kilomentor | 16 January, 2009 22:05
Does microwave heating have a place in process development? In June of 2008 I attended a conference that raised this question quite a lot. The question I kept asking anyone who cared to talk about it was simply, what can microwave heating do that other methods cannot?
Microwave heating heats liquids or solids that have polar bonds. The heating comes from the flipping back and forth that the bonds dipole does trying to stay aligned with the radiation.
This heat is generated right inside the solution so that it does not have to be transmitted from the heated walls of a reactor. Thus the walls of a vessel heated by microwaves are cooler than the bulk material.
If you have a reaction that requires a high temperature and which shows degradation/impurity formation from charring on the hot walls, microwave heating can help you. Small scale reactions display the largest wall effects because they have a greater proportion of wall area to total volume, so scaling up is likely to help you anyway, even without microwave heating. The problem is that as one scales up it takes longer to reach your desirable high operating temperature. Microwave heating can help because, since the entire solution is heated from inside itself, energy can be delivered more rapidly and the temperature can be rapidly ramped up. Of course an alternative exists if you donít want to use the technology. The solvent could be heated to a high temperature before mixing with the substrate, which could be added as a slurry.
Aficianados of the technology may tell you that because the energy is being directed into specific molecules (the ones with the dipolar bonds) special reactive effects may be at work but other experienced people quietly say not to believe it. It reminds me of the efforts years ago trying to get special activation by irradiating particular IR frequencies.
What we know is that the technique is being used a lot in discovery chemistry because it works well on a small scale and it is fast. Where microwave synthesis could save process chemists time by using the larger scale continuous flow microwave devices to make kilogram quantities of lead compounds without changing the discovery chemistry. This coud save experimentation time that can be redirected to improving the real process without dividing our attention. Microwave synthesis can keep those urgent requests for a few kilograms at bay, while we work out the synthesis as it ought to be done.