ion-pairing spectrophotometric method has also been described [14]. This method involves the use of bromphenol blue, bromcresol purple, bromcresol green, bromthymol blue, or methyl orange, and extracting the ion-pair species into chloroform. The absorbance of the yellow product is determined at 420 nm. 

The products formed in deamination depend on the lifetimes of the different ion-pair species, which, in turn, depend on the structure of the amine and the nature of the solvent. The counter-ion in such reactions can conveniently be varied by using N-nitrosoamides as starting materials. These compounds react to give diazonium ion-pairs in reactions that closely resemble amine deamination, and the lifetime of the ion-pairs can be studied by using lsO-labelled ni-trosoamides (see Scheme 2, XH = solvent or added nucleophile). 

The question of different types of ion pair, similar to those characterised in anionic polymerisation (14), has not yet arisen. Solvents are generally more polar than those used in polymerisations proceeding by carbanionic intermediates, and it could be that the major ion pair species is a loose or solvent separated entity. As far as conductance measurements (Ka) are concerned it is not possible to make a distinction between loose and contact ion pairs. For such investigations the use of other analytical methods becomes necessary (49). 

The thermodynamic data of Tables 2 and 3 are remarkably selfconsistent, and correlate well with data for the various phenyltrimethyl ammonium perchlorates in 1,2-dichloroethane originally studied by Denison and Ramsey (44). Clearly dissociation is nominally exothermic though the actual enthalpy changes are quite small. Solvation of the free ions, therefore, is also small (in these solvents) but exceeds that of the ion pair species. The overall entropy changes confirm some ordering of solvent molecules on dissociation, the loss in entropy for this process being more than sufficient to balance the entropy increase associated with... 

Another important type of observation from conductance experiments involves the effect of small quantities (1-2% W/W) of additives on dissociative equilibria. Obviously in any study where the equilibrium between two reactive species is relevant the possible disturbance of that equilibrium by traces of other components in the system is very important In kinetic studies of polymerisations it is usual to carry out experiments in which the concentrations of both initiator and monomer are varied systematically. While it is anticipated that the former will effect the position of equilibrium between free ions and ion pair species, it is easily overlooked that the latter may also have a significant influence, particularly if the polarity of the monomer differs substantially from that of the solvent... 

This behavior can be understood by the assumption that two different types of ion pairs exist in a thermodynamic equilibrium and add the monomer with different rate constants. The less reactive contact ion pair (tight-ion pair) and the more reactive solvent separated ion pair (loose-ion pair), which is more stable at lower temperatures (31). The curved lines show the transition from one ion pair species to the other. Thus, the polymerization mechanism can be described by this scheme ... 

The thermodynamic parameters of the equilibrium between the two ion pair species and those for the dissociation of the solvent separated ion pair, obtained from kinetic and conductivity measurements, are listed in Table II. The agreement of the values determined by the two independent methods is quite satisfactory. 

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