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.

A natural bond orbital analysis of aryl‐substituted polyfluorinated carbanions: negative hyperconjugation

“…(Table S7). [19] There is a linear correlation between the two quantities for respective carbanion series, supporting that the E(2) values for π(Cα-C1) ! σ*(Cβ-F) can be related to the negative hyperconjugation of the Cβ-F bond.…”

“…σ o and Δσ − R are the normal substituent constant and the resonance substituent constant in the gas phase (Table S7), respectively, [18,19,38] and r − is the resonance demand parameter representing the degree of π-delocalization of the negative charge into the aryl π-system. A typical Yukawa-Tsuno correlation is illustrated in Figure 11.…”

Gas‐phase Acidities of 2‐Aryl‐2‐chloro‐1,1,1‐trifluoroethanes and Related Compounds. Experimental and Computational Studies

“…Accordingly, it is necessary to separate the contribution of β‐fluorine negative hyperconjugation from the apparent acidity values in order to elucidate details of the electronic effect of substituent on acidity. For this purpose, an empirical approach reported previously could be useful for the present analysis …”

“…This is consistent with geometric features, that is, a large elongation of the C β F bond (0.137 Å for 1b [H] and 0.186 Å for 2b [H] ) and a large contraction of the C α C β bond (−0.116 Å for 1b [H] and −0.135 Å for 2b [H] ) on deprotonation. Although the magnitude of the | ΔG o β‐F | values of the C 2 F 5 group in 1b is again larger than those in 2b and 3b , the former value decreases with increasing electron‐withdrawing ability of substituent in a similar manner to that in 2b and 3b , being consistent with the tendency observed previously …”

“…Plot of ρ or ρ el versus gas‐phase acidities or GA el of the respective parent carbon acids (Ar = Ph). Open circles, ArCH(Cl)R. Open triangles, ArCH(F)R. Closed circles, ArCH 2 R and ArCH(R) 2 …”