Sciencemadness Discussion Board

Microwave transperant solvents

User - 22-1-2010 at 06:43

Hi,

Lately ive been increasingly interested in microwave chemistry.
There is a lot to discover/try etc.

First of all , has anyone got a list of microwave transparent solvents?
Or any documentation that can provide supporting data.

Secondly, How could i check whether solvents aren't heated or react by the radiation without testing it.



Physics are not really my area.
Using mwr might be more physics than chemistry.
If anyone can help me , thanks a lot.


(since solvents are often organic i thought it would be best to post it here)


[Edited on 22-1-2010 by User]

crazyboy - 22-1-2010 at 07:18

"Non-polar solvents (e.g. toluene, dioxane, THF) are microwave transparent and reactions in these solvents can be heated only if other components in the reaction mixture respond to the microwave energy, i.e. if the reaction mixture contains either polar reactants or ions. When using less polar solvents, more concentrated reaction mixtures might be preferable. "

From the first hit on Google.

bahamuth - 22-1-2010 at 07:23

Dipole solvent are not transparent to microwaves, one should expect at least.

Cannot give much advise other than except just to try and irradiate it and see if any changes occur, temperature, color, smell, bp., Abs.(Absorbency spectrum), refractive index etc.

They should be anhydrous to get true results, or very low in water content.

Nicodem - 22-1-2010 at 13:59

Quote: Originally posted by User  
Lately ive been increasingly interested in microwave chemistry.
There is a lot to discover/try etc.

You are in for a big disapointment. There is not much left to discover as there was little to discover already in the first place. And because MW assisted synthesis was one big fad, and this seems to still persist up to these days, there was already too much published on this topic, sadly big part of it just crap papers.
The MW reactors are still expensive equipment, especially considering they are nothing else but fast heating autoclaves. They might be easy&clean&controllable to use for routine work when compared to classical closed cap vessels immersed in oil bath, but that is about it. If you have a dozen thousands of euros surplus to pay for that easy&clean&controllable then they are indeed nice toys to play with.
Quote:
First of all , has anyone got a list of microwave transparent solvents?

You can find plenty of tables with dielectric constants for common solvents on the net. The one provided by S*g*a is quite commonly stuck on the walls of laboratories. The dielectric constant is not so constant and it changes with increasing frequencies, but this data will still give you the general picture.
Quote:
Secondly, How could i check whether solvents aren't heated or react by the radiation without testing it.

You read the temperature from the display. Every MW reactor is equipped with an IR sensor for temperature measurement or else it could not regulate power.
Quote: Originally posted by crazyboy  
"Non-polar solvents (e.g. toluene, dioxane, THF) are microwave transparent ....

THF and dioxane absorb MW energy just fine. Toluene is a bit problematic. I have to set the power limit of the MW reactor to the max for it to reach temperatures of >120°C (no way to get up to 200°C). Yet, the low MW absorption of solvents is rarely a problem, because most solvents used in organic synthesis have at least some minimum dielectric constant even at the MW frequencies. But if you need to heat something in toluene at 200°C you can always add a drop of a better MW absorbing solvent (DMF, DMSO, alcohols, or a tiny amount of a ionic liquid).

chief - 23-1-2010 at 05:36

Any fat/oil poorly absorbs, and so then probably gasoline etc. ...

User - 23-1-2010 at 06:07

Iam not going to reveal too much about what I am planning to do but it is about extractions from organic material.
Mwr is good at breaking cel structures.
I am thinking about constructing a write up when my current testing comes to some conclusions, its looking good so far.

Low flameability would be nice for indoor experiments :)
Total transparency would be first choice but as long as the solvent doesn't heat up like hell it can be workable.
For example ethylene glycol gets warm in a MW but it takes effort.
Rather loose some solvent than rebuilding the device to add a reflux condenser.

Fleaker - 24-1-2010 at 08:39

Nicodem, I can think of many polymerizations that will work in minutes in the microwave but will take hours (or days) done conventionally. It is very useful for making many things quickly. Why the disdain for microwave irradiation?





For reference to the thread starter: http://cem.com/page.php?2

Nicodem - 24-1-2010 at 11:41

Quote: Originally posted by Fleaker  
Nicodem, I can think of many polymerizations that will work in minutes in the microwave but will take hours (or days) done conventionally. It is very useful for making many things quickly. Why the disdain for microwave irradiation?

