Acetone— tetraalkylammonium halides

Evans, Zawoyski and Kay analysed data for R4N salts in acetone (AC) " with the Fuoss-Onsager equation. They found Ka decreases with cation size, and for the anions, association decreases in the order Bu4NBr(i = 264) > I-(143) NOg > CIO4 (80) > Pic-(17). This agrees with data for methylethylketone. The fact that association of Bu4NC104 in AC, benzonitrile, and methylethyl-ketone corresponds to = 4.85 A for the three solvents, indicates formation of contact ion pairs. Tetraalkylammonium halides in dimethyl-formamide (DMF) have small association constants when the data are evaluated with Shedlovsky s eqn. 5.4.10. When the data for Me4NPic in is assessed with Fuoss and Hsia s eqn. 5.2.31, a is 6.0 A. 

The densities and volumetric specific heats of some alkali halides and tetraalkylammonium bromides were undertaken in mixed aqueous solutions at 25°C using a flow digital densimeter and a flow microcalorimeter. The organic cosolvents used were urea, p-dioxane, piperadine, morpholine, acetone, dime thy Isulf oxide, tert-butanol, and to a lesser extent acetamide, tetrahydropyran, and piperazine. The electrolyte concentration was kept at 0.1 m in all cases, while the cosolvent concentration was varied when possible up to 40 wt %. From the corresponding data in pure water, the volumes and heat capacities of transfer of the electrolytes from water to the mixed solvents were determined. The converse transfer functions of the nonelectrolyte (cosolvent) at 0.4m from water to the aqueous NaCl solutions were also determined. These transfer functions can be interpreted in terms of pair and higher order interactions between the electrolytes and the cosolvent. 

The nucleophilic reactivity of the lithium salts changes in the same order as in protic solvents (I > Br > Cl cf. Table 5-15). However, the order is completely reversed for the ammonium salts (Cl > Br > I ), and this latter order is the same as that found in dipolar non-HBD solvents such as A,A-dimethylformamide [278]. The small lithium cation, with its high charge density, has a strong tendency to form ion pairs with anions, whereas the electrostatic interaction between the large tetraalkylammonium ion and anions is comparatively weak. Quaternary ammonium salts, therefore, should be practically fully dissociated in acetone solution. Thus, the reactivity order obtained with these salts corresponds to that of the free, non-associated halide ions. On the other hand, the sequence obtained with the lithium salts also reflects the dissociation equilibria of these salts in acetone solution [279]. 

Examination of the Sn2 reaction between ethyl tosylate and halide ions in hexamethylphosphoric triamide (fir = 29.3) with a variety of counter ions [Li , [n-C4H9)4N ] has shown that the rates obtained with Hthium salts are always higher than those with the corresponding tetra-n-butylammonium salts [341]. This is in contrast to the situation observed in acetone [279]. This means that, in this particular solvent, lithium salts are more dissociated than tetraalkylammonium salts. This has indeed been confirmed by conductivity measurements [341, 342], The lithium cation apparently has specific interactions with strong EPD solvents such as [(CH3)2N]3PO cf. Section 3.3.2). 

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