Halide ions solvation

H 72.8, O 141.0 and I 295.2 kJ mol . Consistent with this the compound CsAu has many salt-like rather than alloy-like properties and, when fused, behaves much like other molten salts. Similarly when Au is dissolved in solutions of Cs, Rb or K in liquid ammonia, the spectroscopic and other properties are best interpreted in terms of the solvated Au ion (d °s ) analogous to a halide ion (s p ). 

The theory of the structure of ice and water, proposed by Bernal and Fowler, has already been mentioned. They also discussed the solvation of atomic ions, comparing theoretical values of the heats of solvation with the observed values. As a result of these studies they came to the conclusion that at room temperature the situation of any alkali ion or any halide ion in water was very similar to that of a water molecule itself— namely, that the number of water molecules in contact with such an ion was usually four. At any rate the observed energies were consistent with this conclusion. This would mean that each atomic ion in solution occupies a position which, in pure water, would be occupied by a water moldfcule. In other words, each solute particle occupies a position normally occupied by a solvent particle as already mentioned, a solution of this kind is said to be formed by the process of one-for-one substitution (see also Sec. 39). 

Persulfate (41) reacts with transition metal ions (e.g. Ag, Fe21, Ti31) according to Scheme 3.42. Various other reduetants have been described. These include halide ions, thiols (e.g. 2-mercaptoethanol, thioglycolic acid, cysteine, thiourea), bisulfite, thiosulfate, amines (triethanolamine, tetramethylethylenediamine, hydrazine hydrate), ascorbic acid, and solvated electrons (e.g. in radiolysis). The mechanisms and the initiating species produced have not been fully elucidated for... 

Large concentrations of halide ions, preferably iodide, favour the formation of /ra/i5-stilbene from benzaldehyde and benzyltriphenylphosphonium halides in methanol with methoxide as base, whereas large concentrations of methoxide ions slightly favour formation of the m-isomer. These effects have been explained by the preferential solvation of P+ by halide ions, leading to greater reversibility of betaine formation. Methoxide ions, on the other hand, are preferentially solvated by methanol. 

Mass spectrometry has been used to study the energetics of solvation and has shown that the enthalpies of attachment of successive water molecules to either alkali metal or halide ions become less exothermic as the number of water molecules increases (Kebarle, 1977). The Gibbs free energies of attachment for water molecules have also been found to be negative. 

A variation of the halide affinity approach was used by Riveros et al. in the investigation of the enthalpy of formation of o-benzyne. Reaction of bromo- or iodobenzene with base in an ICR leads predominantly to the formation the expected M-1 anion, but also leads to the formation of solvated halide ions (Eq. 5.15). By using substrates with known halide affinities, it was possible to assign limits to the enthalpy of formation of the benzyne product. Ultimately, the experiment is comparable to that outlined in Eq. 5.14, although the acidity and halide affinity measurements are made in a single step. 

Changing the solvent in which a reaction is carried out often exerts a profound effect on its rate and may, indeed, even result in a change in its mechanistic pathway. Thus for a halide that undergoes hydrolysis by the SN1 mode, increase in the polarity of the solvent (i.e. increase in e, the dielectric constant) and/or its ion-solvating ability is found to result in a very marked increase in reaction rate. Thus the rate of solvolysis of the tertiary halide, Me3CBr, is found to be 3 x 104 times faster in 50% aqueous ethanol than in ethanol alone. This occurs because, in the S,vl mode, charge is developed and concentrated in... 

The factors that promote unimolecular, as opposed to bimolecular (E2), elimination are very much the same as those that promote SN1 with respect to Sw2, namely (a) an alkyl group in the substrate that can give rise to a relatively stable carbocation, and (b) a good ionising, ion-solvating medium. Thus (a) is reflected in the fact that with halides, increasing El elimination occurs along the series,... 

Molecules of the protic solvent, water, solvate a halide ion by forming hydrogen bonds to it. 

Solvation stabilizes the transition state leading to the intermediate carbocation and halide ion more it does the reactants => the free energy of activation is lower. 

The results of a recent investigation of the dependence of the cleavage rate constant upon the solvent of two similar anion radicals, those of 3-nitrobenzyl chloride and 3-chloroacetophenone,85 may likewise be interpreted as the outcome of a competition between the Lewis acid solvation of the developing halide ion and of the negatively charged oxygen atoms in the initial state. 

