I was running some good old brute force reactions in the lab the other day, the kind with rock-solid reactants and products. The way to get such reactions to go, if they're a bit slow on you, is of course to heat them up. One of my Laws of the Lab, formulated back in grad school, was "A slow reaction at room temperature is Nature's way of telling you to reflux that sucker".
That's not always true - there are reactants that won't put it with that sort of treatment and find something else to do, just as there are products that are unstable to the heat that might have been used to make them. (That last situation is a natural for flow chemistry, by the way, where you might be able to get the products out of the hot zone before they have a chance to do something else). But for the things I was doing, and for many other kinds of reactions, a good blast of heat can be just the thing.
The microwave reactor is a good way to put this into practice. Seal up your reaction in a vial and tell the thing to heat up the contents to, say, 120C for half an hour. Reaction done, or not? If not, then maybe another half hour - or maybe you should set one up where you hit it at 140C for a shorter time? Or 160? Why not? You might have a bunch of five- or ten-minute reactions ready to go, and you won't know until you crank on them a bit. You might also have a shortcut to a tube of blackened gorp, but how else do you find out that you've gone too far? The nice thing about the sealed microwave vials is that they can take a good amount of pressure. You can use "normal" solvents at higher temperature than you would ordinarily. My limit is acetonitrile at about 190C in a small vial, which is about triple its standard boiling point, and gives (in my case) a pressure of about 17 or 18 atmospheres in the tube.
Now, this can take some getting used to, for less experienced chemists. One of the things that is drummed into students in the lab is the Never Heat a Closed System, and there are clearly a lot of good reasons for caution. But sometimes heating a closed system is just the thing. There are several lab-scale gizmos to allow sealed-tube reactions to be run more safely, for just these Need For Heat reasons. Another nice thing about a sealed tube is that your reactants (and products) can't get away. Running stuff in decalin or sulfolane (classic high-boiling solvents) can put you in a situation where the reaction is merrily boiling away in the flask, but some of your own materials are fleeing up the condenser in terror, likely to whoof off and vanish out the fume hood exhaust if you keep it up.
I would be a lot more circumspect about such conditions if it weren't for the robustness of the commercial microwave platform. People run stuff like this all the time, so you can blast away with more confidence. Not that you can't blow one out, especially if there's an exothermic reaction waiting to take off on you. You'll want to sneak up on a new reaction to make sure that it's not waiting for you with one of those thermodynamic jack-in-the-boxes. And keep in mind that I'm a discovery chemist. A fifty-milligram reaction is fine by me. Proposing to the scale-up group, though, that they run a bunch of sealed acetonitrile reactions at 190C will get you a different reception. You can do that stuff on larger scale, though, if you're truly motivated. That's what those big solid metal reactors with the screwed-down tops are for, but that's also what pressure monitors, blast shields, and differential scanning calorimeters are for, too. Scale matters - it matters a lot, and a liter of hot acetonitrile (much less fifty liters) under high pressure is a very different thing than a couple of mLs in a thick-walled vial. The latter could easily be one of a dozen routine reactions queued up in a microwave rack, but the former could easily be your last sight on this earth, and you'd better plan accordingly.