Keras
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Synthesis of trichloroethylene
Folks,
I'd like to synthesize some trichloroethylene (i.e. C2HCl3, partly because I'd like to use it as a solvent, partly as a
precursor to chloroacetic acid).
Wikipedia says it’s possible to synthesize a precursor to TCE, namely C2H2Cl4, by reacting acetylene with
chlorine over a solution? bed? of iron chloride at 90 °C. Tetrachloroethane is then dehydrochlorinated with calcium hydroxide, forming calcium
chloride, water and TCE.
Acetylene and calcium hydroxide are produced by the dissolution of calcium carbide in water.
Chlorine, as anyone knows, is readily obtained from bleach and hydrochloric acid.
Has anyone ever attempted this? I’m somewhat uneasy at the idea of warming acetylene to 90 °C. Set up would be a two-neck flask filled with lumps
of CaC2 and FeCl3. Chlorine is led from an Erlenmeyer flask and water is dripped from an overhead dropping funnel (isobaric
type). The simultaneous presence of calcium hydroxide and tetrachloroethane would directly lead to TCE.
I’m concerned by the fact that chlorine and acetylene often react violently, as evidenced by the famous underwater fireworks demo. In this
case, the reaction would somehow have be tamed down.
Opinion?
[Edit: apparently some other source claims that 60 °C is enough.]
[Edited on 21-8-2018 by Keras]
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Tdep
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Mood: PhD is done! It isn't good but it's over lol
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Quote: Originally posted by Keras  |
I’m concerned by the fact that chlorine and acetylene often react violently, as evidenced by the famous underwater fireworks demo. In this
case, the reaction would somehow have be tamed down.
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"often". I'm going to go with "pretty much always", especially when you preheat everything. 100% this is nuts. I'm curious at how it was done in an
industrial setting. Either 1) some careful control of the stoichiometry (ie one reactant in huge excess, would have to be a huge excess of acetylene
I'd think?) allows it to react in somewhat ease or 2) it is violent but they build the tanks so that they can cope and the industrial chemists all die
of stress before they hit 40. Probably a combination of both.
Anyway, not something I see a home chemist pulling off.
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JJay
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| Quote: Originally posted by zhonglanindustry | I think you can use this method
Ethylene is directly chlorinated to obtain a mixture of tetrachloroethane and pentachloroethane, which is subjected to gas phase cracking to obtain
trichloroethylene and tetrachloroethylene.
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You can make ethylene by dehydrating ethanol at red heat or with sulfuric acid at ~160 C IIRC. The tricky part is regulating the ethylene and chlorine
ratio. Also, I'm guessing it helps if the gases are dry.... or maybe you can just react them wet and oxidize the chlorohydrin to chloroacetic acid.
[Edited on 22-8-2018 by JJay]
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Keras
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| Quote: Originally posted by Tdep | | 100% this is nuts. I'm curious at how it was done in an industrial setting. Either 1) some careful control of the stoichiometry (ie one reactant in
huge excess, would have to be a huge excess of acetylene I'd think?) allows it to react in somewhat ease or 2) it is violent but they build the tanks
so that they can cope and the industrial chemists all die of stress before they hit 40. Probably a combination of both. |
Catalysts can turn seemingly violent reactions into more gentle ones. Take for example the oxidation of gaseous ethanol into ethanal in air on a hot
copper ribbon. The ribbon glows red hot, yet ethanol fumes will not ignite.
There is a clear threshold effect in Underwater fireworks: the flashes you get are intermittent, indicating that a certain amount of
acetylene has to accumulate before the combustion takes place (there seems to be a pretty large threshold of activation). If the catalyst can act
before that critical amount is reached, maybe the reaction can proceed in a steady, peaceful flow?
Anyway, I intended to fill the flask with chlorine, so the chlorine would be in excess, and react only small quantities of acetylene at a time.
Alternatively, I could dissolve acetylene in acetone and maybe use sulfuryl chloride as a source of Cl2? The boiling point of acetone is
too low, but maybe at 40 °C we might get something?
[Edited on 22-8-2018 by Keras]
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Keras
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| Quote: Originally posted by JJay |
[/rquote]
You can make ethylene by dehydrating ethanol at red heat or with sulfuric acid at ~160 C IIRC. The tricky part is regulating the ethylene and chlorine
ratio. Also, I'm guessing it helps if the gases are dry.... or maybe you can just react them wet and oxidize the chlorohydrin to chloroacetic acid.
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I’m very wary of producing ethylene by dehydrating ethanol over 160 °C, because first you'd have to warm the mix up, and before reaching 160 °C,
the same mixture produces diethyl ether, whose autoignition temperature is precisely 160 °C. Did anyone ever attempt this?
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JJay
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I would certainly suggest not letting any air into the mix, since that could decrease yields.
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Keras
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| Quote: Originally posted by JJay | | I would certainly suggest not letting any air into the mix, since that could decrease yields. |
What reaction are you referring to?
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JJay
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At what point do you think it is acceptable to allow air into the process?
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Keras
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I’m not sure if you refer to ethylene chlorination or acetylene chlorination, as both have been mentioned.
BTW, what do you think of using heated Al2O3 to dehydrate ethanol into ethene, such as suggested here?
[Edited on 22-8-2018 by Keras]
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JJay
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I think that a wide array of catalysts will function to dehydrate ethanol into ethene at sufficient temperatures. A catalyst is not even really
necessary. The idea of chlorinating acetylene seems rather dangerous to me; I personally would not attempt it without blast shields.
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Keras
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| Quote: Originally posted by JJay | | The idea of chlorinating acetylene seems rather dangerous to me; I personally would not attempt it without blast shields. |
Even if the water only slowly drips onto the calcium carbide?
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JJay
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No matter how slowly the water drips, you're still going to have a flask full of acetylene.
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Keras
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Acetylene is quite lighter than chloride (it’s even lighter than air). Unreacted stuff should "float" up and escape. I could connect the top neck of
the flask to an underwater exhaust?
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