University of Graz, Austria
A chemist muses on the technique of microwaving flowing chemical reactions.
A hot topic in synthetic chemistry is continuous-flow processing involving ‘microreactors’ or related devices. Making molecules using a continuous-flow process has several advantages over the more conventional batch approach: for example, reaction conditions can be easily scaled up to production capacities without the need for further optimization.
Chao-Jun Li at McGill University in Montreal and Michael Organ at York University in Toronto, both in Canada, and their colleagues combined continuous-flow processing, using standard glass capillaries, with microwave irradiation to perform extremely fast chemical reactions (G. Shore et al. Chem. Eur. J. 16, 126–133; 2010). The team generated propargyl amines, which are important synthetic intermediates, using a three-component reaction.
To achieve efficient microwave heating, the researchers coated the insides of the flow tubes with thin films of metals such as gold and copper. Analysis by scanning electron microscopy showed that these metal films typically consisted of clusters of metal nanoparticles, which have been shown to catalyse a variety of valuable synthetic transformations.
What struck me in this study was the achievement of such high reaction temperatures — in excess of 900 °C on the outer surface of the glass tube — with comparatively low levels of 2.45-gigahertz microwave irradiation.
Reactions at such high temperatures are extremely fast. One could speculate that, in these conditions, some chemical transformations might occur in the gas phase, as seen in flash vacuum thermolysis, a technique that is used to perform high-temperature organic reactions in the gas phase. This could potentially open up an easy way to perform high-temperature gas- or liquid-phase reactions in a scalable manner.