Rates of substitution of the hydrated beryllium cation by hydrogen fluoride and fluoride ion

Abstract

The rate of formation of the monofluoroberyllium species Be(OH₂)₃F⁺ was measured by a conductimetric stopped-flow technique. At acid concentrations exceeding 0.01M two parallel reaction paths were identified: Be(OH₂)₄²⁺ +F- ↔ Be(OH₂)₃F⁺ (k₁,k_-1) Be(OH₂)₄²⁺ +HF- ↔ Be(OH₂)₃F⁺H⁺(k₂,k_-2) At 25º and an ionic strength of O.2M, k₁ = 724 ± 95 l. mole^-l sec^-1 and k2 = 73 ± 3 l. mole^-1 sec^-1. Both substitution paths exhibit activation energies of 8.9 ± 0.8 kcal/mole. The data are interpreted in terms of an S_N11P mechanism which is also proposed for the analogous Be²⁺ + SO₄^2- reaction. Outer-sphere ion pairs of Be(OH₂)₄²⁺ with respectively F-, HF, and SO₄^2- undergo substitution with the same activation energy. A linear free energy relation exists between the rate of formation of the inner-sphere complexes from the outer-sphere ion pairs and the accompanying free energy changes.