Carbon acidity. 71. The indicator scale of lithium ion pairs in tetrahydrofuran

An equilibrium lithium ion pair indicator scale has been established for eight hydrocarbons that form solvent-separated ion pairs in THF. The scale covers the pK/sub a/ range from 18.49 to 23.84 relative to 9-phenylfluorene at pK/sub a/ = 18.49. This range allows the study of enolates and other highly stabilized anions under realistic synthetic reaction conditions. Equilibria were monitored with UV-vis spectroscopy, and the indicator anion spectra are reported. Thermodynamic measurements of the indicator equilibria yield small ..delta..S/sup 0/ values indicating that all of the solvent-separated ion pairs must have similar structures and solvation. Comparison of these thermodynamic parameters with those measured for the analogous cesium ion pairs points out the more complex nature of organocesium ion pairing. Aggregation studies indicate that the organolithium salts exist predominantly, if not exclusively, as monomers in the concentration range employed (10/sup -3/-10/sup -4/ M).