ADDITIVITY OF SUBSTITUENT EFFECTS IN THE FLUOROARENE SERIES - EQUILIBRIUM ACIDITY IN THE GAS-PHASE AND DEPROTONATION RATES IN ETHEREAL SOLUTION

Benzene and all mono-, di-, tri-, tetra- and pentafluoro substituted d erivatives thereof were equilibrated with the corresponding aryl anion s in the gas phase. perfect additivity of the substituent effects on t he acidity was observed. The basicities were diminished by 12, 6 and 4 kcal/mol depending on whether the supplementary fluorine atom occupie d the ortho, meta or para position with respect to the deprotonation s ite. On the other hand, substituent effects on the rates of hydrogen/m etal interconversion processes level off with increasing numbers of ha logens. For example, while the free energy of activation of sec-butyll ithium promoted lithiation of fluorobenzene is at least 4 kcal/mol sma ller than that of benzene, the difference was found to shrink to less than 2.5 kcal/mol when pentafluorobenzene and 1,2,3,4-tetrafluorobenze ne were compared. (C) 1997 Elsevier Science Ltd.