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 25� 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 SN11P mechanism which is also proposed for the analogous Be2+ + SO42- reaction. Outer-sphere ion pairs of Be(OH2)42+ with respectively F-, HF, and SO42- 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.