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Author: Subject: Questions about working at very low temperatures
Sergei_Eisenstein
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[*] posted on 25-12-2005 at 04:49
Questions about working at very low temperatures


I will soon have the opportunity to conduct experiments at very low temperatures. I have found ( :D ) a 25L Dewar vessel to store liquid nitrogen and an acquaintance can help me with solid CO2. The only problem is my lack of experience, since I have never conducted experiments at temperatures lower than 0°C. So I have a few practical questions about performing this kind of reactions.

Imagine I would like to make a functionalized aryllithium by halogen-lithium exchange. The precursor is 4-iodobenzonitrile and 4-cyanophenyllithium can only be generated at cryogenic temperatures (-100°C) with e.g. n-BuLi. At higher temperatures, n-BuLi will also react with the cyano functional group. This temperature can be attained with an ether/liquid nitrogen bath. But how do I have to make this ether/LN bath? Do I have to add LN to ether in a Dewar reaction vessel, or do I add ether to LN, or does it not make a difference (especially regarding safetey - it is safer to prepare dilute acid by adding the acid to water and not water to the acid)? Also, how to I sustain this temperature for prolonged time? Is it sufficient to add some more LN to keep the temperature at -100°C?

An idea I had was to connect the 25L Dewar with coiled tubing (I can get some old copper, nickel and stainless steel tubing that has been used for gas chromatography). The liquid is cooled by LN circulating through the coil and a reaction flask can be put inside the coil. Perhaps a few of these coils can be connected in series.



(just to get an idea of the coil)

I would greatly appreciate any practical advice you can give me concerning the topic.
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[*] posted on 25-12-2005 at 07:09


As long as you add one to the other slowly, it shouldn't really matter whether you add ether to LN or LN to ether. If you mix them too rapidly you'll get lots of boiling, and hence splashing. If all the LN boils off during the reaction, simply add some more to the bath. It'll still be cold and so it shouldn't boil up too much when you add. But as long as you add a decent amount at the start it should last you a while, and you probably won't need to add more unless you're doing something very exothermic, or very long.
The main things to remember are don't give glass big thermal shocks, don't let ice plug your container ("BANG!"), and remember that LOX will build up and can be a hazard.

A "funny" thing happened while I was in my university lab a few weeks ago. Someone was using a thermos of LN as a cold trap during a high vacuum distilation, and somehow managed to break it. It was quite spectacular, there was a fairly loud "pop!" which made everyone jump (and caused two or three other people to break whatever they were holding!), and the guy who broke the thermos instantly disappeared in a cloud of fog! Luckily I'd just finished transfering a solution of selectride (lithium tri-sec-butylborohydride) into my flask, or I might have spilled it. I have heard that it can be quite spectacular if you spill a lot (the flames are such a lovelly shade of green ;)), but I don't want to be the one to do it!




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Mr. Wizard
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[*] posted on 25-12-2005 at 07:55


Add the ether to the LN, but slowly. Here is the rational: If you add a drop of hot grease to water it won't splatter, because the material that makes the vapor is water, and grease doesn't contain much heat. If you added one drop of water to a vat of hot grease the water would submerge beneath the surface of the hot grease and then turn to a gas and throw the hot grease upwards. This is to be avoided. In this analogy, ether is the 'hot' grease. There will still be boiling but at least the rate of heat input will be determined by how quickly you pour in the ether. The same rational applies to adding acid to water vs adding water to acid. One drop of acid will not heat up a volume of water, but one drop of water under the surface of a volume of acid may generate heat and a steam bubble and back splatter. Wear facial protection, gloves, and keep ignition sources away, including static electricity. Ether is famous for these problems, especially in cold dry weather.
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[*] posted on 25-12-2005 at 10:02


Something I learned the hard way during my ammonia liquefaction using acetone/dry ice: Get all your components as cold as possible before combining them. In your case I guess the only thing to have precooled would be the ether since the N2 in the dewar would already be cold.

I had warm acetone in a warm dewar. When I added the dry ice chunks I suddenly had a great deal of CO2 gas generation causing acetone to spill out all over the place.