Quantum yields for the formation of 141 from 138 in TFE-MeCN were estimated by transient absorption actinometry (Table l).62 The data refer to solvated carbocations (141) since ion pairs (140) are too short-lived for detection on the ns time scale. The modest to poor yields of 141 could be due to predominant ion-pair recombination (140 -> 142), or to parallel protonation (139 — 140) and insertion (139 — 142). Picosecond LFP studies on photoheterolyses of A CH-X in MeCN revealed that the ratio of collapse to escape (k /ki) for [Ar2CH+ X-] is slightly affected by p-substituents (H, Me, OMe) and by X (Cl, Br).66 In contrast, 4>M1 was found to increase by a factor of 17 as p-H (138d) was replaced with p-OMe (138a).62 Hence the ion-pair hypothesis seems difficult to reconcile with the effect of p-substituents on unless the strong nucleophile RO in 140 behaves differently from the weakly nucleophilic halide ions. 

Halide ion mobilities follow the expected trends (Fig. 9). Chloride ion, with the smallest crystallographic radius of the three halides considered, is the most mobile in solvents such as nitrobenzene and dimethylformamide, where anion solvation is expected to be small. In these solvents the Walden products are large. 

Table 7. AG°, Aff and AS° for the solvation of halide ions in DMSO and in water 12) ...

We shall now turn our attention to the results obtained for the hydration of alkali- 128,130,isi, 158) and halide ions 135,131,133,139 which have been the focus of all kinds of theoretical calculations. Some features are clearly evident from the gas-phase solvation data (Table 8). 

One more difference between ionic liqnids and conventional organic solvents of the C-Cl cleavage should be mentioned. Normally, the intrinsic solvation of the developing halide ion disfavors the cleavage via the entropy term. Such a term cannot be significant in the ionic liquids composed of very bulky cations and anions. 

Protonated forms of the large-ring macrocycle [24]Ng02 (5) and related compounds have been shown to be active as synthetic phosphorylation catalysts in ATP synthesis. It is likely that in this case the substrate enters the macrocyclic cavity to some extent, or is enveloped by it. Evidence for this possibility comes from the crystal structure of the chloride salt of 5-6H (Figure 1) in which a chloride ion is enveloped within a cleft formed by the boat-shaped conformation of the macrocy-cle. The crystal structure of the nitrate salt of 5-4H has also recently been determined and the host again adopts a boat-like conformation as it interacts with the anion. The hydrochloride salt of the smaller [22]Ng binds two chloride anions above and below the host plane in a similar way to 1. Molecular dynamics simulations indicate that the pocket-like conformation for 5-6H is maintained in solution, although Cl NMR experiments demonstrate that halide ions are in rapid exchange between the complexed and solvated state. 

When the apparently penta-coordinated diarsine complexes just described are dissolved in solvents more polar than nitrobenzene, they tend to dissociate into halide ions and bivalent cations, thus becoming 2 1 electrolytes (119). The effect is most marked with the platinum compounds. It has been shown that solvation effects might be less with platinum than with palladium, and so, other things in the equilibria being equal, it can also be concluded that the bonding of further ligands by a square-planar complex is much weaker with platinum than with palladium. Square-planar nickel complexes are of course the most ready to take up further ligands. 

Table 6.2 shows the detachment energy of one water molecule from a hydrated halide ion cluster [41]. The strength of the water-halide interactions is reduced as the ionic radius increases in the order of Fspecific adsorption in an electrochemical environment. It is clear that the nonspecific adsorption behavior of F is due to its strongly bound solvation shell. Due to... 

Water detachment energy from solvated halide ion clusters [41]. 

Solvent for Displacement Reactions. As the most polar of the common aprotic solvents, DMSO is a favored solvent for displacement reactions because of its high dielectric constant and because anions are less solvated in it (87). Rates for these reactions are sometimes a thousand times faster in DMSO than in alcohols. Suitable nucleophiles include acetylide ion, alkoxide ion, hydroxide ion, azide ion, carbanions, carboxylate ions, cyanide ion, halide ions, mercaptide ions, phenoxide ions, nitrite ions, and thiocyanate ions (31). Rates of displacement by amides or amines are also greater in DMSO than in alcohol or aqueous solutions. Dimethyl sulfoxide is used as the reaction solvent in the manufacture of high performance, polyaryl ether polymers by reaction of bis(4,4,-chlorophenyl) sulfone with the disodium salts of dihydroxyphenols, eg, bisphenol A or 4,4,-sulfonylbisphenol (88). These and related reactions are made more economical by efficient recycling of DMSO (89). Nucleophilic displacement of activated aromatic nitro groups with aryloxy anion in DMSO is a versatile and useful reaction for the synthesis of aromatic ethers and polyethers (90). 

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