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

The SN1 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.