Gas-Phase Acidities Of Aryl-Substituted Fluorinated Alkanes. The Nature Of Beta-Fluorine Negative Hyperconjugation

Gas-phase acidities (GA) of 2-phenyl- and 2-p-nitrophenyl-substituted fluoroalkanes, ArCH2Rf and ArCH(R-f(1))R-f(2), were calculated at the B3LYP/6-311+G(d,p) level of theory. The GA values of the fluoroalkanes having no fluorine atom at the beta-position of the conjugate acid anion center were correlated linearly with the corrected number of fluorine atoms contained in the fluorinated alkyl group (R-f), indicating that these acidities were determined by accumulated inductive effect of fluorine atoms. The GA values of ArCH(CF3)(2) 1 and ArCH(C2F5)(2) 2 also conformed to the line, indicating negligible or no contribution of beta-fluorine negative hyperconjugation to the stability of both carbanions. On the basis of this linear relationship, the extent of beta-fluorine negative hyperconjugation involved in the acidity of fluoroalkanes having beta-fluorine was evaluated quantitatively. It was concluded that the contribution of beta-fluorine negative hyperconjugation to acidity of aryl-substituted fluoroalkanes is complementary to stabilization effects by the aryl group as well as accumulated inductive effect of fluorine atoms contained in the fluorinated alkyl group.