Sciencemadness Discussion Board

First reaction (safety question) - CaC2 to C2H2 to C6H6 (via thermal polymerisation)

Halcyon - 5-10-2010 at 03:13

I'm looking to learn a little (or a lot) about (primarily organic) chem, teaching myself what I can and practising where I can.

After doing a couple of simple things like fractionally distilling oils/ethanol and concentrating acetic acid from vinegar (by converting to a salt and distilling with H2SO4) I thought that this might be an interesting reaction to try, which would leave me with a product I might be able to use for future reactions.

The proposed setup is shown below. My quick drawing is dodgy, but the idea is clear enough - Generate acetylene from calcium carbide and water (slowly added in short increments to control process flow and also prevent acetylene escaping through the sep funnel), run it through a red-hot copper tube, condense the benzene, and pipe unreacted acetylene to atmosphere a distance away (in this case, out a window).



Now the logistics are easy enough - the 1/4" copper tubing is sealed to the glass at each end with a thermometer adapter, keck clips and grease are used to seal the glass-to-glass joints, and a section of copper tubing is heated to red-heat with a bunsen burner.

Obviously, I've got concerns about generating acetylene right next to an open flame. Could anyone advise me on this?

I've set the apparatus up (dry) and tested it for leaks, and it seems like a perfect seal at far higher pressures than the system will ever experience, but given that it doesn't seem too much different to a ketene lamp, I wonder if I've missed something obvious.

The room will be well-ventilated, the excess acetylene will be exhausted away from the apparatus, the gas-bottle feeding the bunsen burner will be isolated from the room in which the reaction is taking place and accessible if something goes wrong, and the production of acetylene will be very slow.

Cheers in advance for any advice. (any thoughts on what interesting reactions can be done with benzene and other obtainable reagents would also be appreciated).

per.y.ohlin - 5-10-2010 at 03:31

This reaction is extremely dangerous. Even if you manage to seal the apparatus and flush out all the oxygen, it could still explode. Acetylene is unstable, not just flammable.

From http://www.msha.gov/alerts/hazardsofacetylene.htm
Quote:

Flammable range: Acetylene has a very wide range of flammability. The lower flammable limit (LFL) is typically listed as 2.5% and the upper flammable limit (UFL) is listed as 81%. Although acetylene will not undergo combustion at concentrations above the UFL, it can undergo an explosive decomposition reaction, even at concentrations of 100%.


There are much safer ways to produce benzene. Take a look at http://www.sciencemadness.org/member_publications/benzene_pr... .

mewrox99 - 5-10-2010 at 03:32

Seems like a bad idea. There's a lot that could go wrong.

metalresearcher - 5-10-2010 at 03:37

Good idea by some safety issues apply. C2H2 + air is VERY explosive.. ANd C6H6 is flammable and toxic.
So the C2H2 in the heater copper tube should be absolutely oxygen free.

But I didn't know that thimerization of C2H2 works by just heating in to red hot.

Halcyon - 5-10-2010 at 03:40

Quote:
Although acetylene will not undergo combustion at concentrations above the UFL, it can undergo an explosive decomposition reaction, even at concentrations of 100%


Damn, how did I miss that? I'm sure the MSDS I read didn't mention that...

Time for more research and a rethink, thanks guys.

watson.fawkes - 5-10-2010 at 04:38

Quote: Originally posted by Halcyon  
Time for more research and a rethink, thanks guys.
Good, because you'd also have gotten the formation of copper acetylide.

arsphenamine - 5-10-2010 at 05:29

Quote: Originally posted by per.y.ohlin  
This reaction is extremely dangerous. Even if you manage to seal the apparatus and flush out all the oxygen, it could still explode. Acetylene is unstable, not just flammable.
Compressed acetylene was a time bomb before the tanks were packed with glass wool to shorten the mean free path of the molecule; apparently, if unobstructed, it picks up enough velocity/energy to initiate voilent decomposition.

Some acetone is also added in to reduce the available acetylene at pressure.
Handled thusly, it was no longer classified as an explosive by shipping and trade organizations.

smuv - 6-10-2010 at 20:50

Well you certainly picked an interesting reaction.

Before you scrap this, do some tests igniting acetylene, to get a feel for how dangerous this is. I have played with calcium carbide and don't remember explosions at all, just sooty yellow flames. Also you can easily replace copper tubing with steel or stainless tubing, which is available at all auto parts stores as brake and fuel line. Do a patent search to get an idea what specific conditions are required for this reaction.

One thing to keep in mind, is that calcium carbide makes pretty impure acetylene due to calcium phosphide contaminants. The acetylene therefore contines diphosphine and phosphine. I do not know to what extent these would interfere with the reaction, but they can be removed.

Attached is a paper that quantifies and describes removal of the common contaminants from acetylene.

Attachment: acetylene_contaminants.pdf (1.1MB)
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