Equilibrium Ion Pair Acidities of Polyhalogenated Benzenes in THF. Extrapolation to Benzene¹

The equilibrium cesium ion pair acidities of six polyfluorobenzenes at 25 degrees C and six polychlorobenzenes at -20 degrees C were determined in THF. For fluorinated benzenes the additive (negative) effects of fluorine on pK (partial equilibrium factors) are o = 5.2, m = 3.0, and p = 1.4. From these the cesium ion pair pK (per H) of benzene is extrapolated to be 44.8 at 25 degrees C. For chlorobenzenes the additive contributions for o-, m-, and p-chlorine are 4.2, 2.7, and 2.1, respectively. The corresponding pK of benzene is 47.0 at -20 degrees C. Aggregation studies show that in the concentration range 10(-)(3)-10(-)(4) M the cesium salt of 1,2,4,5-tetrachlorobenzene and the cesium and lithium salts of 1,2,4,5-tetrafluorobenzene are monomeric. The pK of benzene on the Li scale is extrapolated to be 39.5.