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More Details On Microwave Effects

Chemists find little difference in reaction rates between microwave and conventional heating once reaction temperatures are achieved

by Stephen K. Ritter
June 16, 2014 | A version of this story appeared in Volume 92, Issue 24

Palladium-catalyzed cross-coupling reactions have previously been reported to proceed substantially more quickly with microwave irradiation than when they are heated to the same temperature by conventional heat sources such as an oil bath. A number of researchers have investigated this phenomenon and come to a range of opinions on the exact cause. New data from Andrew W. Holland and coworkers of Idaho State University support the consensus view that microwaves induce only a thermal effect and that nonthermal radiation effects are not at play (React. Kinet. Mech. Catal. 2014, DOI: 10.1007/s11144-014-0733-z). Unsatisfied with prior explanations for faster microwave Buchwald-Hartwig aminations, Holland’s group constructed experimentally difficult kinetic profiles of reactions and found that the rate improvement achieved by microwave heating occurs mostly in the beginning of a reaction and is magnified by catalyst deactivation. The faster rates can be reproduced in an oil bath simply by mixing preheated reaction components, which mimics the short time it takes to ramp up to the reaction temperature. “Such conclusions take nothing away from the value of microwave ovens as tools in synthetic laboratories,” the researchers write. “Microwaves often remain far more convenient than alternatives—but they illustrate that the advantages of microwaves can often be enjoyed using more traditional methods.”


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