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

• Published in: Journal of physical organic chemistry 2016-10, Vol.29 (10), p.523-531
• Main Authors: Mishima, Masaaki, Abboud, José-Luis M., Fujio, Mizue, Suenaga, Masahiko, Koch, Heinz F., Koch, Judith G.
• Format: Article
• Language: English
• Subjects: ary-substituted fluoroalkanes; DFT calculation; Fluorine; gas-phase acidities; negative hyperconjugation; substituent effect
• ISSN: 0894-3230; 1099-1395
• DOI: 10.1002/poc.3576

The gas‐phase acidities (GA) of 2‐aryl‐2‐chloro‐1,1,1‐trifluoroethanes (1a), 2‐aryl‐2‐fluoro‐1,1,1‐trifluoroethanes (2a), and related compounds, XC6H4CH(Z)R where Z = Cl (1) or F (2) and R = C2F5 (b), t‐C4F9 (c), C(CF3)2C2F5 (d), C(CF3)2Me (e), Me (f), H (g), were investigated experimentally and computationally. On the basis of an excellent linear correlation (R2 > 0.99) of acidities of 1c, 1d, 1e, 1f and 2c, 2d, 2e, 2f where there is no fluorine atom at β‐position to the deprotonation site with the corrected number of fluorine atoms contained in the fluorinated alkyl group, the extent of β‐fluorine negative hyperconjugation of the CF3 and C2F5 groups (ΔGoβ‐F) was evaluated. The GAel values given by subtraction ΔGoβ‐F from the apparent GA value were considered to represent the electronic effect of the substituent X. The substituent effects on the GAel values and GA values for 1c, 1d, 1e, 1f and 2c, 2d, 2e, 2f were successfully analyzed in terms of the Yukawa–Tsuno equation. The variation of resonance demand parameter r− with the R group observed for various XC6H4CH(Z)R was linearly related to the GA (GAel) value of the respective phenyl‐substituted fluorinated alkanes. On the other hand, the corresponding correlation for the ρ values provided three lines for ArCH(Cl)R, ArCH(F)R and ArCH2R, respectively. These results supported our previous conclusion that the r− and ρ values are governed by the thermodynamic stability of the parent ion (ring substituent = H). Other factors arising from an atom bonded to the acidic center also influence the ρ value. Copyright © 2016 John Wiley & Sons, Ltd. The β‐fluorine negative hyperconjugation of the CF3 and C2F5 groups (ΔG0β‐F) and electronic effect of the aryl group (GAel) involved in the gas‐phase acidities of 1a,b and 2a,b were evaluated. The substituent effects on the GAel and GA values of various aryl‐substituted fluoroalkanes were analyzed.