Microwave irradiation - a closer look at heating efficiencies

Microwave irradn. is rapidly evolving into a common heat source in different areas of chem. including medicinal and org. chem. as well as polymer chem. The major advantages of the use of microwave irradn. are the often obsd. faster and cleaner reactions and sometimes changes in selectivity. Although the majority of beneficial effects might be explained by the faster heating, higher attainable temps. due to closed reaction vessels and the more homogeneous heat profile, in some cases selective heating results under microwave irradn. results in changes in selectivity or it allows chem. that cannot be performed with conventional heating. Besides these well-documented advantages of the use of microwave irradn., it is often claimed that microwave irradn. is a fast and efficient heat source. To investigate the validity of the claimed efficient heating under microwave irradn., we examd. the efficiency of microwave heating various solvents under microwave irradn. using a monomode microwave reactor. Examples of temp.-time profiles and the corresponding efficiency curves are shown in the Figure. In addn., the effects of adding salt or passive heating elements on the heating efficiency were studied. The heating efficiency for water was further examd. using different reaction vols. as well as monomode and multimode microwave synthesizers. [on SciFinder (R)]