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September 30, 2009
Microwaves Aren't Magic
Many synthetic chemists these days use microwave reactors to speed up their reactions, especially metal-catalyzed couplings. But there's been a debate ever since the technique became popular about why it works so well. Some people think that microwave irradiation is just a very efficient and fast way to heat up a reaction, while others have hypothesized some sort of microwave-specific effect, outside of the heating behavior. Metal catalysts have been particular favorites for this possibility.
The former view has been gaining ground, though, and I think we can now say that it's won. A new paper from the lab of microwave chemistry pioneer Oliver Kappe has an ingenious way to settle the argument. They've fabricated a microwave reactor vial out of silicon carbide. It's chemically inert and has very high thermal conductivity, but SiC is completely opaque to microwave frequencies. Reactions run in this vessel heat up just as quickly as those run in the same-sized glass tube, and reach the same internal pressures and working temperatures. But the contents experience no microwave irradiation at all.
Kappe and his co-workers ran a wide variety of reactions head-to-head in the two kinds of vial, including a range of metal catalysts. No differences were observed in the yields, purities, or side products for any of eighteen different types of reaction. That's good enough for me: unless someone can come up with a weirdo outlier catalyst, there is no nonthermal microwave effect on organic chemistry.
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