MSAD acidity scale

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. 

In this approach, compounds that are sufficiently basic that they can be used to depro-tonate weak acids just outside of the water limit, but whose conjugate acids are just acidic enough to be measurable in water, are the key to linking all organic acids to a water pKa scale. Two of the most important compounds used in this way are the acids cyclopentadiene and 9-phenylfluorene, whose pfCgS are 16.0 and 18.5, respectively. A carbon acidity scale based upon 9-phenylfluorene was developed by Cram, and named the MSAD scale (named after McEwen, Streitwieser, Applequist, and Dessy). This scale is the one commonly used to define pKa values of near 18 to near 40. Values above 40 can be determined using an electrochemical method developed by Breslow. 

A value of about — 7 cal.deg-1.mole-1 for the smaller carbon acids seems reasonable3 (that for the dissociation of HCN in aqueous solution is — 7.4), and combination of this value with the standard enthalpy change yields a value of 56 kcal.mole-1 for the standard free energy change at 298 °K for reaction (1), and hence an equilibrium constant of approximately 10-41. This is in excellent agreement with the pAT of methane of 40 on the MSAD scale (see Section 2., p. 7) and lends support to the calculations shown in Table 1. 

The acidities of carbon acids have been extensively discussed before, especially by Cram5 who has constructed a scale of pK values based on the work of Mc-Ewan, Streitwieser, Applequist, and Dessy and known as the MSAD scale. Table 3 shows a selection of these pK values, and also values given by Reutov and co-workers6 derived from polarographic measurements. It is clear from the values shown in Table 3 that substituents such as a-phenyl, a-benzyl, and a-carbethoxy increase the extent of carbanion formation from carbon acids very considerably,... 

A further point of interest is in the assigned pK values to the unsubstituted alkanes. Although the MSAD scale and the polarographic scale do not agree very well in this region, the two scales give the same sequence of acidities, i.e. 

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