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Derek Lowe The 2002 Model

Dbl%20new%20portrait%20B%26W.png After 10 years of blogging. . .

Derek Lowe, an Arkansan by birth, got his BA from Hendrix College and his PhD in organic chemistry from Duke before spending time in Germany on a Humboldt Fellowship on his post-doc. He's worked for several major pharmaceutical companies since 1989 on drug discovery projects against schizophrenia, Alzheimer's, diabetes, osteoporosis and other diseases. To contact Derek email him directly: Twitter: Dereklowe

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« Nobel Season 2009 | Main | Ignoring Patents? »

September 30, 2009

Microwaves Aren't Magic

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Posted by Derek

Many synthetic chemists these days use microwave reactors to speed up their reactions, especially metal-catalyzed couplings. But there's been a debate ever since the technique became popular about why it works so well. Some people think that microwave irradiation is just a very efficient and fast way to heat up a reaction, while others have hypothesized some sort of microwave-specific effect, outside of the heating behavior. Metal catalysts have been particular favorites for this possibility.

The former view has been gaining ground, though, and I think we can now say that it's won. A new paper from the lab of microwave chemistry pioneer Oliver Kappe has an ingenious way to settle the argument. They've fabricated a microwave reactor vial out of silicon carbide. It's chemically inert and has very high thermal conductivity, but SiC is completely opaque to microwave frequencies. Reactions run in this vessel heat up just as quickly as those run in the same-sized glass tube, and reach the same internal pressures and working temperatures. But the contents experience no microwave irradiation at all.

Kappe and his co-workers ran a wide variety of reactions head-to-head in the two kinds of vial, including a range of metal catalysts. No differences were observed in the yields, purities, or side products for any of eighteen different types of reaction. That's good enough for me: unless someone can come up with a weirdo outlier catalyst, there is no nonthermal microwave effect on organic chemistry.

Comments (17) + TrackBacks (0) | Category: Chemical News


1. cookingwithsolvents on September 30, 2009 7:16 AM writes...

Thanks yet again for an extremely interesting and useful paper!

I've always been in the "faster heating" camp, myself.

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2. RB Woodweird on September 30, 2009 8:36 AM writes...

I can't access ACIE and am limited to reading the abstract. (A separate, off-topic rant about the giant suckitude that is Wiley is inserted here.) A while ago, I had the opportunity to try out a microwave unit for a few weeks before we bought one for another lab, so there are a couple of things that bother me. Maybe reading the whole paper will clear these up in future.

Using the same glass vial reactor that fits in the microwave well, I ran the same reaction in an oil bath at X degC and in the microwave programmed to bring the vial up to and maintain it at X degC. I found a big difference in reaction time and yield between the two. Again, the whole "magic microwave" vs "really efficient localized heating" controversy was brought up as I discussed these results within the company and with the manufacturer's technical people. The manufacturing chemists were at that time also leaning away from any participation by the microwave energy in bond breaking, and I really didn't have the need to care about why my reactions were working better than in an oil bath. They just were. (It was my impression at the time that the energy necessary to break a bond and the energy available in a microwave photon are just way to different for there to be any magic going on.)

I guess what bugs me about the abstract is that the authors run their reaction in a microwave-blocking vessel. Are they claiming that there is no difference between using a microwave and using an oil bath? Because that is simply and demonstrably false. In the simplest example, one that was demonstrated to me the day the service guy dropped off the demo unit and tested it out, a vial of deionized water put in the microwave and irradiated with a pretty good amount of energy will heat up very sluggishly. You can barely see the temperature increase. But add a pinch of salt to the water and it will shoot up to a boil in no time. I thought this showed that microwave energy was being captured in the solution. I doubt that salted and unsalted water are going to heat that differently in an oil bath.

I guess I have this nagging feeling that the experiments with the SiC tube don't prove that there is no microwave effect. They just prove that you can heat up a reaction in a microwave reactor using a SiC tube and it is the same as using an oil bath.

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3. Ty on September 30, 2009 8:52 AM writes...

all talking about bulk temperature of the solution. thers't nothing magical about the microwave itself per se, but I'm a believer of local heating. transient, extreme temperature should still be at work on the surface of SiC tube, I imagine.

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4. Andy on September 30, 2009 9:02 AM writes...

