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Published on: October 10, 2016
Equilibrium Ion Pair Acidities of Polyhalogenated Benzenes in THF. Extrapolation to Benzene(1)
Manolis Stratakis1, Peng George Wang, Andrew Streitwieser
1Department of Chemistry, University of California, Berkeley, California 94720-1460.
Abstract:
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.
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