Gas‐phase acidity of 1,1‐bis(trifluoromethanesulfonyl)propane derivatives and related compounds: experimental and theoretical studies

The gas‐phase acidity (GA) of a series of 1,1‐bis(trifluoromethanesulfonyl)propane derivatives, Tf 2 CHCH 2 CH(R 1 )R 2 , and Tf 2 CHCH 2 Ar (Ar =  phenol derivatives ) was determined by measuring proton‐transfer equilibria using a Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometer and by computing the free energy of the deprotonated carbanions and the corresponding neutrals. The effects of R 1 and R 2 or Ar groups on the acidity were examined. In Tf 2 CHCH 2 CH(R 1 )R 2 , the GA values calculated using the conformers of neutral molecules that are free from the intramolecular hydrogen‐bonding interaction between the acidic hydrogen atom of the Tf 2 C H moiety and R 1 or R 2 group and between the hydrogen atom of the C H R 1 R 2 moiety and the SO 2 CF 3 group were correlated in terms of an equation, GA = −17.0Σσ I  + 3.4Σσ α  + 299.5. On the basis of this correlation, it was elucidated that the intramolecular hydrogen bonding or dipole–dipole interaction in the neutral molecule weakens significantly the acidity. In Tf 2 CHCH 2 Ar ( 5 , 6 and 7 ), the GA was strengthened by the strong hydrogen‐bonding interaction between the phenolic hydrogen in the aromatic moiety and the SO 2 CF 3 group in the conjugate anion compared with that in the neutral molecule. Copyright © 2014 John Wiley & Sons, Ltd.