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Microwave and Chems

D_DOOD

D_DOOD

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Joined
Nov 15, 2004
Messages
281
I know for a fact that microwave heats things by making water molecules to "vibrate", and "vibration" of molecules is composes the temperature.
the thing I don't know is how microwave affects molecules other that h2o, such as random drugs or even amino acids and vitamins- suppose the said compound isn't heat sensitive, should microwaving it be ok?(not a specific question)
 
In terms of chemical reactions I think there is evidence to support that they proceed at an accelerated rate in the microwave. I would guess this compound 'X' would be ok in the microwave if it doesnt react on heating.

BTW chemical reactions occur frequently on heating food/preparing meals. Mixing carbohydrates, proteins and various flavorings around in a cooking pot really does cause lots of new compounds to be created. That is why it is the chef as well as the ingredients that make the dish.
 
Microwaves will make any molecule with an overall dipole moment flop back and forth. For most applications, this is just water, but if you were to put a beaker of liquid ammonia in a microwave (and keep the ambient temperature low enough that it would remain liquid), it would also heat up. Liquid methane however would not, since it is perfectly tetrahedral (and since the bond dipole moment between C and H is negligible), and therefore has no overall dipole moment.

Since amino acids and drugs are more complex than water or methane, there is no quick answer to what would happen. First things first, it is often quite hard to get amino acids completely dry, and if there is any water present the sample will heat up. But chances are for a drug the individual dipole moments will not cancel out, but would likely make something more complex like a quadrupole. Depending on how things are arranged, this would behave like a dipole at larger scales (ie. if you aren't looking at a single molecule in isolation) and it would heat up, although it would probably need more energy to do so since it weighs so much more than water.
 
I actually thought it was due to the microwave energy causing the OH bond to vibrate. If it was just due to a dipole moment, there wouldn't be any reason why it has to be 2.48GHz (give or take 50MHz). OH bonds have a peak in their absorbtion spectrum of 2.48 GHz. I don't think it would work with liquid ammonia because of the different absorbtion spectrum of N-H bonds (you can also heat anhydrous isopropanol, courtesy of the O-H bond, in a microwave, though it's best not to do it for too long)
 
Well, I'm hardly an expert in mirowave design. I can't prove that what you're saying is incorrect, but I know that a micrwave oven sets up orthagonal electric and magnetic fields. The magnetic field doesn't cause any acceleration (it can change the direction of a moving charged particle, but the tangential speed remains constant), but the electric field does cause oscillation of the dipole.

What actually happens is that each end of the dipole is pulled in opposite directions. The positive end is "pushed" and the negative end is "pulled" (assuming the convention of drawing electric field lines originating at the positive source of the field). This causes a torque, which is the cross product of the electric dipole moment and the electric field. The rotational motion caused by the torque will stop however once the dipole is perpendicular to the field, which is why a constant electric field is not used. The field is varied regularly with a frequency which puts it in the microwave range of EM radiation, which allows optimal rotation back and forth. In other words, it takes ~ 1/(2.48 x 10^9) seconds for one complete motion, which is why the field polarity (or direction of the field lines) is reversed ~ 2.48 x 10^9 times per second. Intensity of the field could be easily varied to increase the magnitude of the torque produced, and thus the amount of power generated by the motion (which would determine the rate at which the water heated up).

However, this could just be another side of the coin which you were arguing. By modelling the orthogonal fields as photons one could argue that the photons would be absorbed, causing a specific excitation of an electron which would then relax, releasing the energy as a bond vibration. Like I said, I'm no expert at this. However the field explanation would allow for any molecule with an overall electric dipole to be heated, while the photon explanation would only allow for molecules with an electronic configuration permitting an excited state (which is not a nonbonding orbital) with the precise amount of energy supplied by a photon in the microwave range.

Either way, chances are that D_DOOD's molecule of interest would not be damaged (assuming that it doesn't decompose when heated).
 
There used to be plenty of references to using microwaves in organic synth at the H***. From what I remember, a lot of addition and substitution reactions are accellerated using microwaves.

As for the intricate mechanism behind it, I only understand the very basics behind it; if anyone could point me at a paper or two that goes into detail, I'd be grateful.

Dave:
If I understand correctly, you're saying that the basis behind how it works is comparable to the way they make dipoles resonate in nuclear magnetic resonance imaging?
 
fastandbulbous- Sort of, but not really. In NMR the external field causes any of the nuclei with nonzero net spin (for most purposes nuclei with spin 1/2 are used, spin 1 nuclei like boron behave like magnetic quadrupoles and tend to mask other signals) to align with the field. In this situation there are two possible states: the nucleus is aligned with the field, or it is oriented opposite to the field. Obviously the lower energy state is when the nucleus is oriented parallel to the field.

The signal is then generated by causing the nuclei to flip. This is done by introducing photons (in the AM radio range if I remember correctly) which are absorbed by the nucleus, which now have enough energy to flip into the antiparallel position. However, this is not stable, and the nucleus soon flips back to the parallel position, releasing another photon of slightly less energy. This photon is detected, and is used to generate the NMR signal.

For microwaves, it isn't a magnetic field which matters but rather an electric field. Long story short, it takes advantage of the electrostatic attraction/repulsion of an electric dipoole in an electric field, which causes a similar re-orientation as in NMR. In a microwave however, this re-orientation is not constant (whereas in NMR the external field is constant at around 5 Tesla, AFAIK), since the field's polarity is varied constantly. This ensures more or less constant motion of the dipole, which on the macro scale results in the heating of the material.

There's a pretty good explanation (as well as a LOT more detail) at the link in Grignard's reply.
 
thanks alot guys.
say I'm making my own L-theanine extracts, should the microwave damage it in any way?
(the extract is for a home-made mix party-hangover pills)
 
I have no clue. If you were to find the IUPAC name you could do a chemfinder search to find the melting point, which would give a clue to its stability for the temperature range in question. I would guess that it would be fine, but it's really just a guess.

What makes you think that theanine will do any good as an afterparty remedy?
 
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