Heterogeneous catalysis in solution. Part 20.1—Catalysis by silver bromide and other solids of the solvolysis of t-butyl bromide in 80 vol% ethanol + water

Abstract

The S_N1 solvolysis of 2-bromo-2-methylpropane (t-butyl bromide) in 80 vol%(55.2 mol%) EtOH + H₂O at 25 °C was found to be catalysed by silver and several insoluble silver and mercury salts but not by BaSO₄, SiO₂ or platinum. The relative catalytic effectiveness of these solids could be largely explained by Pearson's SHAB concept. The kinetics of the AgBr-catalysed process were studied in detail by a pH-stat technique. The results fitted a model in which t-BuBr molecules and product Br^– ions competitively adsorbed on the surface according to Langmuir isotherms. Calculations showed the surface reaction to be ca. 10⁵ times faster than the homogeneous solvolysis and to possess a much lower activation energy. The heterogeneous reaction was found to be selective, forming no alkene product and a higher ratio t-BuOEt:t-BuOH than in the bulk solution. These findings have been discussed in terms of the structure of the interfacial region.