Boffis - 28-11-2013 at 13:17
The first question I hear you ask is "why would anyone what to prepare this compound?". After all it was once a byproduct of the manufacture of
tetrachloroethylene and carbon tetrachloride and is probably just as toxic.
However, I am interested in Squaric acid and the squarate ion (C4O4)2- and according to the patent US 4104308 it is fairly easily prepared from
1,1,2,3,4,4-hexachloro-1,3-butadiene via reaction with morpholine (and probably other secondary) and then hydrolysis to the product.
The problem is the hexachlorobutadiene starting material, while it is available from various suppliers it is not readily available and is now rather
expensive. Then an interesting idea occurred to me after reading Garage_chemist's excellant post in the Prepublication section covering the
preparation of unsym-heptachloropropane and hexachloropropene (from the reaction on Org Syn attached below). Could the reaction be carried out using
tetrachloroethylene and aluminium chloride as in the Org. Syn. document but substituting the chloroform with trichloroethylene to give me
heptachlorobuta-1-ene? This could then be striped of one molecule of HCl as described by Garage chemist to give the required hexachlorobutadiene (in
theory!).
I can see problems with the first reaction, for instance:
1) in the absence of chloroform would the tetrachloroethylene react with itself? Depending on the mechanism it may be possible to control the outcome
using stages addition of one reaction. In other words is the presence of a hydrogen atom on the second molecule essential?
2) trichloroethylene, while it has one hydrogen as does chloroform, it is an unsaturated compound so would it tend to react with itself to give
hexachlorobutene (and hence pentachlorobutadiene) or a complex mixture of tetrachlor-tetrachlor; tetrachlor-trichlor and trichlor-trichlor
condensation products?
3) What is the mechanism? Presumably the aluminium acts as a lewis acid genarating a chlorocarbenium ion. But does it do this via a reaction with the
chloroform or the tetrachloroethylene? This is important to my original question because if the reaction is via the tetrachloroethylene then it may be
possible to use an excess of this compound as the solvent and then then add the trichloroethylene stepwise. If on the other hand the initial reaction
is with the chloroform when this is replaced with trichloroethylene will a reaction occur? Or being an unsaturated compound anyway will it simply
react with its self?
Does any one have any ideas about this reaction.
A second possibility is an idea put forward by Unintentional Chaos in the same thread "Does anyone know if a second addition of CHCl3 could be carried
out on the hexachloropropene?" This would give nonochlorobutane but then an idea occurred to me; would it be possible to replace chloroform with
dichloromethane to give initially octachlorobutane and then strip off two molecules of HCl to get the required hexachlorobutadiene. (The two hydrogens
may end up on adjacent carbon atoms in which case a cumulene (-C=C=C-) would result but this may re-arrange to a more stable 1,3 diene anyway, or it
may just fragment!).
Any thoughts anyone?
Attachment: Synthesis of Squaric acid US 4104308.pdf (159kB)
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Attachment: CV2P0312.pdf (120kB)
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Boffis - 3-12-2013 at 07:46
Thanks to Solo I have been able to answer the questions I posed in my earlier post. Have waded through some 18 paper on the subject it is clear that
the aluminium chloride forms an addition compound with the chloroform (and also with Carbon Tetrachloride) and that this reacts with the
perchloroethylene (and also with trichloroethylene and 1,2 dichloroethylene) to give predominately one product or a separable mixture. longer chain
compounds often gave complex mixtures of isomers that were often difficult to separate.
@Unintentional Chaos one of the paper describes exactly the question you posted "Does anyone know if a second addition of CHCl3 could be carried out
on the hexachloropropene?"; the answer is yes.
Unfortunately it would appear that dichloromethane does not react and indeed the author uses this compound as a solvent for these reaction. To answer
my own question it does not appear that hexachlorobutadiene is available via this route from readily available chlorinated solvents (unless someone
knows of a convenient source of 1,1,2 trichloroethane. It appears that some saturated poly chloroethane and related higher hydrocarbons also form
reactive intermediates with aluminium chloride.
Tetrachloroethylene by itself does not react readily and the product is mostly resin, this seems to be fairly consistent for unsaturated chloro
compounds unless they contain a -CHCl2 or -CHCL-CCl3 units within the chain.
Another important side reaction with some compounds is de-hydrochlorination basically the loss of HCl to form another C=C double bond. Some other
later papers started to look at other related condensations with aldehydes and ketones and also investigate the products of hydrolysis of the
polychloro-compounds.
Boffis - 1-3-2014 at 18:01
Well as no one has commented on my previous posts I'll try again with another possible route to hexachlorobutadiene.
Do any think that it might be possible to oxidatively dehydrogenate and dimerise trichloroethylene to hexachlorobutadiene and if so any suggestions as
to an oxidizing agent and conditions? Basically the reaction would be:
Cl2C=CClH + HClC=CCl2 ==> Cl2C=CCl-CCl=CCl2 + 2H
Any thoughts; better still a reference? I have a done a literature search in what I have access to but most of the reactions are additions across the
double bond or the removal of HCl. One idea that occurred to me is to dissolve say bromine in the trichloroethylene and then reflux with a suspension
of chalk to neutralise the HBr generated before it can add to the double bond. Soluble alkalis would probably generate too much hydrolysis. Would the
bromine add to double bond faster than remove the hydrogen? Would iodine be a better bet? Iodine being a solid would be difficult to add incrementally
over the reaction period though (bromine can be added drop wise). Other oxidising agent that would cut the risk of addition are thinks like Oxone and
acetone.
[Edited on 2-3-2014 by Boffis]
Boffis - 3-3-2014 at 07:47
@ Mr_Magnesium Thank you for you U2U, I have reviewed the patents before but I had not seen the Russian paper which contains some interesting details
but the main issue with all of these is they are aimed at industrial type high temperature chlorination reactions of already part chlorinated butanes.
This raises two problems, firstly the issue of the practicality of high temperature chlorination in a home lab and secondly the source of the
polychlorobutane feedstock.
The purpose of my posts above was to look at home-lab do-able preparations. I still think a modification of Garage_chemist's procedure may be the
answer but the route is complex because it appears to require two stages to build up the C4 chain because the work of Boesken seems to indicate that
the C2 chains required (tetrachloroethylene and trichloroethylene) don't react easily though 1,2 dichloroethylene and tetrachloroethylene do to give
mainly a hexachlorobutene. The chlorinated of this compound to Octochlorobutane and then double dehydrohalogenation using a method similar to G_C's
may yield the required butadiene but this is not certain (some experiments are required and will be conducted as I have now acquired the necessary
dichloroethylene).
The oxidative couple idea offers a much more elegant route but I have never heard of a comparable reaction with halocarbons in liquid phase reactions
nor can I find much basic chemistry for trichloroethylene. The most useful I have found was published on this web site by Woelen and its reactions
with alkalis etc. If anyone knows of such a reference I would be interested to hear of it (I have already checked such refes as Ullmanns but they are
always slanted towards the industrial scale reactions).