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Substituent effects on acidity of aryl‐substituted fluoroalkanes: a computational study
Author(s) -
Mishima Masaaki,
Abboud JoséLuis M.
Publication year - 2016
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.3489
Subject(s) - substituent , chemistry , hyperconjugation , aryl , fluorine , polar effect , medicinal chemistry , deprotonation , electronic effect , alkyl , benzene , inductive effect , stereochemistry , steric effects , organic chemistry , ion , molecule
The gas‐phase acidities ( GA ) of various aryl‐substituted fluoroalkanes, XC 6 H 4 CH(R 1 )R 2 , 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 R 1 and R 2 . Their changes in acidity of 1 and 2 having two strong electron‐withdrawing groups (CF 3 or C 2 F 5 ) 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 ( R 2 > 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 ( ΔG o β‐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 GA el values obtained by subtraction ΔG o β‐F from the apparent GA value were successfully correlated in terms of the Yukawa–Tsuno equation. The obtained ρ el and r − el values were linearly related to the GA el 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.