I would not say that I'm disdainful toward microwave reactors themselves, but I sure am disdainful toward the tasteless abuse of these machines to get all kind of crap science published just because it is so hip. Maybe this f(r)a(u)d never approached the disgustfulness of the green chemistry fad, but it sure did its best to deteriorate science just one additional step further. The bullshit that got published on MW assisted synthesis is misleading to many and User is one fine example. He knows near to nothing about microwave reactors, I'm quite sure he does not even have access to any, and from his questions it is obvious he is was mislead badly into believing it does some kind of magic. And this is not even the first time. We already had one member who even went so far as to believe that kitchen MW ovens can be used instead of programmable MW reactors. Needless to repeat again how dangerous it can be to put a closed vessel into a MW oven. A kitchen MW oven has no IR thermometer coupled with its power control, it has no pressure sensor for a closed vessel, it has no adequate ventilation to prevent flammable solvent vapours from simply exploding... and moreover there is no rational reason to heat a reaction mixture with MW up to an unknown temperature under such uncontrolled conditions when you can use proper methods like reflux or oil baths or whatever else. A MW reactor has about nothing in common with a kitchen MW oven, except the magnetron.
Just to clear up things, about one third of all reactions I run (at the job), I do so in a microwave reactor, particularly tests on new reactions on a 0.5-5 mmol scale. I admit that it is a much more convenient and controllable way of heating up reactions, it is cleaner and faster than oil baths, and heterogeneous reactions often proceed faster than just using closed cap vessels at the same temperatures. There are many benefits and even though it was demonstrated already several times that microwave heating has no influence on the kinetics of homogenous reactions, I still prefer to use microwaves over classical heating wherever applicable. I think soon the majority of labs will use this technology on routine basis. But this does not mean I'm going to eat every shit that gets published about it. If it smells like shit and tastes like shit, then it most likely is shit - and crap science is full of shit.

kmno4 - 24-1-2010 at 13:19

Quote: Originally posted by User  
Hi,

Lately ive been increasingly interested in microwave chemistry.
There is a lot to discover/try etc.

First of all , has anyone got a list of microwave transparent solvents?
Or any documentation that can provide supporting data
.............]

There is a lot of literature devoted to MW in chemistry.
For example this book: "Microwaves in Organic Synthesis" (Wiley, 2002).
Quote: Originally posted by Nicodem  

...But this does not mean I'm going to eat every shit that gets published about it. If it smells like shit and tastes like shit, then it most likely is shit - and crap science is full of shit.

Heh, I agree. Especially journals in kind of Syth.Comm. etc
are full of joyful creativity in question of MW.

turd - 24-1-2010 at 13:21

What follows is a little bit off-topic, please be indulgent with this whacked out freak... :)

I have no doubt that all you say is correct for the synthesis of organic molecules, nevertheless I have to take up the cudgels for standard kitchen MWs. These things absolutely rock for inorganic reactions in a home setting! With the correct technique, you can easily reach 1000°C with them, and thus make reactions in molten NaCl or similar reaction media. When you consider the insane prices suppliers charge for muffle type furnaces, kitchen MWs are a godsend.

End of off-topic transmission...

Panache - 28-1-2010 at 22:08

Quote: Originally posted by Nicodem  
Quote: Originally posted by Fleaker  
Nicodem, I can think of many polymerizations that will work in minutes in the microwave but will take hours (or days) done conventionally. It is very useful for making many things quickly. Why the disdain for microwave irradiation?

I would not say that I'm disdainful toward microwave reactors themselves, but I sure am disdainful toward the tasteless abuse of these machines to get all kind of crap science published just because it is so hip. Maybe this f(r)a(u)d never approached the disgustfulness of the green chemistry fad, but it sure did its best to deteriorate science just one additional step further. The bullshit that got published on MW assisted synthesis is misleading to many and User is one fine example. He knows near to nothing about microwave reactors, I'm quite sure he does not even have access to any, and from his questions it is obvious he is was mislead badly into believing it does some kind of magic. And this is not even the first time. We already had one member who even went so far as to believe that kitchen MW ovens can be used instead of programmable MW reactors. Needless to repeat again how dangerous it can be to put a closed vessel into a MW oven. A kitchen MW oven has no IR thermometer coupled with its power control, it has no pressure sensor for a closed vessel, it has no adequate ventilation to prevent flammable solvent vapours from simply exploding... and moreover there is no rational reason to heat a reaction mixture with MW up to an unknown temperature under such uncontrolled conditions when you can use proper methods like reflux or oil baths or whatever else. A MW reactor has about nothing in common with a kitchen MW oven, except the magnetron.
Just to clear up things, about one third of all reactions I run (at the job), I do so in a microwave reactor, particularly tests on new reactions on a 0.5-5 mmol scale. I admit that it is a much more convenient and controllable way of heating up reactions, it is cleaner and faster than oil baths, and heterogeneous reactions often proceed faster than just using closed cap vessels at the same temperatures. There are many benefits and even though it was demonstrated already several times that microwave heating has no influence on the kinetics of homogenous reactions, I still prefer to use microwaves over classical heating wherever applicable. I think soon the majority of labs will use this technology on routine basis. But this does not mean I'm going to eat every shit that gets published about it. If it smells like shit and tastes like shit, then it most likely is shit - and crap science is full of shit.


