Acidity of hydrocarbons. Ionisation constants for some substituted 9-phenylfluorenes

The pKa values of a series of substituted 9-phenylfluorenes have been measured. In conjunction with earlier studies, the results enable the establishment of an H– acidity function for 25–97% aqueous dimethyl sulphoxide containing tetramethylammonium hydroxide. It is indicated that this scale does not parallel the acidity function based on the ionisation of aniline indicators in the more aqueous solutions. Substituents in the fluorene ring exert a greater influence on hydrocarbon acidity than those in the 9-phenyl nucleus. The correlation of substituent effects on hydrocarbon acidity, by use of a modified Hammett equation, suggests that the 9-phenyl ring is displaced from planarity with the fluorene nucleus by ca. 39°, close to the value predicted earlier by molecular orbital calculations.