A recent C&E News article brings up the topic of microwaves as magical or “a box that heats.” For a fun read I turn your attention to the article.

With the article we have come to enjoy the usual cast of characters — Oliver Kappe and Greg Dudley. This article attempts to keep the shots at bay but you can still feel the tension — love it — that’s how things progress. Let me simply state the positions and then comment: Oliver Kappe – the microwave provides a mechanism for reactions to proceed quicker by accelerating the bulk temperature of the reaction medium (notwithstanding some of the inorganic hydrothermal nucleation). He has gone on to explain that we often have trouble getting the correct temperature to explain subtle differences in reaction rates, such as hot spots from heterogeneous catalytic activity as an example, but that the microwave energy applied results in heat that is measured as the bulk temperature of the reaction. Dudley on the other hand has published work illustrating an open-vessel microwave acceleration of an ionic benzylation in a non-polar, non-microwave absorbing solvent — toluene. Their position is that the rate is much faster than explained by the bulk temperature reaction rate and that things are happening at a molecular level — or selective heating in the reaction.

Part of the issue comes in the form of scale, sufficient agitation to call something “homogeneous” and the type of microwave power that is being applied. There is a significant number of papers in the literature that suggest single-mode microwaves can get a reaction to accelerate differently than a multimode. This is partly true, but only in that if the microwaves are evenly distributed in the cavity and more importantly in the reactions themselves AND sufficient agitation to call them equal — then there should be little to no difference between the two approaches. The problem with that is that organic chemists tend not to do things the same way from lab to lab, so agitation and measuring temperature (major differences between the commercial microwave companies) SO it complicates the picture or the canvas painted if you will.

The article goes on to show an additional example of a reaction that is accelerated past explanation (acceleration with high enantiomeric excess maintained vs. thermal conditions). It is an interesting read — the use microwave power but cool the reaction as well, complicating the story. I hope however, that people take notice and it provides a possibility for chemists to utilize microwaves for more chiral development — we haven’t done enough in this area and there is opportunity.

I find it fascinating that there is some pushing of the envelope rather than simply using the technology without understanding the process….too much of that going on. My hope is that someone will be able to provide a way to measure temperatures of nucleation events in the inorganic field to show differences in temperatures between the nucleation and the bulk temperature. Maybe it would produce more hand waving, but the evenly distributed particles formed in these reactions are different than conventional heating…would be an interesting DOE. And with all the talk on temperature — what about the effects of added pressure or generated pressure during the course of a reaction? Let me clarify – we can now raise the boiling points of solvents in the course of a reaction by adding pressure — can the catalysts or “hot spot heating” simply accelerate the reaction? I imagine there are some differences here that are noteworthy, much like ultrasound in combination with microwave.

 

Advertisements