Microwave Chemistry's Thermal Effect
Clever experiment shows that microwave heating’s rate
enhancement of organic reactions is due to the heat, not
specific microwave interactions
Microwave heating has become a popular way to speed up chemical reactions, but it has
been unclear whether the rate enhancements are thermal effects or specific microwave
effects. C. Oliver Kappe, David Obermayer, and Bernhard Gutmann of the University of
Graz, in Austria, used silicon carbide reaction vials in a microwave reactor to show that the
effects in most cases are probably purely thermal (Angew. Chem. Int. Ed., DOI:
10.1002/anie.200904185). Silicon carbide strongly absorbs microwave radiation, so any
reaction occurring in a SiC vial is shielded from the microwaves themselves and only
experiences the heating of the vessel. The researchers obtained virtually identical results in
both glass and SiC vials for 18 previously published microwave-assisted reactions,
including Mizoroki-Heck couplings, alkylation of triphenylphosphine with benzyl chloride,
and Newman-Kwart and Claisen rearrangements. In addition to determining that the
microwave enhancement is generally a thermal effect, the researchers found that the high
thermal effusivity of SiC helps remove heat from exothermic reactions.