Picric-A
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Reactions in a 'Sealed Tube'
Several reactions i have come across, such as condensations ect. are required to be performed in a sealed glass tube (eg, at 105 deg. C ). These
reactions are all taken from old org. chem books like Brauer.
My question is, how exactly is this done?
What vessel is sealed and how is this done?
Safety- Heating something like EtOH in a sealed vessel at 110 deg C of course poses lots of safety hazards like explosions and fires. Are there any
modern alternatives to using a 'sealed tube'?
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JohnWW
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If reactions in "sealed tubes" are going to involve heating, or exothermic heat evolution, with thermal expansion (although this would probably be
against air or an unreactive gas above the liquid), or else evolution of gases, the "sealed tubes" would also have to be designed as pressure vessels
able to withstand the appropriate pressures.
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The_Davster
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You may be able to get by using a microwave tube. You can buy them with one time use aluminum lids which are then crimped on with a special tool. I
have used these (albeit in a professional microwave oven that monitors pressure) at temperatures exceeding the boiling point of the solvent within.
An easier method would be flame sealing a glass tube. My glassworking ability has deteriorated through disuse so I would be sure that one can make a
seal that would not create a weak spot.
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entropy51
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The older organic lab manuals in the forum library have some detailed instructions for preparation and handling of sealed tubes. Strict adherence to
technique is not a bad idea, if you value your eyesight and facial features and don't enjoy surgery and blood transfusions. Commercial varieties are
available.
It's not for no reason that these sealed reaction vessels are commonly known as "bombs". Some of us here have used them, and a search will turn up
some threads.
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sonogashira
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It is an interesting question.. I have never known what benefit it has cf. higher boiling solvent or longer reaction time.
Does the heat of vaporization of the solvent above its bp have a big effect of the rate maybe? Or is it mainly pressure effect?
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Ozone
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Remember-heating anything in a sealed tube creates a potential bomb--expecially if the ensuing reaction is exothermic and/or produces gas. Sealed tube
polymerizations have "gone wrong" catastrophically (see attached*), but good safety practices can make it all managable.
Remember, a fire is only a problem if you did not expect it to occur.
*Sorenson, W.R. and Campbell, T.W. (1968). Preparative Methods of Polymer Chemistry, 2nd ED. Interscience Publishers, John Wiley & Sons.
LCCC#67-29543, pp.9-10.
Cheers,
O3
 
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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entropy51
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Quote: Originally posted by sonogashira  | It is an interesting question.. I have never known what benefit it has cf. higher boiling solvent or longer reaction time.
Does the heat of vaporization of the solvent above its bp have a big effect of the rate maybe? Or is it mainly pressure effect?
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There is just no other method for some preps, for example methylation of aniline using methanol, which does
not proceed appreciably at the boiling point of methanol. Goes just fine when heated under pressure, however. The same goes for preparation of some
amines from the alkyl halide and ammonia. Very old and quite effective methods, but definitely not for sissies. (I've done them, but they scare me to
death. )
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Picric-A
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Nice attachment Ozone, thanks.
@Entropy - I am indeed attempting to prepare 2-butanamine from 2-chlorobutane and ammonia. In this case no other solvent can be used 
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entropy51
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There is a procedure that sometimes works, especially for tertiary amines (not the case here, but you could try it on a small scale.) Like most
methods it gives a mixture of primary, secondary and tertiary amines. For primary amines alone Gabriel's synthesis is the way to go.
Dilute 28% NH4OH to 7.4 g NH3 per 100 gm of H2O, which is the solubility of NH3 in H2O at 100 C. Warm in a flask fitted with reflux condenser and
addition funnel to 75 C and add the alkyl halide slowly. With luck it will reflux. Reflux for several hours and make it basic and separate the
organic layer, dry on NaOH and distill.
The reference is Butler & Benjamin, A Simple Method for the Preparation of Tertiary Amines, J Chem Ed, 38:191 (1961)
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ScienceSquirrel
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I have done some 'bomb' type chemistry in a university lab and I would do almost anything to avoid a sealed tube reaction due to the complications and
risk.
The chemicals were loaded into the tube and the tube was sealed. The tube was taken up to the 'bomb room' where it was loaded in to a steel tube
furnace inside a vented steel box. The box was closed and the power was switched on. The tube was heated for the required time and then the furnace
was switched off.
Finally the tube was allowed to cool to room temperature, then it was cooled in dry ice / acetone.
The tube was opened wearing full full blast kit.
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IrC
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I spent much time experimenting with making Iridium Carbide in sealed tubes. I gave up on glass and went to high quality quartz tubing. I would seal
one end and load it with fine Ir dust and carbon dust. Some distance away I inserted a metal screen and after that hooked up a vacuum pump. Heating
the tube allowed vacuum to seal the other end after which very high temperatures was applied to the rotating tube. It was fairly easy and quick to
construct each time and helped me improve at glass working. I found this was a great way of producing compounds of high purity, insofar as home
experimenting goes.
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Fleaker
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Quartz is hell to work with unless you've got the tools.
I've had an incident before with a sealed quartz tube which exploded, destroying a tube furnace. Now, I would examine any sealed tube I make with
polarized light to look for stress/cracks, or have a glassblower do it for me.