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Fleaker
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[*] posted on 25-12-2005 at 17:43


Most of it is common sense when working with cryogenic materials. An obvious consideration is to make sure you are protected properly so you do not spill any upon yourself. Do not forget that liquid nitrogen can cause severe frostbite if you spill a lot on you (mainly on an extremity). I've worked with LN2 on quite a few occasions and I can forewarn you about this: pouring liquid nitrogen into glass can shatter it just as easily as heating it quickly. I always use HDPE or a plastic when doing anything with cryofluid. You will want to add the ether to the liquid nitrogen which will be boiling anyways (it boils for about an hour or two after it is pumped in the container which reminds me, don't let them cheat you at the pump! When they say they'll put 25L in, expect to see 10-20, the rest evaporates as it cools the dewar!). I would suggest working in styrofoam but since you are using diethyl ether, that obviously won't work :P

Mr. Wizards analogy is a good one, as is Magpie's experience with adding the acetone to CO2.

BTW, nice score on the 25L dewar! Those usually run about $700-1000

[Edited on 27-12-2005 by Fleaker]
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[*] posted on 25-12-2005 at 19:39


Don't be too afraid of splashes - it's perfectly safe to pour liquid nitrogen over your hand. When I was doing materials sciences my supervisor took me too see one of the electron microscopes in the department, and there was a small dewar of LN there. He told me to put my hand out and I, not knowing what he was going to do, did it. And he poured about a litre of LN over my hand. It feels cold, but it forms a cushion of gas around you so that it never actually touches your skin.
If it soaks into clothing then it might give you a nasty burn, but a bit splashing about is OK. Of course, any solid objects cooled to that temperature should NOT be touched with bare skin!!!




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[*] posted on 25-12-2005 at 20:11


Two problems I can see - you might freeze the ether solid, or you undercool it, and the reaction doesnt take place (or only very slowly). The only way I can see this working is to take the reaction vessel with a long receptacle, and dip it into N2 whenever the temperature goes above -100 deg C. I certainly would NOT put N2 into ether, you are going to get massive boiling. Neither the other way round, the ether will be flashfrozen instantly and sink as ether droplets to the bottom. I triied those very things with ethanol - pouring it into a beaker of N2 and such. So what you need is a thermometer which measures such low temps, then you should be set.

On the note of safety- yeah, it's safe if you spill it on your hand. Most injuries however occur when the N2 enters your clothes, and THEN it doesnt quickly boil away. I once got a splash into my shoe...ouch!
Pyrex/duran glass is fine with N2in my opinion. I once took a 200 ml pyrex beaker, and filled it (at RT) with N2 - no breaking, nothing. I took this beaker and put it onto a scale, to see how fast it was losing weight. The rate of loss was about a gram per 10 seconds (~ 80 ml of N2 gas per second :D).

Also beware that the ether will likely go viscous at such low temps, which might make mixing difficult. Freezing point is at -116 deg C, so I suppose underfreezing it wont be a problem. Without a thermometer, I'd probably keep the ether just on the edge of being frozen (by dipping the reaction vessel periodically into N2).


[Edited on 26-12-2005 by chemoleo]




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[*] posted on 26-12-2005 at 06:22


Thanks for the tips and advice!

I found an interesting article: A versatile low-temperature thermostat (Industrial & Engineering Chemistry 8(2) 1936 149) (see attachment). It discusses how to make your own thermostat for use with a dry ice bath.

Attachment: thermostat.pdf (166kB)
This file has been downloaded 656 times

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[*] posted on 26-12-2005 at 17:05


Despite having LN poured on your hand I wouldn't get too careless or worry free around it. If the liquid gets on your clothing and wicks into the fabric, it will get very cold very quickly. Even worse, liquid ether at cryogenic temps will not form a protective vapor barrier, it will just suck up heat. The dermatologists don't use LN to freeze flesh and warts because it doesn't work. Just because you can wave your hand through a flame quickly doesn't mean fire is harmless. Be careful. Life is tough enough with ten fingers and two eyes. Scar tissue doesn't do much to make you an evolutionary success ;-)
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[*] posted on 27-12-2005 at 11:22


It actually takes a good amount of time for the ether to freeze in LN2. It would take far too long to cool it by merely putting it in a container and immersing it (kind of like a double boiler concept for chocolates but in this case gentle cooling, not heating). Adding the ether to the LN2 would be advantageous because any left over LN2 can be decanted off and put back into the dewar. Plus, when the ether becomes too warm, you can simply add more LN2 to drop the temperature assuming you have a thermometer that works well. I don't see why using ethanol is out of the question, does the procedure specifically call for ether?

