Substituent effects on acidity of aryl-substituted fluoroalkanes: a computational study

Abstract

The gas-phase acidities (GA) of various aryl-substituted fluoroalkanes, XC6H4CH(R1)R2, were calculated at the B3LYP/6-311 + G(d,p)//B3LYP/6-311 + G(d,p). The acidity values of alkanes having a common substituent X varied significantly with the change of R1 and R2. Their changes in acidity of 1 and 2 having two strong electron-withdrawing groups (CF3 or C2F5) at the deprotonation site and 8, 9, 10, 11 having no fluorine atom at β-position were linearly correlated with the corrected number of fluorine atoms contained in the fluorinated alkyl group (R2 > 0.999). On the other hand, the GA values of β-fluorine substituted alkanes (3, 4, 5, 6, 7) deviated in a stronger acid direction from the line. The enhanced acidity was attributed to the additional stabilization of the conjugate anion caused by the β-fluorine negative hyperconjugation. The magnitude of β-fluorine negative hyperconjugation of the fluorinated alkyl group (ΔGoβ-F) given by the deviations from the line decreased with increasing electron-withdrawing ability of substituent X on the benzene ring, indicating that β-fluorine negative hyperconjugation competes with the electronic effect of the substituent X. The GAel values obtained by subtraction ΔGoβ-F from the apparent GA value were successfully correlated in terms of the Yukawa–Tsuno equation. The obtained ρel and r−el values were linearly related to the GAel value of the respective phenyl-substituted fluoroalkanes, supporting our previous conclusion that the ρ and r− values for the substituent effect caused by the electronic effects of the substituent on the acidity are determined by the thermodynamic stability of the parent ion (ring substituent = H). Copyright © 2015 John Wiley & Sons, Ltd.