I too can't access ACIE, so can't really comment on that paper, but I think what people talk about when they refer to microwave-specific effects can be a variety of different things. A while ago there was the suggestion that microwaves might affect bond rotations, leading to different product mixtures in certain cases, but I think it was a report by Mingos that clearly explained why that wasn't, and couldn't be, the case. This is the sort of thing that is meant by 'non-thermal' microwave effects.
Then there are effects that are thermal, but difficult to obtain in an oilbath. I believe that even under non-sealed conditions the temperatures of the bulk solvent can get above the boiling point, because the heating is so rapid, and heat transfer isn't efficient enough to get to the surface molecules (very hand wavey, but it's something like that.) Microwave specific, perhaps, but certainly not non-thermal.

Also, it is often difficult to compare like to like anyway, due to the efficiency of some solvents of being heated up in the microwave. Hence, some reactions will work in the microwave more easily than otherwise, but not really microwave specifically.
Finally most MW reactions are done in sealed vessels anyway, which allows higher temperatures to be reached, although this of couse can be replicated by using the same vessel in an oil bath.

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5. weirdo on September 30, 2009 11:03 AM writes...

RBWoodweird: They weren't comparing the SiC tubes to oil bath reactions. They were using two different reaction vessels (SiC vs. Pyrex) in the exact same microwave reactor, using the exact same heating programs. And they saw no difference in almost every case. Not knowing the nuances of the ability of microwaves to get through SiC, I can't comment on the conclusions; but if the authors are correct than it seems pretty solid.

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6. RB Woodweird on September 30, 2009 11:28 AM writes...

weirdo: Thanks. That makes more sense.

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7. Sili on September 30, 2009 11:51 AM writes...

I love smart people.

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8. DevicesRus on September 30, 2009 1:31 PM writes...

I am confused. If the microwaves can't get through the SiC vial, how did the material in the water get heated?

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9. labdog on September 30, 2009 1:39 PM writes...

DeviceRus: I have the same question for the author.

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10. anon on September 30, 2009 2:20 PM writes...

That's in the paper, the microwaves heated the vial (cause electron movement in the SiC which is a semi-conductor, the resistance to electron flow generates heat) and the heat from the vial was trasferred into the solution just like when you heat a reaction in a rbf you heat the flask with the oil and the flask heats the solution.

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11. Polymer Bound on September 30, 2009 6:25 PM writes...

This isn't his first paper debunking microwaves. Kappe did another important study where he (his students) measured the internal temperature in microwave reactions with a fiber-optic probe and found that 1) there are huge temperature gradients across microwaved reaction mixtures and 2) the internal temperature is several degrees hotter than the temperature reported by the external IR sensor.

Sealed tube reactions conducted in an oil bath, at the "real" reaction temperature of the microwave reactions gave identical results.

Science wins again.

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12. PorkPieHat on September 30, 2009 6:43 PM writes...

In the early days of microwaves, it was posited that the rate accelerations could also be a micro-localized high pressure effect. Is it clear that this possibility has been ruled out with the recent experiments?

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13. Chemhack on September 30, 2009 8:54 PM writes...

@ PorkPieHat: micro localized high pressure occurs when using ultrasound where micro bubbles implode generating at the same time heat.

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14. microwaveguru on October 1, 2009 10:00 AM writes...

In the early 1980's we measured, and reported on, temperatures, measured with fiber-optic thermometry, in excess of 120 C in water being heated in a microwave oven. These temperatures certainly have a string effect on reaction rates. I don't know how often they occur in microwave-heated reaction.

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15. Lu on October 1, 2009 3:52 PM writes...

It all comes down to energy. I, too, believe, that microwaves cannot affect chemical bonds directly.
But what if there is an electrical field redistribution with its enhancement on tiny spikes on a rough surface of a metal catalyst?
(Have you ever put a mug with a metal rim in a microwave?)
Can this energy redistribution/ionization/activation be the source of microwave effect?

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16. Anonymous on October 1, 2009 6:08 PM writes...

weirdo said "They weren't comparing the SiC tubes to oil bath reactions. They were using two different reaction vessels (SiC vs. Pyrex) in the exact same microwave reactor, using the exact same heating programs."

But if the SiC tubes are completely opaque to microwaves, then the heat is being transferred from the walls of the tube, just like a pyrex tube in an oil bath. So it seems like the point is that they *are* comparing SiC to oil bath.

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17. Anonymous on October 14, 2009 4:54 PM writes...

There are examples of cooled microwave reactions (Google turns up a few). And in my experience, it's better to have continuous low power microwave energy rather than bursts of high power microwave energy. It might come down to more even heating, but it could be the microwaves are doing something themselves other than bulk heating.

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