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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IrC
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| Quote: Originally posted by Fleaker | Quartz is hell to work with unless you've got the tools.
I've had an incident before with a sealed quartz tube which exploded, destroying a tube furnace. Now, I would examine any sealed tube I make with
polarized light to look for stress/cracks, or have a glassblower do it for me. |
Just FYI but I found that the Diamond wheel Dremel makes ($15 includes arbor) turning at high speed cuts quartz tubing like slicing butter. So much
easier than cutting glass and cuts are ultra smooth. Just takes very high temperatures to work is the down side but other than this I think I like
working with quartz more than glass.
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turd
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| Quote: | | Safety- Heating something like EtOH in a sealed vessel at 110 deg C of course poses lots of safety hazards like explosions and fires. Are there any
modern alternatives to using a 'sealed tube'? |
Oh my, oh my, the organikers and their sissy temperatures...  At a reaction
temperature of 110°C I wouldn't even consider using a glass tube, unless the reaction medium is something as tasty as H2SO4/HNO3. <230°C is the
perfect temperature for steel autoclaves with Teflon inlays. At 110°C I'm not even sure the steel autoclave is needed - a thick walled Teflon
container with a screw cap may suffice. These can be made from a Teflon rod by anyone who knows how to operate a lathe and shouldn't be missing in
any lab.
As to fused silica ("quartz") tubes:
| Quote: | | I've had an incident before with a sealed quartz tube which exploded, destroying a tube furnace. Now, I would examine any sealed tube I make with
polarized light to look for stress/cracks, or have a glassblower do it for me. |
You must be doing something seriously wrong. Either your tubing is too thin, of miserable quality, or your method of sealing them is wrong as in you
thinned it too much (or you're doing something silly like using fluorides or heating beyond 1370 K). I've done literally hundreds of experiments in
fused silica tubes at 1170-1270 K and not a single one of them exploded in the oven. The few that broke were seriously corroded and quenched from 1270
K with ice water.
| Quote: | | Just takes very high temperatures to work is the down side but other than this I think I like working with quartz more than glass.
|
Damn right. "Quartz" glass and your run of the mill Duran/Pyrex glass don't even play in the same league. Duran breaks if you look wrongly at it,
fused silica at 1270 K can be thrown into ice water.  The high softening
temperature is an advantage for clumsy people like me, BTW, since if you note that there is a serious risk of implosion during sealing you can remove
the burner and it will solidify immediately, whereas with normal glass it's typically too late. I'd say the only disadvantage is the price.
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Fleaker
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| Quote: |
You must be doing something seriously wrong. Either your tubing is too thin, of miserable quality, or your method of sealing them is wrong as in you
thinned it too much (or you're doing something silly like using fluorides or heating beyond 1370 K). I've done literally hundreds of experiments in
fused silica tubes at 1170-1270 K and not a single one of them exploded in the oven. The few that broke were seriously corroded and quenched from 1270
K with ice water. |
Yes, I did do something wrong. It's called working thick wall high quality quartz with oxymethane, not oxyhydrogen as one should. Oxyacetylene is
acceptable, although dirty, but really, quartz is best worked with oxyhydrogen. Also a good example of why one can seal smaller tubes and have no
incident occur, but move up in diameter (recall well the definition of pressure!). My seal held a very hard vacuum, and it was not overly thin
(several mm). Nonetheless, that was where the material failed.
I'm well aware of the thermal properties of quartz. It's very impressive stuff. Quenching from 1270 K seems a bit like showing off.
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
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solo
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.......I've have always used a sealed tube inside a sealed metal pipe with good results and avoided the risks of exploding glass.....a much safer
method to do this type of sealed reactions.......solo
It's better to die on your feet, than live on your knees....Emiliano Zapata.
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DJF90
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Its not that safe considering that the sealed metal pipe could also fragment, sending metal shards everywhere (see Ozone's post above with picture
:O). Using a sleeve of glass or polymer cloth is a much better solution as it can flex to absorb the impact, containing the glass shards from the
tube.
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entropy51
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solo, please post an outline of your methods, such as type of tube, sealing methods, heating methods, and so forth. Thank you.
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turd
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| Quote: Originally posted by Fleaker |
Yes, I did do something wrong. It's called working thick wall high quality quartz with oxymethane, not oxyhydrogen as one should. Oxyacetylene is
acceptable, although dirty, but really, quartz is best worked with oxyhydrogen. |
Oh yes, definitely. With the right H2/O2 burner it all becomes nicely manageable.
| Quote: Originally posted by Fleaker | | I'm well aware of the thermal properties of quartz. It's very impressive stuff. Quenching from 1270 K seems a bit like showing off.
|
I'm interested in polymorphs that are stable at > 1170 K. The only way to obtain them at RT is quenching from high temperatures. Therefore breaking
of the tube when quenching is preferred since contact with water cools the sample much faster.
Note that even with the highest vacuum you will never get more than ~1 bar pressure difference. A sealed "quartz" tube should take a few bar.
| Quote: | | .......I've have always used a sealed tube inside a sealed metal pipe with good results and avoided the risks of exploding glass.....a much safer
method to do this type of sealed reactions.......solo |
Something similar has to be done when working under supercritical conditions. The tube is placed in an steel autoclave filled with an inert
hydrocarbon solvent to provide back pressure from the outside.
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