Wow a rant from Nicoderm, however i do agree, a large no. of the papers seem to emanate from India where i believe entire departments where formed to simply repeat know experiments in a microwave and publish the data, calling it science. That was a joke but with some truths thrown in.

One point missed in this discussion is the elevated boiling points achievable in a monomode microwave (or a conventional microwave with the rotating platform removed and the sample positioned on one of the nodes). It allows for substantially increased solubility when doing recrystallisations. Although i have not read any research published on this i have used the technique multiple times and it works exceptionally well, especially if the sample is left in the microwave to cool (this minimises the chance of the solution boiling over when removing it.)

edit--another point i have noted over the last few years of using a microwave as a hotplate is that certain brands of glassware appear to absorb the radiation better (and thus heat up) than others. The reasons behind this are fairly easily explained by virtue of the components of the different brands. i must keep record from now on when i observe the phenomena. From a rather weak perspective i believe the duran glasses absorb poorly whilst those from england (i believe in times gone past the UK used to make varieties of pyrex/borosilicate) absorb well. However this observation is weak. Does anyone have any similar observations, i have little Kimax glass so i have no observations regarding it.


[Edited on 29-1-2010 by Panache]

User - 29-1-2010 at 05:13

I indeed noticed the same thing about glass heating up.
Funny how some types of glass stay absolutely cold and others get warm, not really hot.
Could the thickness of the glass be involved or is just the material responsible?

Ceramic's also heat up, these can get very very hot, burnt my fingers a couple of times.
Really wonder what temperatures can be achieved.

Just for the sake of clarity.
I dont believe any paper that seems to be official.
There is probably a lot of crap written.
But this doesnt change my line in the first post.
There is a lot to discover and test.
For example I am really a fan of dehydration in the MW.
Even domestic microwaves are capable of quite a lot.



[Edited on 29-1-2010 by User]

Synthesis Technologist - 29-1-2010 at 06:20

Having worked with most of the currently available lab-scale MW reactors, i can tell you that nothing is as convenient as working with a monomode reactor having an autosampler.

Currently none of the big pharma companies works without microwave reactors.

The >25.000 - 35.000 € for example a single mode reactor from Biotage, Anton Paar or CEM will cost are only a small investment compared to the increase in productivity / and saves a lot of costly working time. - You are making many reactions 100-1000 fold faster than they would be at RT/reflux temperature.

A big majority of reactions with suitable substrates (achiral compounds, thermally stable) can be accelerated by raising the temperature by 100 to 200 °C to cut reaction times from days or hours to minutes or seconds.

For example you save a lot of time when you are doing reaction optimization, especially when you have an autosampler. You can program a matrix of conditions and the instrument will do the rest over night.

I agree with Nicodem in that the seen accelerations in reaction rate are a pure consequence of the arrhenius law, so you can of course achieve similar results with a thermally heated autoclave.

But still, having a contactless heating method has some advantages when it comes to the integration of an autosampler, for example.

Currently i am not aware of any small-scale autoclave system which could replace a single mode microwave reactor in terms of automization possibilities, heating rates, cooling rates and temperature control features.


[Edited on 29-1-2010 by Synthesis Technologist]

chemoleo - 29-1-2010 at 18:04

Just as an example - I read about the reaction of epoxides with ammonia in methanol to form triethanol amine derivatives- conventional heating at 50 deg C takes about 6 days, with rel poor stereoselectivity - while - so the paper claimed - in the MW, it takes about 3 hrs (at 120 degC)... with apparently greater stereoselectivity - not sure how the latter would be achieved - comments?

bbartlog - 29-1-2010 at 18:15

I'd think that any improved stereoselectivity would be due to less opportunity (time) for rearrangement and not due to the primary mechanism.

Organikum - 31-5-2010 at 06:32

Quote: Originally posted by Nicodem  
Quote: Originally posted by Fleaker  
Nicodem, I can think of many polymerizations that will work in minutes in the microwave but will take hours (or days) done conventionally. It is very useful for making many things quickly. Why the disdain for microwave irradiation?

