Dissociated ion pair

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. 

Many alkyl halides and arenesulfonates (R—X) undergo first-order solvolysis in ionising solvents (SOH) and the essential experimental features maybe accommodated by the generic mechanism shown in Scheme 4.4. The mechanism includes reversible ionisation of the covalent substrate to give a contact (intimate) ion pair, R+ X-, which undergoes internal return or nucleophilic capture by the solvent (in some cases, there is evidence for the intervention of solvent-separated ion pairs, and even fully dissociated ion pairs, but the minimalist mechanism of Scheme 4.4 serves our present purpose adequately) [13]. 

For a C-C bond to cleave, there must be an acceptor substituent at one end and a donor at the other. The potassium aldolates form highly dissociated ion pairs and cleave easily. They give the thermodynamically controlled aldol products. When using lithium salts, the donor character of the C) function is reduced and the retroaldol process is retarded. Hence, the kinetic products are also observed. 

This intermittent growth model was further elaborated [334-341], and ethylene polymerisation systems were devised to involve consecutive equilibria appearing to convert alkylaluminium halide and metallocene first into donor-acceptor complexes [scheme (61)] equivalent to contact (inner) ion pairs and then into separated (dissociated) ion pairs [scheme (62)] [297,338] ... 

The generally lower stereospecificity in cationic polymerizations compared to anionic polymerizations has been attributed by Natta (188) to be due to more easily dissociated ion pairs, large gegen-ions having several equal atoms (e. g., chloride) available for coordination with the carbonium ion, and to isomerization reactions. Therefore, just as was discussed previously for anionic polymerizations, cationic polymeriza-... 

The three sets of A Eg values observed in this work indicate that, in addition to the sets of AEg values defined by Kennedy and coworkers (J3), other values are also possible and that these values are determined by the Lewis acidity of the coinitiator and the nature of the initiator. With very strong Lewis acids, no significant differences in AEg values can te observed possibly because highly dissociated ion pairs are formed under all conditions. With weaker Lewis adds AEg values are sensitive to subtle changes in the counter anion, for example, changing the halogen from Cl to Br in Et3AlX increases the AEg from 2.0 to 4.0 kcal/ mole. 

Apparently neither electrostatic interactions nor reduced diffusibility of protons is the major cause for the decrease in the proton transfer rate. As these effects are dominating in ion-pair recombination and ion-pair separation, we have to focus our attention to the primary event in proton dissociation ion-pair formation. In this reaction, the hydrogen of the OH bond of the excited parent molecule forms a hydrogen bond with the nearest H2O molecule, which itself is hydrogen bonded to other water... 

The conductance study of (Cs+,T )BPh4 in THF led to its dissociation constant, of 1.1 10-4 M at 20 °C. A perfect linear relation was obtained for kp plotted vs. /,fraction of dissociated ion-pairs, yielding k 5.6 M-1s-1 and 1 10-6 M at 20°C. Hence, the kryptated cesium salt is about four times as reactive as the kryptated potassium salt, and the dissociation constant of the former salt is three times larger than of the latter. 

All successful water models make use of a distribution of point charges rather than of point multipoles. The main reason is that the directional properties of intermolecular hydrogen bonds can be obtained efficiently with oifly 3 or 4 point charges. Furthermore, Goldman and Backx [39] have shown that model molecules with such distributions of point charges are more effective as solvents (for instance in their ability to dissociate ion pairs) than molecules with equivalent point dipoles and quadrupoles. 

It thus appears as if a radical mechanism cannot satisfy the observations. The alternative has been suggested that Step 4 occurs as a hydride ion abstraction with the formation of a dissociated ion pair, HOa and R. If this is the case, it would require unusually large electron affinities for the HOa radical or unusually small ionization potentials for the R-radical to account for the data. I don t believe that independent measurements of these quantities can be adduced to support such a contention. Nevertheless, an intimate ion pair mechanism cannot be completely ruled out at the moment. 

Such results exemplify the analytic use of NMR of a quadrupolar nucleus. It Is not mandatory to be able to make a detailed Interpretation of the NMR parameters, chemical shifts and llnewldths, for each of the chemlced species In the solution to which the quadrupolar nucleus can attach Itself. If these NMR parameters differ sufficiently between these various species, relative concentrations can be obtained and such Information can be quite valuable. Re rdlng the structure of the ion pairs themselves, at present Na NMR Is restricted to distinguishing between tight, loose or dissociated Ion pairs, which It can do rather reliably (24). But obviously. It Is to be complemented by h and NMR If Information about the geometry and the electron distribution In the anion Is desired (25-27). 

This methodology has been also used to extract subtle mechanistic details of the dissociation mechanism of an NaCl ion pair in liquid water [12]. In this case, committor distributions revealed that rearrangement of solvent molecules around the dissociating ion pair significantly contributes to the free energy barrier separating the contact state from the dissociated state. 

The degree of association of the ion pairs depends also upon the nature of the counterion and on the temperature of the reaction medium. Completely dissociated ion pairs allow chain growth to take place free from the influence of counterions. The carbon cations simply add directly to the double bonds of the incoming monomers. Propagation rates for such reactions are greater than for those with tight ion pairs. ... 

TMSBr, a 1 1.4 mixture of axial and equatorial THPs (N + M) was formed. To explain this reversal of selectivity, the authors hypothesized that SnBrs and the oxocarbenium formed a dissociated ion pair thus favoring the equatorial addition (Scheme 41). We have to point out that substituted allenes can be used and, depending on the configuration of the double bond, the stereochemistry at C5 can be controlled. 

Several mechanisms involve the formation of a more or less dissociated ion-pair which reacts with the epoxy compound to form the ester without any contribution of a cyclic species. 

The solutions in acetic acid contain scarcely dissociated ion-pairs owing to its low dielectric constant. Some reactions lead to solvolysis products, such as FeCl(RCOO)2. Partial hydrolysis is found to occur with ferric and aluminium chloride, titanium(IV), niobium(V) and tantalum(V)-chlorides, while halides of arsenic(III), zirconium(IV), thorium(IV) and uranium(IV) are completely solvo-lysed. The high reactivity is undoubtedly due to the presence of acetate ions, and ethylacetate gives many more adducts with acceptor molecules than does acetic acid. 

Several extensions and modifications of the electrolyte theory in the first half of the twentieth century should be mentioned Bjeiium [14] introduced the concept of limited electrostatic dissociation (ion pair formation), Onsager and Fuoss extended the DH approach and the ideas of Debye about the electrophoretic and the relaxation effect on transport properties such as electrical conductivity and diffusion coefficients [15]. As already mentioned, the DH description is also the basis of one of the two constituting parts of the DLVO theory in colloidal chemistry. 

DctIcx relationship Since the fraction of dissociated ion pairs increases with the concentration of redox sites, the apparent diffusion coefficient exhibits a steep increase with an increasing Cx value. 

How can such an unusual effect be explained The currently accepted answer is that there are two kinds of ion pairs contact ion pairs and solvent-separated ion pairs. A solvent-separated ion pair includes one (or perhaps several) solvent molecule between the cation and anion. It is distinct from a completely dissociated ion pair... 

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