Phosphonium salt pKas

Most of the pKa values for the phosphonium salts have been determined by potentio-metric measurements in the case of / -ketophosphonium salts562 (Table 13). The acidity of these phosphonium salts depends not only on the R and R groups of the carbonyl chain, but also on the other substituents linked to the phosphorus562 564. Particularly for the cyclic phosphonium salts, the pKa value can vary significantly depending on the different steric and electronic (even transannular) effects on the phosphorus atom564 ... 

In addition, the acidities of phosphonium salts vary in relation to the solvent in a typical way for onium salts in the case of various water-ethanol mixtures, the acidity decreases as the proportion of ethanol increases and then reincreases in pure ethanol562. Further, there are great differences between the pKa values measured in two different solvents562 ... 

The results indicate that the kinetic acidity is markedly lower for phosphonium salts than for the corresponding nitroalkanes, but cannot be correlated with pKa values. In contrast, the measurement of kinetic acidity using double potential step chronoamperometry180 allowed the determination of pka values for a series of phosphonium salts corresponding to semi-stabilized or non-stabilized ylides ... 

We and others have found that CH2CI2 is an excellent solvent for acidic dihydrogen and dihydride compounds. Methylene chloride has a low dielectric constant so that monocationic complexes will usually form 1 1 ion pairs below 0.01 M and higher aggregates above this concentration. Several pK/ of metal hydride complexes in CH2CI2 anchored to the pKa fl of phosphonium salts or other acids have been reported (e.g. see Table 1.2 from our work) they fall in the range from about -5 to 12 [23,30,49-52]. These are actually ion-pair pK values because they have not been corrected for the effects of ion-pairing. 

Johnson and Amel (5J) studied the pKa values of a series of phenacylonium salts and found that the acidifying effects of the onium group increased in the following order arsonium < phosphonium < sul-fonium. The acidifying effects of the onium atom substituents increase in the order methyl < n-butyl < phenyl. 

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