For many years there has been debate over whether there is a specific microwave effect on chemical reactions or if it’s just a thermal effect. A couple of years ago I took lecture course on microwave and ultrasound chemistry. The course covered a few papers on the existence of a microwave effect and concluded that there isn’t anything special going on—microwaves just give very efficient and fast heating compared to normal convective heating in an oil bath or dry-syn block.
I found course particularly interesting, so whenever I see a paper on the subject I at least read the abstract to see if anything has changed. Angewandte Chemie have recently published a paper titled Microwave Effects in Organic Synthesis—Myth or Reality? (DOI: 10.1002/anie.201204103) by C. Oliver Kappe, Bartholomäus Pieber, and Doris Dallinger.
They looked at two recently published papers that allegedly found a specific microwave effect. Both claimed microwave irradiation significantly enhanced the reaction rate or yield in a way that couldn’t be replicated by regular heating to the same temperature.
Summarising a few pages: Kappe et al. couldn’t replicate the findings and argue that the problem lies in poor temperature management. To test the existence of a specific (non-thermal) microwave effect you need to run the same reaction twice at the same temperature, one with microwaves and the other normally (e.g. with an oil bath).
However the researchers who report a microwave effect use external infrared temperature probes, which record a lower temperature than the bulk reaction mixture. Microwaves heat more efficiently than the normal heating, so the microwave reaction will give you a higher yield and both vessels are in fact not at the same temperature. Instead you must use fibre optic temperature probes placed inside the reaction vessels. Doing this eliminates any microwave specific effect. To quote:
Importantly, we firmly believe that the existence of genuine nonthermal microwave effects is a myth, as all our attempts to verify these often claimed “magical” microwave effects during the past decade have failed.
It’s a good read and, I think, a nice example of science at its best. I’m also glad I read it because a colleague and I had, for some reason, been looking at getting a microwave flow reactor—which would be completely pointless, as all the benefits of microwaves in batch chemistry (high pressures and homogeneous heating) can be readily achieved in flow using normal convective heating. If anyone could tell me why such an apparently pointless bit of kit exists, I’d like to know…
Reference: C.O. Kappe, B. Pieber and D. Dallinger, Microwave Effects in Organic Synthesis—Myth or Reality?, Angewandte Chemie International Edition, 2012. DOI: 10.1002/anie.201204103