Over the years, the use of microwave chemistry has decreased whilst Flow Chemistry is taking its place. Over 20 years ago, microwave chemistry was the optimal path for chemists to take. Microwave chemistry offered the ability to quickly overheat and pressurise reactions generally up to 20 bar on a small scale, and it was easy to get started with as the kitchen microwave was easily accessible.
The use of microwaves to heat a reaction vessel eliminates the need for a vessel jacket or oil/water bath. Microwaves can heat evenly throughout the volume of the vessel (providing it is sufficiently thin), preventing the problem arising from conventional heating which takes a ong time for the core of the reaction vessel to achieve the target temperature, reducing temperature gradients. Microwave chemistry also provides instant heating and can, at the press of a button, instantly remove the heat source. Microwave heating may have some advantages over conventional methods of heating (e.g. oil baths), including:
Although past microwave chemistry papers suggested the option of heating specific molecules in a reaction, it was soon agreed that the repartitioning of thermal energy from targeted molecules to the rest of the reaction would be so rapid as to eliminate any effects. Solid phase reactions, however, show much higher heat transfer resistances, opening up the enticing possibility of selective “hot spots” heating by microwave chemistry.
Two microwave effects are proposed: “specific microwave effects” and “non-thermal microwave effects”.
“Specific microwave effects” are those that cannot be easily emulated by conventional heating methods, such as the removal of vessel wall effects, or the selective heating of specific components of the reaction.
To explain unfamiliar observations in microwave chemistry, “non-thermal microwave effects” have been proposed and are not supposed to require the transfer of microwave energy into thermal energy. Even though the existence of non-thermal microwave effects is debated, the reference to them in various publications in the past helped boost the importance of microwave chemistry.
The rapid rise in continuous flow chemistry publications has coincided with a decline in microwave chemistry papers, and it isn’t a coincidence.
The graph above shows a sudden drop in the number of publications referencing “microwave synthesis” in 2015
One of the main disadvantages of microwave chemistry is the difficulty of scale-up because realistically, microwaves can only be used in reactors of up to a few litres, which means researchers will have to find an alternative way to heat the vessel beyond this scale and therefore may change the whole setup or reaction. Many attempts have been made to produce continuous flow microwave systems, but these are costly and provide no advantages over standard heating modules for flow chemistry.
Continuous flow chemistry systems can be compared with bathroom taps - turn it on and the flow could fill up a tiny cup, but leave the flow running and you could fill a bath. It is this ease of scale-up that makes continuous flow chemistry such an appealing synthetic technique to chemists who need to consider the scale-up of their reactions.
Microwave chemistry gained its popularity through an efficient heat transfer and the ability to provide virtually instantaneous heat, but this is also easily achieved by continuous flow chemistry systems. The capacity to rapidly cool reactions is what sets continuous flow chemistry systems apart from microwave chemistry.
One of the biggest names in continuous flow chemistry, Oliver Kappe, published a paper in 2013 along with his team strongly refuting the existence of non-thermal microwave effects. They claimed that after more than a decade of intense research in this area, they concluded that nonthermal microwave effects simply do not exist. They affirmed there were no doubts that there will be many more claims to the existence of these effects in organic chemistry (and in other fields) in the future. But, unless those claims are independently verified, they would caution the scientific community against taking the existence of those effects for granted.
For small-scale experimental batch chemistry, microwave chemistry is still an excellent and easily accessible method, but for anything beyond that, it has major disadvantages.
There is an explanation as to why laboratories that were concentrated on microwave chemistry have moved to continuous flow chemistry. In continuous flow, rapid heating/cooling and high-pressure reactions are easily accomplished, and the miniaturised design of the flow chemistry systems makes it suitable for optimising experimental chemistry and reaction. Combining the many benefits offered by continuous flow chemistry and its easy automation, integrating continuous flow chemistry into a lab is a better option than using batch microwave chemistry techniques.
Researchers at the Institute of Applied Synthetic Chemistry (Vienna, Austria) and the Department of Chemistry at Durham University (Durham, UK) have successfully translated the batch microwave process of synthesizing methyl glycosides via Fischer glycosylation into a continuous flow chemistry procedure. Fischer glycosylation – developed in the early 1890s as the earliest glycosylation protocol – still remains one of the most valuable preparative methods for simple glycosides. In 2005, Bornaghi et al. reported on the microwave-acceleration of Fischer glycosylation to overcome the long reaction times required under the classical conventionally heated processes. While this decreased reaction times, it introduced a scale-up limit; the translation of this batch technique to continuous flow has removed the scale-up limit without compromising on the reaction time improvements it gained. Read the open access paper here.
In this article, you will learn about the predictions in continuous flow chemistry and how it can increase lab performance. You can also discover the 7 things to keep in mind when adopting flow chemistry and 5 Benefits of Automated Chemistry Systems.
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