Questions about working at very low temperatures

I will soon have the opportunity to conduct experiments at very low temperatures. I have found ( ) 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.

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!

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

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]

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 ).

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]

Cryogenic temperatures