Acid–Base Equilibria and Salt Formation
Acid–Base Equilibria and Salt Formation
Acid–base equilibria involving salt formation and the generation of reactive intermediates, such as enolates, are discussed. The absence of the solvated proton in the equilibria means that, in contrast to individual ionization constants, they are relatively insensitive to solvent basicity. The intrinsic tendency to salt formation between carboxylic acids and amines is drastically reduced in most solvents compared with water. At higher concentrations in alcohols and alcohol–water mixtures, however, this is compensated for by high levels of ion-pair formation. In polar aprotic solvents, charge-forming equilibria are strongly inhibited by a combination of low equilibrium constants and a weaker tendency for ion-association. The ionization of carbon acids, such as ketones, by neutral bases, even the strongest known neutral bases, phosphazines, is severely limited. In non-polar solvents, such as chlorobenzene, chloroform, and carbon tetrachloride, there is no observable formation of free ions, but a high degree of proton transfer between common acids and bases to form ion-pairs frequently occurs.
Keywords: salt formation, substituent effects, alcohols, mixed solvents, aprotic solvents, ketones, phosphazenes, ion-pairs, chlorobenzene, chloroform
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