Adsorption structures and mechanisms of hydrogen—deuterium exchange reactions of compounds with acidic protons

Abstract

Using a microcatalytic pulse reactor containing deuterium oxide-pretreated MgO, Sm 2O 3, La 2O 3 or BPO 4 as catalysts, deuterium uptake reactions of various ketones, propanal, nitromethane and acetonitrile were examined. The reaction mixtures were separated by gas chromatography and then analyzed by mass spectrometry. In contrast to alcohols, carbonyl compounds undergo hydrogen—deuterium exchange reactions not only when base catalysts (MgO, Sm 2O 3, La 2O 3), but also when BPO 4, a solid acid, are used. Beginning at ∼60 °C, more than 10% deuterium uptake was detected. With increasing temperature deuterium uptake passed through a maximum. The lower deuterium uptake into the reactants at higher temperatures is attributed to two factors: at 200 °C less D 2O is adsorbed on the surfaces, and secondary reactions ( e.g. condensation) start to consume deuterium, forming side products. As in studies on alcohols, deuteration windows were defined for ketones and propanal that appeared between 80 and 160 °C using catalysts with strong basic sites (E1cB catalysts, solid bases) such as MgO, Sm 2O 3 or La 2O 3. In acetonitrile and nitromethane, deuterium uptake was lower than into carbonyl compounds. Using BPO 4, a solid acid, deuterium uptake into carbonyl compounds was lower compared with solid base catalysts, but higher with nitromethane and acetonitrile as reactants. In all reactants used, deuterium exchange took place in the α position to the carbonyl, nitro or cyano group. Applying the model of EPA—EPD interactions of reactants and surfaces, a common mechanism for deuterium exchange reactions into CH acid compounds could be proposed.