CARBON ACIDITY .92. EQUILIBRIUM ION-PAIR ACIDITIES OF POLYHALOGENATEDBENZENES IN THF - EXTRAPOLATION TO BENZENE

The equilibrium cesium ion pair acidities of six polyfluorobenzenes at 25 degrees C and six polychlorobenzenes at -20 degrees C were determi ned in THF. For fluorinated benzenes the additive (negative) effects o f fluorine on pK (partial equilibrium factors) are o = 5.2, m = 3.0, a nd p = 1.4. From these the cesium ion pair pK (per H) of benzene is ex trapolated 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, respe ctively. The corresponding pK of benzene is 47.0 at -20 degrees C. Agg regation 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 lithi um salts of 1,2,4,5-tetrafluorobenzene are monomeric. The pK of benzen e on the Li scale is extrapolated to be 39.5.