Gas-Phase Acidity of Polyfluorinated Hydrocarbons. Effects of Fluorine and the Perfluoroalkyl Group on Acidity

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Abstract
Gas-phase acidities of polyfluorinated hydrocarbons have been determined by measuring proton-transfer equilibria and by computing the free energies of deprotonated carbanions and the corresponding neutrals. An excellent linear relationship between acidities and the accumulated inductive effects of fluorine atoms contained in a molecule was observed for the perfluoroalkyl-substituted neopentanes, (Rf1)(Rf2)(Rf3)CCH3, and polyfluorinated bridgehead carbon acids where the contribution of negative hyperconjugation of the Cβ–F bond to the stability of the conjugate anions is absent or negligibly small. On the basis of this relationship, the extent of β-fluorine negative hyperconjugation involved in acidities of polyfluorinated hydrocarbons could be evaluated quantitatively. The negative hyperconjugation was found to be negligibly small in the stable tertiary polyfluorinated carbanions while in the less stable primary and secondary carbanions the contribution of this effect is present certainly, indicating that the negative fluorine hyperconjugation is complementary to the stabilization by the accumulated inductive effect of fluorine atoms. The extent of negative hyperconjugation was found to increase in order of CF3CH2− < C2F5CH2− ≈ C3F7CH2− < i-C3F7CH2−, being qualitatively consistent with the elongation of the Cβ–F bond by deprotonation. The effect of α-fluorine on acidity was found to change complicatedly with the carbanion, e.g., the α-fluorine substitution in CF3CH3 strengthens acidity, no effect in (CF3)2CH2, and strengthens again acidity in (i-C3F7)2CH2. Such varying effect of α-fluorine in the polyfluorinated hydrocarbons would be caused by a subtle balance of effective electronegativity of an anionic center carbon, a varying p–p lone pair repulsion depending on the net negative charge at the formal charge center carbon, and the change in ability of β-F negative hyperconjugation caused by α-F substitution.

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