Microwaves do enhance reaction rates

Microwave ovens have become popular tools to facilitate chemical reactions, but it has been unclear if the observed enhanced reaction rates result from rapid heating or from interactions of microwaves with the bonds of the reactants. Chemical engineer Roshan Jachuck of Clarkson University, Potsdam, N.Y., and his coworkers have now shown that microwave radiation indeed has a significant impact on reaction rates (Green Chem. 2006, 8, 29). Jachuck's group designed and built a continuous capillary microreactor that can be maintained at a constant temperature while being irradiated with microwaves. The researchers used the reactor to study the iron-catalyzed conversion of benzyl alcohol to benzaldehyde. The reactor produced benzaldehyde in about 75% yield under optimized conditions, which equated to a reaction time of about 15 seconds and a flow rate of about 1 mL/minute. The key observation was that increasing the microwave intensity at constant temperature increased the yield of benzaldehyde. Jachuck envisions that frequency-tunable microwave microreactors could be used for lab- or pilot-scale projects or for production of low-volume active pharmaceutical ingredients.