[Enthalpy]

For my personal information (sorry for not bringing you further):

Do microwaves bring anything special to the reactions, or are they only a convenient means of heating?

[Corribus]

In principle, if you heat a mixture of reactants to some temperature, it doesn't matter how you do the heating. However in a microwave because the vessel is pressurized, you are able to heat to a much higher temperature. The practical benefit is that the reaction takes less time to accomplish - this is the basis of a pressure cooker, after all. However, because the temperature is quite a bit different, the reaction mixture will behave differently as well. The relative rates of the primary reaction and unwanted side reactions will change to some degree. Not having done a lot of microwave reaction chemistry, I couldn't speculate on how much difference there is, but I have heard reports from colleagues of some reactions working better in a microwave, and other reports of some reactions working worse. I suspect it's very much case-specific whether the product mixture will be better, worse, or unchanged compared to conventional heating methods.

[curiouscat]

In principle, if you heat a mixture of reactants to some temperature, it doesn't matter how you do the heating. However in a microwave because the vessel is pressurized, you are able to heat to a much higher temperature.

So is that the same as a conventional autoclave? Even with a mantle & a pressure vessel you could get to high pressures right?

I was wondering about two other modes:

(a) Whether like in photochemistry there was any chance of a direct mode transfer of energy to the bond breaking of interest?

(b) If the local T / P conditions (as opposed to global averages) are much higher in a microwave? i.e. say a reaction was only significant above 900 C a 500 C average T microwave may have tiny pockets of 900 C?

[Corribus]

(A) Mode-selective chemistry applies primarily to vibrational or vibronic modes. I can't imagine how microwaves, which access primarily rotational states, could drive mode-selective reaction routes. But I'm open to counterarguments.

(B) This is one reason why microwaves for chemical applications are engineered to minimize hot spots. Vigorous stirring should also help to distribute heat energy throughout the reaction mixture. Of course, the larger your reaction vessel is, I could see heat distribution issues being a larger problem. Paritcularly in large samples, microwave penetration to the inner parts of the sample will be very inefficient. (Think how long it takes to microwave a big bowl of soup! The outside is scalding hot and the inside is still practically frozen.) Solving those kind of problems is why we keep you chemical engineers around.

[curiouscat]

Solving those kind of problems is why we keep you chemical engineers around.

Ah yes. Thanks for reminding me that I can be actually useful on some rare occassions. 

Jokes apart, my point was: Could a hot spot be a feature in some cases rather than the bug that it usually is? I wonder.

[Enthalpy]

If not for other purposes, you'd all be useful to bring chemistry light (and delight) to me 

RF and microwaves being a mere means of heating, this was the way I believed to understand them. In addition to the low photon energy, the too short free flight time precludes selectivity: with EM wave periods of 10µs to 0.1ns, a shock every ps in a liquid or 0.1ns in air means that energy is redistributed before it accumulates. Consistently, we observe no resonances, and for instance the absorption by water is accurately modelled by the lossy orientation of water molecules plus ionic conduction.

Though, surprises do happen, and for instance the coherer used over a century ago isn't still well explained
http://en.wikipedia.org/wiki/Coherer
so if non-thermal effects were observed in chemical reactors, the next task would be to interpret them - physics as well is an experimental science...

A catalyst of palladium or metal wire would more likely show non-thermal effects in microwaves than Pd(II) complexes.

Hot spots, big volumes: just take a lower frequency to obtain even heating. Call it radiofrequency instead of microwaves if you want. Already used in food processing to dry, cook... Polarization losses are bigger at 2.45GHz but ISM power is produced at any frequency: ~200kHz, 13.56MHz, 434MHz... as you wish.