Ion pairs acidity

Relative equilibrium ion-pair acidities have been determined by Streit-wieser and Scannon (434) for thiazole and several heterocyclic compounds by reference to hydrocarbon indicators. The pK values for the... 

Very recently equilibrium ion-pair acidities of substituted diphenylmethanes have been measured in cyclohexylamine188. The meta series gives a normal Hammett plot (p = 9.69)... 

Other experimental values are available for ion pair acidities defined by the transmet-allation reaction of equation 3, where the acid R H of known pKa serves as a reference, and are thermodynamic in nature. 

A few measurements are available that relate to the ion pair acidity of ethylene and some other alkenes. Ethylene is difficult to metallate directly, but vinyl bromides and iodides undergo facile transmetallation with alkyllithium reagents. Applequist and O Brien determined the equilibrium constants of transmetallation exchange reactions as a measure of relative acidity (equations 6 and 7)25. 

Cram26 had developed an acidity scale based on the ion pair acidity and used this and other measures (such as the acidity function technique) in compiling his so-called MSAD acidity scale, named after W. K. McEwen, A. Streitwieser, D. E. Applequist and R. E. Dessy. The scale used 9-phenylfluorene (pK = 18.5) as its standard and is considered at least approximately to refer to the dilute aqueous solution as the standard state. On this scale ethylene is assigned a pK value 0.5 units lower than benzene however, in another early compilation27 ethylene is 1 pK unit higher than benzene. In an updated MSAD scale, ethylene was found to be 1 pK unit less acidic than benzene6,28,29. 

Due to the high interest in metalation reactions with lithium amide or alkyllithiums, an indicator scale of lithium ion pairs in THF has been developed119. Aggregation studies have indicated that organolithium species exist predominantly, if not exclusively, as monomers in the 10-3-10-4 M concentration range. Particular attention has been devoted to the lithium and caesium ion-pair acidities of diphenylamine in THF120 that, at 25 °C, have been found to be 19.05 and 24.20, respectively. 

Here, the measurements lead to values that are best described as ion-pair acidities. Although it is not possible to directly relate these values to true pAia values because they do not reflect the stability of the bare carbanion, they do provide useful data, particularly for very weak acids, and are relevant to many synthetic applications of carbanions because the solvents and concentrations employed in synthetic work generally lead to ion pairing. 

Figure 3.2. Plot of ion-pair acidities in THF and versus free ion values in...

Figure 3.4. Plot of the rate of tritium exchange in lithium cyclohexylamide/cyclohexylamine versus cesium ion pair acidity in cyclohexylamine. The following species are included (1) p-biphenyldiphenylmethane (2) di-p-biphenyhnethane (3) triphenyhnethane (4) diphenyl-methane (5) p-methylbiphenyl.

The second equilibrium ion-pair acidity constant (pK2) of 9,10-dihydrodibcnz[a,/z]-anthracene (DBDHA) (2) in THF at 298 K has been found to be considerably lower than for other 9,10-dihydroanthracene (DHA) derivatives and the dependence on the counter ions sodium (pK% 28.5) and potassium (pK2 30.4) is less marked as a consequence of extended >, 71-conjugation of the dianion (3) with the outer benzene rings.5 The disodium, dipotassium, and dirubidium salts exist as contact ion triplets and the dilithium salt as a solvent-separated triplet. 

Equilibrium contact ion-pair acidities have been reported for monomeric lithium and caesium salts of several sulfones and a sulfoxide in TE1F.19... 

The experimental response may deviate from that expected under ideal conditions for the following reasons. First, there may be interactions between the adsorbed molecules that cause the surface activity to differ from the surface concentration [13,14]. Alternatively, double-layer effects, ion-pairing, acid-base dissociation, and dispersion of the formal potentials can cause similar deviations. A non-zero peak splitting may indicate intermolecular interactions between the redox centers or that switching the redox composition triggers a structural change within the supramolecular assembly, e.g. adsorbate reorientation or the formation of... 

Ion-pair acidity is an important parameter to probe highly diluted solutions, as demonstrated by the sophisticated visible or near-UV absorption methods developed mainly by Streitwieser and coworkers. This analytical approach afforded the pXa values and aggregation states of a large set of cesium and lithium enolates. Its principle relies... 

Interestingly, no correlation could be observed from their monomer ion-pair acidities (pAT0 in THF) and the second-order rate constant for the monomer in their reaction with m-chlorobenzyl bromide (Table 2, right), a linear relationship occurs when the corresponding cesium salts are alkylated with methyl tosylate. On the other hand according to the authors, this accounts for the fact that the lithium cation is as important as the basicity of the enolate. 

Alkylation takes place at the most acidic position of a reagent molecule for example, acetoacetic ester (CH3COCH2COOEt) is alkylated at the methylene and not at the methyl group, because the former is more acidic than the latter and hence gives up its proton to the base. However, if 2 equivalents of base are used, then not only is the most acidic proton removed, but also the second most acidic. Alkylation of this doubly charged anion (a dianion) occurs at the less acidic position, in this case the second most acidic position (see p. 513). The first and second ion pair acidities of (3-diketones has been studied. 

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