Regarding the vapor layer: it is a similar idea behind wetting your hand and quickly immersing it in molten lead: the heat of vaporization and the general heat capacity of water is so high, it protects your flesh. Eitherway, there's a limited amount of time. The chief danger is accidental prolonged immersion since spills on bare skin slide right off thanks to that vapor layer. And yes, getting it on clothes will definitely be detrimental to the situation. That's the reason why cryo gloves are impermeable.

[Edited on 28-12-2005 by Fleaker]
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[*] posted on 27-12-2005 at 11:33


Something else to consider - asphyxiation hazard. Nothing to sneeze at when working with 25L of LN2 in a confined space (basement, lab).

Also, lots of materials, including stainless steel and plastics, have a nasty habit of getting very brittle at those temperatures, beware of structural failures.




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[*] posted on 28-12-2005 at 11:47


In case someone is interested, here is a paper with some references on how to make Ar-M reagents when aryl contains substituents such as ester, cyano, chloro, azido, or nitro group (!). Ths is still sometimes considered impossible, but can be done with low temp. litium-halogen excgange as Sergei said, anyways the paper by Knochel et al (J. Am. Chem. SOC. V ol. 114, No. 10, 1992):

http://rapidshare.de/files/9991156/Preparation_of_Highly_Fun...

Chemoleo, to avoid solvent-freezing at such low temperatures a mixture THF/ether/pentane (4:1:1) is used.


[Edited on 28-12-2005 by Sandmeyer]




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[*] posted on 28-12-2005 at 22:01


..humm it would be very interesting to know what the going price on a cyro freezer would be? second hand anyone?



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[*] posted on 29-12-2005 at 04:40


LOTS.

Making an air liquifier shouldn't be too hard though. All you really need is a strong air pump and various bits of copper tubing...
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[*] posted on 29-12-2005 at 06:46
Cryogenic temperatures


I would suggest trying ether-dry ice instead of ether or ethanol-liquid nitrogen. This first combination is said to reach a temperature of -100C. This way you may be able to avoid the hassle of storing LN2, and it will also make preparation and maintenance of the cooling bath simpler.

I had read some time ago about the very low temperature ether-CO2 reached, and googling for this gave a very useful list, from a BioForum thread. Some of the values are certainly not what I would have expected, but several others I know are correct. Here it is below:
Quote:
Some Useful Laboratory Cooling Mixtures

Mixture Mixture temperature (Centigrade)

