 |
|
Speeding up Organic Reactions with Microwaves
|
|
By the early 1980s people had been using microwave ovens to heat and cook food quickly for many years. Microwave ovens had revolutionized the way in which meals were prepared at home and in restaurants. But, while cooking is chemistry, no one had looked at the use of microwaves to provide the "heat" required to speed up chemical reactions, particularly organic chemical reactions. Part of the reluctance to use microwaves for organic reactions was the theory that water was required to absorb the microwaves. And organic molecules and water usually don’t get along!
But in 1982, Dr. Frank Smith from Laurentian University’s Chemistry Department and Dr. John Bozic from Inco Ltd. in Sudbury, Ontario began investigating the use of microwave ovens to speed up the dissolution of ore so that the percentage of metals in the ore could be determined faster. This approach turned out to be effective and the time consuming dissolution step in these analyses was reduced significantly. Later, discussions with Ken Westaway and Richard Gedye, two professors from Laurentian University’s Chemistry Department, led them to wonder whether microwaves could also be used to speed up other reactions. Since both Gedye and Westaway were organic chemists, it was decided to see if microwaves could be used to speed up organic reactions. This pioneering work began in the summer of 1985 and was followed in early 1986 by a publication in Tetrahedron Letters, a journal for reporting exciting new developments in organic chemistry. This research showed that several different types of organic reactions could be done several times faster in a sealed Teflon® container in a microwave oven than they could by using the normal technique of refluxing the reaction mixture. Teflon® containers were used because they allow microwaves to pass through the container and into the reaction mixture. In fact, one reaction, a reaction known as an SN2 reaction between para-cyanophenoxide ion and benzyl chloride occurred over 1200 times faster in the microwave oven than it did under reflux. To put this in perspective, a reaction which took over 16 hours under reflux was done in 35 seconds in the microwave oven.
This method was shown to increase the rate of a wide variety of organic reactions and to work whenever polar molecules were used. That is, microwaves did not just excite (heat) water molecules that are responsible for the heating in the microwave oven in cooking, but would excite (heat) any molecule that had a polar bond. This is obviously why it works on such a wide range of reactions.
These workers also investigated the factors that affected the rate increases in the microwave oven reactions, showed that the reactions were, if anything cleaner (had less undesired side products) when the microwave method was used, and that the yields in the reactions were usually slightly higher than those obtained under reflux conditions.
Recently, a paper by Westaway and Gedye won the "Outstanding Paper Award" for 1995 from the International Microwave Power Institute. The research in this paper showed that microwaves do not have a special "activating" effect on the molecules but that the reactions occur more rapidly in the microwave oven because i) the reaction mixture is heated very rapidly, and ii) the reaction mixture is heated to a higher temperature, i.e., superheated under pressure, in the sealed Teflon® reaction vessels used for the microwave reactions.
This research has provided a new approach to the synthesis of organic compounds. One very important use of this method is the synthesis of labelled molecules with short-lived radioactive isotopes that can be used as tracers for medical tests. Another use is for the disposal of toxic wastes that are difficult to decompose in furnaces. Microwave radiation is also being used to produce polymers more rapidly than can be made by using conventional methods.
|
