Aprotic solvents anion activity

In mineral and synthetic engine oil formulations, hydrogen bond formation between anionic or molecular bases (A, B) and their conjugate acids HA or BH+, have a heavy impact on the activity of acids and their salts. For example, in mixtures of carboxylic acid with excess of sodium carboxylate in hydrocarbon and aprotic solvents, the activity of the acid is greatly decreased because of homoconjugation ... 

In Sj 2 reactions and Sj,Ar reactions in which bond-formation is the rate-determining step, the differential solvation in the transition state is usually much smaller than the differences in levels of solvation of the anionic nucleophiles between protic and aprotic solvents. Therefore in dipolar aprotic solvents the activation energy AE is lowered and these reactions consequently proceed a good deal faster in such solvents (Figure 2). This is true of azide ion which is in general a stronger kinetic nucleophile in aprotic than in protic solvents. 

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 acetyUde ion, alkoxide ion, hydroxide ion, azide ion, carbanions, carboxylate ions, cyanide ion, hahde 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). 

Table 8-8 gives some nonelectrolyte transfer free energies, and Table 8-9 lists single ion transfer activity coefficients. Note especially the remarkable values for anions in dipolar aprotic solvents, indicating extensive desolvation in these solvents relative to methanol. This is consistent with the enhanced nucleophilic reactivity of anions in dipolar aprotic solvents. Parker and Blandamer have considered transfer activity coefficients for binary aqueous mixtures. 

Electrodimerization of activated alkenes in aprotic solvents occurs by radical-ion, radical-ion coupling. There is ample evidence for steric inhibition to this process. In contrast to the low reactivity of 11,4-methylbenzabnalononitriIe radical-ion dimeiises with a rate constant of 5.8 x 10 M s in dimethylformamide containing tetraalkylammonium ions [48]. Dimethyl maleate radical-anion diraerises faster than dimethylftimarate radical-anion by a factor of lO in dimethylformamide [49]. 

For adequate reaction rates, a high concentration of iodide anion is necessary. The cation portion of the salt appears to have little or no effect on catalytic activity or reaction selectivity. Inorganic iodides (such as potassium iodide) are the obvious first choice based on availability and cost. Unfortunately these catalysts have very poor solubility in the reaction mixture without added solubilizers or polar, aprotic solvents. These solubilizers (e.g., crown ethers) and solvents are not compatible with the desired catalyst recovery system using an alkane solvent. Quaternary onium iodides however combine the best properties of solubility and reactivity. 

The values of ds in seven dipolar aprotic solvents have been reported to be 80 + 5 pm for cations and 44 4 pm for anions [10]. The MSA is also used in treating ionic activity coefficients in a recent study [11], the change in solvent permittivity with electrolyte concentration was taken into account in addition to the change in ionic radius, and excellent agreements were obtained between the experimental and theoretical results for 1 1 electrolytes of up to 2.5 M. 

A change from a protic to an aprotic solvent can also affect the acidity or basicity, since there is a difference in solvation of anions by a protic solvent (which can form hydrogen bonds) and an aprotic one.158 The effect can be extreme in DMF, picric acid is stronger than HBr,159 though in water HBr is far stronger. This particular result can be attributed to size. That is, the large ion (C N CsE C)- is better solvated by DMF than the smaller ion Br-.160 The ionic strength of the solvent also influences acidity or basicity, since it has an influence on activity coefficients. 

These effects are particularly pertinent in type (I) -> (II) reactions, where the high anion activity is required to overcome the lattice energy of the non-molecular precursor (I). Thus many non-molecular metal halides, sulfides and thiolates which are insoluble and unreactive in aqueous media will dissolve with anionic ligands in aprotic solvents, and clusters can be isolated from the resulting solutions. Anion activation by use of cation complexands to solubilize salts in hydrocarbon solvents is advantageous, as is the use of complexands in the isolation of polymetallate anions.342... 

Anionic polymerizations, when carried out in aprotic solvents, are characterized by the long lifetime of the carbanionic (or oxanionic) sites l2). When neither spontaneous transfer nor termination reactions are involved, the polymers obtained exhibit sharp molecular weight distributions, and their number average degree of polymerization is determined by the [Monomer]/[Initiator] molar ratio, provided initiation is fast as compared to propagation. However, the major advantage of these methods, as far as synthesis is concerned, is the socalled living character of the polymers 12) After completion of the polymerization the active sites retain their reactivity and can be used for functionalizations at the chain end. 

Activation parameters for the decomposition reactions of halonitrobenzene radical anions in aprotic solvents"... 

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