p-Xylene/Liquid nitrogen 13
p-Dioxane/Liquid nitrogen 12
Cyclohexane/Liquid nitrogen 6
Benzene/Liquid nitrogen 5
Formamide/Liquid nitrogen 2
Aniline/Liquid nitrogen -6
Cycloheptane/Liquid nitrogen -12
Benzonitrile/Liquid nitrogen -13
Ethylene glycol/Dry ice -15
o-Dichlorobenzene/Liquid nitrogen -18
Tetrachloroetane/Liquid nitrogen -22
Carbon tetrachloride/Liquid nitrogen -23
Carbon tetrachloride/Dry ice -23
m-Dichlorobenzene/Liquid nitrogen -25
Nitromethane/Liquid nitrogen -29
o-Xylene/Liquid nitrogen -29
Bromobenzene/Liquid nitrogen -30
Iodobenzene/Liquid nitrogen -31
Thiophene/Liquid nitrogen -38
3-Heptanone/Dry ice -38
Acetonitrile/Liquid nitrogen -41
Pyridine/Liquid nitrogen -42
Acetonenitrile/Dry ice -42
Chlorobenzene/Liquid nitrogen -45
Cylcohexanone/Dry ice -46
m-Xylene/Liquid nitrogen -47
n-Butyl amine/Liquid nitrogen -50
Diethyl carbitol/Dry ice -52
n-Octane/Liquid nitrogen -56
Chloroform/Dry ice -61(-77)
Chloroform/Liquid nitrogen -63
Methyl iodide/Liquid nitrogen -66
Carbitol acetate/Dry ice -67
t-Butyl amine/Liquid nitrogen -68
Ethanol/Dry ice -72
Trichloroethylene/Liquid nitrogen -73
Butyl acetate/Liquid nitrogen -77
Acetone/Dry ice -78
Isopropanol/Dry ice -78
Isoamyl acetate/Liquid nitrogen -79
Acylonitrile/Liquid nitrogen -82
Sulfur dioxide/Dry ice -82
Ethyl acetate//Liquid nitrogen -84
Ethyl methyl ketone/Liquid nitrogen -86
Acrolein/Liquid nitrogen -88
Nitroethane/Liquid nitrogen -90
Heptane/Liquid nitrogen -91
Cyclopentane/Liquid nitrogen -93
Hexane/Liquid nitrogen -94
Toluene/Liquid nitrogen -95
Methanol/Liquid nitrogen -98
Diethyl ether/Dry ice -100
n-Propyl iodide/Liquid nitrogen -101
n-Butyl iodide/Liquid nitrogen -103
Cyclohexane/Liquid nitrogen -104
Isooctane/Liquid nitrogen -107
Ethyl iodide/Liquid nitrogen -109
Carbon disulfide/Liquid nitrogen -110
Butyl bromide/Liquid nitrogen -112
Ethyl bromide/Liquid nitrogen -119
Acetaldehyde/Liquid nitrogen -124
Methyl cyclohexane/Liquid nitrogen -126
n-Pentane/Liquid nitrogen -131
1,5-Hexadiene/Liquid nitrogen -141
Isopentane/Liquid nitrogen -160
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Sergei_Eisenstein
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[*] posted on 29-12-2005 at 12:16


At the following website, they show some pictures of temperature measurements of a few mixtures:

http://www.pc.chemie.uni-siegen.de/pci/versuche/english/v105...

The diethyl ether/dry ice combination gives -82.5°C instead of -100°C, the value published in the literature (and which can also be found in Kinetic's list).

[of topic: checking the main page ( http://www.pc.chemie.uni-siegen.de/pci/versuche/english/vers... ), some nice pictures from other interesting experiments are available]
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[*] posted on 29-12-2005 at 22:11


Looking at the list posted by Kinetic, I don't understand how liquid nitrogen can be mixed with cyclohexane or benzene and have a stable temperature above 0C. How can this happen? Am I overlooking something?
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Fleaker
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[*] posted on 30-12-2005 at 19:32


I do not think that list is too reliable. When I saw diethyl ether/dry ice @ -100C I was suspicious since dry ice by itself is -94C tops, and it won't get any colder. After seeing 0C mixtures with LN2, I'm even more doubtful.
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[*] posted on 31-12-2005 at 01:12


Quote:
Originally posted by Fleaker
I do not think that list is too reliable. When I saw diethyl ether/dry ice @ -100C I was suspicious since dry ice by itself is -94C tops, and it won't get any colder. After seeing 0C mixtures with LN2, I'm even more doubtful.


The same temperature and much else of the above can be found in Armarego and Perrin.

Nowhere above (did not read the link) does it say that the mixture of cyclohexane or benzene and cold whatever is particularly rich in the cold stuff. Just enough. No surprise that it doesn't reach lower temperatures then. Latent heat of freezing gives some latitude to keep the temperature in a very narrow range if you cool it just to slush and maintain it there.

Suppose that you want a bath of a particular temperature - you find an organic (that doesn't supercool) with the desired freezing point, keep it slushy (as being frozen solid will mess with heat transfer), and you might be able to keep it close - without a thermometer.
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[*] posted on 10-1-2006 at 11:34


Dri-ice should cool an excess of liquid such as acetone or isopropanol down to -75oC ish. If the the dri-ice is in excess then it might be possible to do the equivalent of an ice + salt bath and get a lower temperature.
If you are using liquid nitrogen the idea is to use a "slush bath", find a liquid that freezes at the temperature that you want and add liquid N2 until you have solid + liquid. The liquid N2 only cools the stuff down and if some dissolves reduces the flammability.
A pentane/ liquid N2 slush bath should be around -100oC and a bit safer than an ether/liquid N2 because of the flash points.

mick
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