Microwave irradiation is electromagnetic irradiation in the frequency range of 0.3 to 300 GHz. All domestic “kitchen” microwave ovens and all dedicated microwave reactors for chemical synthesis operate at a frequency of 2.45 GHz (which corresponds to wavelength of 12.24 cm) to avoid interference with telecommunication and cellular phone frequencies. The energy of the microwave photon in this frequency region (0.0016 eV) is too low to break chemical bonds and is also lower that the energy of Brownian motion. It is therefore clear that microwave cannot induce chemical reactions (O. C. Kappe, Angew. Chem. Int. Ed. 43, 6250-6284, 2004).

Microwave-enhanced chemistry is based on the efficient heating of materials by “microwave dielectric heating” effects. This phenomenon is dependent on the ability of a specific material (solvent or reagent) to absorb microwave energy and convert it into heat.

Traditionally, organic synthesis carried out by conductive heating with an external heat source like an oil bath. This is a comparatively slow and inefficient method for transferring energy into the system, since it depends on the thermal conductivity of the various materials that must be penetrated, and results in the temperature of the reaction vessel being higher than that of the reaction mixture. In contrast, microwave irradiation produces efficient internal heating (in-core volumetric heating) by direct coupling of microwave energy with the molecules (solvents, reagents, or catalyst) present in the reaction mixture. Since the reaction vessels employed are typically made out of (nearly) microwave transparent materials, such as borosilicate glass, quartz, or teflon, an inverted temperature gradient results compared to conventional thermal heating. The very efficient internal heat transfer results in minimized wall effects (no hot vessel surface) which may lead to the observation of so-called specific microwave effects, for example, in the contest of diminished catalyst deactivation. Many inorganic reactions in solution at atmospheric pressure will be accelerated by a factor of up to 10 if the reactions are performed by using microwave heating rather than conventional heating techniques. Where long reflux times are usual, the microwave-heating technique provides an invaluable tool for reducing the time scale.

The Microwave Reactor is equipped with a touch screen used for experimental planning, instrument control and reaction monitoring. The system is equipped with a robot, the cavity insert is automatically inserted and removed by the gripper and sampler arm. When the microwave vial has been inserted into the microwave cavity and the cavity lid has been closed, high frequency microwave (2.45 GHz), generated by the magnetron, heat the reaction mixture. During the heating process, the reaction mixture is continuously stirred by means of magnetic stirring. If stirring is unwanted the magnetic stirring bar is simply omitted. It is also possible to stir the reaction mixture before heating process is started to swirl up the content to improve the microwave absorption optimization and avoid large aggregates of solids that might otherwise cause vial breakage. After processing, the reaction mixture is immediately cooled with pressurized air. When the temperature of the reaction mixture has dropped to 40 °C or 50 °C, the cavity lid is opened and the microwave vial automatically will be removed.

Microwave synthesis is normally conducted under conditions that vary considerably from what is conventionally used in today’s chemistry laboratories. Using Biotage Microwave Cookbook you can browse a selection of popular reactions from Biotage PathFinder (http://www.biotagepathfinder.com/texts.jsp?textName=cookbook). Although microwave synthesis often renders results that are unique the outcome is largely governed by a few, well-known phenomena. With knowledge about these phenomena, your benefits of using microwave synthesis will be greatly enhanced.

What conditions are appropriate when performing microwave synthesis? Biotage microwave systems support a wide variety of different reaction conditions, accommodating different solvents, volumes, concentrations and phases and are characterized by reproducible results.

Under Results you can access all experiments that are available in the memory of the Microwave Reactor: http://microwave.mrl.ucsb.edu. You can view, print, save or delete them.

Please contact Krystyna Brzezinska (kbrzez@mrl.ucsb.edu) to schedule training. Before training starts please read MANUAL.