Comparing microwave-heated reactions in glass and silicon carbide vials suggests that heating effects alone are responsible for the benefits of performing chemistry in a microwave.
Performing chemical reactions using microwave heating has become increasingly popular owing to higher speeds, yields and selectivity than conventional methods. In addition, the ease with which reaction conditions — heating profile, temperature and pressure — can be controlled is attractive. Nevertheless, one vital question has remained unanswered: do the microwaves just heat reactions very rapidly, or do they interact in some highly selective way with reactants to produce the above results?
Now, Oliver Kappe and co-workers from Karl-Franzens University in Graz, Austria, have taken a large step1 towards resolving this issue. They performed eighteen different reactions using both a standard Pyrex vial and a custom-built vial made from silicon carbide. Silicon carbide strongly absorbs microwaves so reactions taking place within that vial were all shielded from the radiation itself, while being heated by conventional heat-transfer mechanisms. For a wide range of solvents, the heating profile using microwave irradiation was similar in both types of vials.
The conversions, yields and selectivities of all the reactions studied were virtually identical in both Pyrex and SiC vials. The benefits of microwave heating remain, but these experiments suggest that, in the majority of cases, the heating alone is responsible for the improved results.
References
Obermayer, D., Gutmann, B. & Kappe, C. O. Microwave chemistry in silicon carbide reaction vials: separating thermal from nonthermal effects. Angew. Chem. Int. Ed. 10.1002/anie.200904185 (2009).
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Davey, S. Silicon carbide solution. Nature Chem (2009). https://doi.org/10.1038/nchem.442
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DOI: https://doi.org/10.1038/nchem.442
