Catalytic Effect of HF

The value of the pH for optimum stability of silicic acid depends on what impurities are present in the solution. Traces of aluminum ions and to a lesser extent, iron, thorium, and beryllium ions tend to offset the effect of fluoride ion by forming complex fluorides and thus retard polymerization in this pH range. Depending on the purity of the solution, the pH of optimum stability may range from 1 to as high as 3-3.5. In silicic acid. solutions free from aluminum impurity, as little as 1 ppm of fluorine has a marked effect on the rate of polymerization in acid solution. 

Her advanced the hypothesis that the polymerization mechanism may involve the temporary expansion of the coordination number of silicon from four to five or six. just as in the case of OH ion. It is not known whether the HF-catalyzed reaction is third order as in the H -catalyzed reaction in the absence of fluoride below pH 2. or 

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The catalytic effect of HF on the fluorination of arenes by XeFg has been discussed by Liebman and Vanderspurt in terms of its polarization of the Xe—F bond. Such catalysis may be operating in the recently reported benzoyloxylation of benzene. Treatment of benzene with benzoic acid and XeFg in excess benzene leads to the reaction ... 

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