Hydroxy- and methoxy-substituted benzenes

In practice, extrapolations of p fR in water have usually used the older acidity function based method, for example, for trityl,61,62 benzhydryl,63 or cyclopropenyl (6) cations.66,67 These older data include studies of protonation of aromatic molecules, such as pKSi = —1.70 for the azulenium ion 3,59 and Kresge s extensive measurements of the protonation of hydroxy- and methoxy-substituted benzenes.68 Some of these data have been replotted as p fR or pKa against XQ with only minor changes in values.25,52 However, for more unstable carbocations such as 2,4,6-trimethylbenzyl, there is a long extrapolation from concentrated acid solutions to water and the discrepancy is greater use of an acidity function in this case gives pA 2° = —17.5,61 compared with —16.3 (and m = 1.8) based on X0. Indeed because of limitations to the acidity of concentrated solutions of perchloric or sulfuric acid pICs of more weakly nucleophilic carbocations are not accessible from equilibrium measurements in these media. 

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