Trityl cations methoxy-substituted

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. 

Michl s views were supported in a recent study of photodissociation of A-(tri-phenylmethyl)anilines by Siskos et al. [112]. These compounds were shown to undergo efficient homolysis of the C-N bond, producing trityl radical with quantum yields of 0.6-0.8, which were independent of solvent polarity. The trityl radical produced either 9-phenylfluorene, triphenylmethane, or trityl cation upon the absorption of another photon, and a detailed mechanistic scheme was proposed for these transformations. Later these workers demonstrated that an increase in the number of phenyl groups in the molecular series PhCH2NHPh, Ph2CHNHPh, and PhaCNHPh caused the fluorescence quantum yield to decrease and the yield of the C-N bond homolysis to increase drastically [134]. An analogous study was conducted on the photodissociation of methoxy-substituted benzyl, benzhydryl. 

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