Traditionally, organic synthesis is carried out by refluxing a reaction mixture using a hot oil bath as a heat source. However, this way of heating a reaction mixture is comparatively slow and energy inefficient, since first the heat energy is transferred from the hot oil bath to the surface of the reaction vessel, and then the hot surface heats the content of the reaction vessel (see Figure 6, entry a). Furthermore, the hot surface can lead to local overheating and to decomposition of sensitive material.
In contrast, microwave irradiation results in energy efficient internal heating by direct coupling of microwave energy with dipoles and/or ions that are present in the reaction mixture. Microwaves pass through the (almost) microwave-transparent vessel wall (see Figure 5, transmission) and heat the reaction mixture on a molecular basis – by direct interaction with the molecules (solvents, reagents, catalysts, etc., see Figure 5, absorption). Due to this direct “in-core” heating (no initial heating of the vessel surface), microwave irradiation results in inverted temperature gradients as compared to a conventionally heated system (Figure 6, entry b).
Furthermore, the conversion of electromagnetic energy into heat energy works highly efficiently and results in extremely fast heating rates – not reproducible with conventional heating. Due to the rapid heating to the target temperature, the formation of byproducts is suppressed. This is another huge advantage of microwave heating, since it means that higher product yields can be achieved and the work-up is simplified.