I would not say that I'm disdainful toward microwave reactors themselves, but I sure am disdainful toward the tasteless abuse of these machines to get all kind of crap science published just because it is so hip. Maybe this f(r)a(u)d never approached the disgustfulness of the green chemistry fad, but it sure did its best to deteriorate science just one additional step further. The bullshit that got published on MW assisted synthesis is misleading to many and User is one fine example. He knows near to nothing about microwave reactors, I'm quite sure he does not even have access to any, and from his questions it is obvious he is was mislead badly into believing it does some kind of magic. And this is not even the first time. We already had one member who even went so far as to believe that kitchen MW ovens can be used instead of programmable MW reactors. Needless to repeat again how dangerous it can be to put a closed vessel into a MW oven. A kitchen MW oven has no IR thermometer coupled with its power control, it has no pressure sensor for a closed vessel, it has no adequate ventilation to prevent flammable solvent vapours from simply exploding... and moreover there is no rational reason to heat a reaction mixture with MW up to an unknown temperature under such uncontrolled conditions when you can use proper methods like reflux or oil baths or whatever else. A MW reactor has about nothing in common with a kitchen MW oven, except the magnetron.
Just to clear up things, about one third of all reactions I run (at the job), I do so in a microwave reactor, particularly tests on new reactions on a 0.5-5 mmol scale. I admit that it is a much more convenient and controllable way of heating up reactions, it is cleaner and faster than oil baths, and heterogeneous reactions often proceed faster than just using closed cap vessels at the same temperatures. There are many benefits and even though it was demonstrated already several times that microwave heating has no influence on the kinetics of homogenous reactions, I still prefer to use microwaves over classical heating wherever applicable. I think soon the majority of labs will use this technology on routine basis. But this does not mean I'm going to eat every shit that gets published about it. If it smells like shit and tastes like shit, then it most likely is shit - and crap science is full of shit.



Personally I preferred the use of a modified household microwave for reactive distillations where often a lot of foaming occurs over the method propagated by Vogels "heat the top of the vessel with a flame torch".

Better results with less risk and hassle and Igor refused to wave the torch for several hours without raise of payment....

And thats just one example.

But it is understood that modifying MWs requires a lot of reading and understanding of the non-intuitive character of MW-radiation beforehand. And a MW-leak tester.

Its just a tool though not the philosophers stone, but it can be an astonishing one. And sometimes disappointing.



[Edited on 31-5-2010 by Organikum]

Nicodem - 31-5-2010 at 06:48

Quote: Originally posted by Organikum  

Personally I preferred the use of a modified household microwave for reactive distillations where often a lot of foaming occurs over the method propagated by Vogels "heat the top of the vessel with a flame torch".

Better results with less risk and hassle and Igor refused to wave the torch for several hours without raise of payment....

And thats just one example.

But it is understood that modifying MWs requires a lot of reading and understanding of the non-intuitive character of MW-radiation beforehand. And a MW-leak tester.

Its just a tool though not the philosophers stone, but it can be an astonishing one. And sometimes disappointing.

I sure hope you are not a ghost or some ashole hijacking other's accounts. In any case your post made me happy for a moment. If it is truly you, welcome back! :)

Ephoton - 31-5-2010 at 07:27

Welcome back orgy :)

leu - 31-5-2010 at 07:41

It's really Organikum who posted that information about the modification of household microwave ovens, he has some experience in these matters :cool:

[Edited on 31-5-2010 by leu]

Attachment: 200612312513_Modification_of_a_commercial_microwave_oven_for_applications_in_the_chemical_laboratory.pdf (427kB)
This file has been downloaded 1198 times

Fleaker - 31-5-2010 at 09:38

"The reports of my death are greatly exaggerated"---Organikum & Mark Twain.

Welcome back to the land of the living!

https://www.sciencemadness.org/whisper/viewthread.php?tid=32...

hissingnoise - 31-5-2010 at 10:33

It's been more than two years, Organikum - I hope you're well. . .
It's good to see you posting again.

ThatchemistKid - 3-6-2010 at 16:12

Solvent tan d
Ethylene glycol 1.350
N,N-dimethylformamide 0.161
Ethanol 0.941
1,2-dichloroethane 0.127
Dimethylsulfoxide 0.825
Water 0.123
2-propanol 0.799
Chlorobenzene 0.101
Formic acid 0.722
Chloroform 0.091
Methanol 0.659
Acetonitrile 0.062
Nitrobenzene 0.589
Ethyl acetate 0.059
1-butanol 0.571
Acetone 0.054
2-butanol 0.447
Tetrahydrofuran 0.047
1,2-dichlorobenzene 0.280
Dichloromethane 0.042
1-methyl-2-pyrrolidone 0.275
Toluene 0.040
Acetic acid 0.174
Hexane 0.020

here is a list of solvents found in the book Practical microwave synthesis for organic chemists on page 15

the value given beside each solvent is called the loss tangent, the higher the loss tangent the more heat generated by the solvent molecules as it is irradiated by microwaves of 2.45GH
There is a draw back of a high loss tangent in that the outer layers of the solvent will absorb most of the energy and the interior will not be penetrated by the microwaves leaving it only to be heated by convection.