Dramatic Effect of the Specific Solvation on the Reactivity of Quaternary Ammonium Fluorides and Poly(hydrogen fluorides), (HF)n*F-, in Media of Low Polarity

Article abstract of DOI:10.1021/jo00263a013

A quantitative study of how the intrinsic reactivity (nucleophilicity and basicity) of the fluoride anion of hexyl4N⁺F^- is affected in solvents of low polarity by the specific solvation of a limited number of water molecules has been performed.The nucleophilicity enhancement is extrapolated to be about 3 orders of magnitude by reducing the specific hydration n of the anion from 8.5 to 0.Such enhancement is much higher (ca100 times) than that obtained, under the same conditions, by dehydrating the other halides.The nucleophilicity scale of anhydrous anions found,F^- >> Cl^- > Br^- > I^-, reflects those well-known in dipolar aprotic solvents and in the gas phase.Comparison in the same hydration range shows that the basicity of the fluoride anion is much more affected by specific solvation than is its nucleophilicity.Extension of this study to quaternary ammonium poly(hydrogen fluorides) Q⁺(HF)n*F^-, where n = 1, 2, provides the following reactivity scale: F^- >> HF2^- > H2F3^-.The increasing stabilization of F^- anion, by interaction with hydrogen fluoride in the sequence F^- << HF2^- < H2F3^-, accounts for the much lower reactivity observed in the case of poly(hydrogen fluorides) with respect to that of the hypothetical anhydrous fluoride (F^- : HF2^- : H2F3^- = 8.5*10⁵ : 1.2*10² : 1).This also explains the different sensitivity of these anions to the specific hydration which decreases in the same order: F^- >> HF2^- > H2F3^-.