Ia. Koppel et al., THERMODYNAMIC ACIDITY OF (CF3)(3)CH AND 1H-UNDECAFLUOROBICYCLO[2.2.1]HEPTANE - THE CONCEPT OF ANIONIC (FLUORINE) HYPERCONJUGATION, Journal of the American Chemical Society, 116(19), 1994, pp. 8654-8657
The gas-phase equilibrium acidity of 1H-undecafluorobicyclo[2.2.1]hept
ane (I), 1H,4H-decafluorobicyclo[2.2.1]heptane (II), and tris(trifluor
omethyl)methane (IV) has been determined by pulsed FT-ICR technique. A
lso, a comparison study of the thermodynamic acidities of I and IV in
DMSO solution has been made. These experiments have long been consider
ed crucial for the evaluation of the role of the anionic (fluorine) hy
perconjugation on the stability of aliphatic fluorosubstituted carbani
ons. Substituent field/inductive, polarizability, and steric effects f
or I, II, and IV can be assumed to be similar. The significantly great
er observed acidity of the open-chain IV, compared to its bridgehead c
ounterparts I and II, can be reasoned to be the inability of the latte
r acids to form planar anionic structures (Bredt's rule) that is a req
uirement for maximum hyperconjugation. Both the new gas-phase and the
DMSO measurements show that IV is by many powers of 10 more acidic tha
n I and II. Together with the existing experimental and the theoretica
l work, important evidence is provided that qualitatively favors the c
oncept of anionic hyperconjugation.