Sauron
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Another "Two-for" Reaction CCl4 + PSCl3
I just love it when both products of a reaction are desirable and hard to obtain.
I previously had the pleasure of posting about one such, in which phthalic anhydride and benzotrichloride react to give almost quantitative yields of
benzoyl chloride and phthaloyl chloride.
Now I have run across another happy reaction:
CS2 + 2 PCl5 -> 2 PSCl3 + CCl4
There is nothing surprising about obtaining CCl4 by chlorination of CS2. But the usual process is a two stage one and is summarized by the unified
equation
3 CS2 + 6 Cl2 -> 3 CCl4 + 6 S
which only goes to about 90% in practice and is rather laborious.
But note that the theoretical ratio of CS2 to CCl4 is 1:1
By using PCl5 as the chlorine source, we get same ratio in theory, and the valuable thiophosphoryl chloride as other product.
This was reported by Rathke (Zeitsch.Chem. 1870, [2],6, 57)but the more accessible paper is by Hofmann in Ann., 115 (4) 264 (1860) and I will have
this shortly.
I found this in A Textbook of Inorganic Chemistry Vol VII Part II by Prideaux (1934)
On the same page it is said that Baudrimont reported that PCl5 can be obtained by chlorination of PSCl3:
2 PSCl3 + 3 Cl2 -> 2 PCl5 + 2S2Cl2
so that is one has no particular use for PSCl3, it can be recycled to PCl5 and the process becomes one for CCl4 only, consuming only CS2.
There may be some Ethiopians in the woodpile not revealed till I receive and translate and study the papers.
The Baudrimont paper is in Ann.Chim.Phys. 1864, [4], 2, 8
which ought to be in BnF Gallica. Yes, got it. It is >62 pages long in total but the reaction in question is on p 7-9.
[Edited on 18-2-2009 by Sauron]
Attachment: Baudrimont_p8.pdf (115kB)
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Sauron
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A related paper from Annalen describes the parallel reaction
CS2 + 2 SbCl5 -> CCl4 + 2 SbCl3 + 2 S
Antimony is a lot easier to come by than red phosphorus and is conserved. The SbCl3 is easily chlorinated back to SbCl5
The CS2 and PCl5 reaction may be reactions under autogenous pressure @ 100 C. The text needs further study.
This is obvious from the bp of CS2. I am uneassy about refluxing that at 3X the bp. Perhaps the reaction with SbCl5 proceeds at a lower temperature.
Or perhaps the reaction with PCl5 can start lower and be ramped up to 100 C as the concentratiion of CS2 declines in favor of CCl4.
[Edited on 21-2-2009 by Sauron]
Attachment: Annalen, 1860, 115, pp264.pdf (165kB)
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Sauron
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I fought my way through the German of that article. The gist is:
PCl5 only reacts with CS2 at temperatures well above 100 C in a pressure vessel, which is not convenient.
Antimony pentasulfide reacts smoothly with CS2 at ordinary temperatures and pressure, and it is particularly recommended to use a catalytic amount of
BsCl5 and to pass in Cl2 (dry) so that the SbCl5 facilitates the reaction, yielding CCl4, SbCl3 and S.
I am awaiting a more precise translation by a native German speaking chemist for further details.
Higher temperatures need to be avoided to suppress formation of SbCl3S2.
This process appears to avoid all the usual byproduct and purification problems associated with CS2 chlorination usinf Fe catalyst. Any Abyssinians in
the kindling? We will see.
[Edited on 22-2-2009 by Sauron]
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Sauron
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Three members responded to translation request, two with brief but cogent synopses, the other with a complete translation and a good job of it too.
This is attached.
Thanks to all concerned: Per and Formatik and special thanks to fractional.
[Edited on 23-2-2009 by Sauron]
Attachment: About the Reaction of CS2 with SbCl5.pdf (21kB)
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Sauron
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Based on the Hoffnab article and translation I would suggest either Hofmann's suggestion of a catalytic use of SbCl5 accompanied by elemental
chlorine, or, if a chlorine generator is not convenient, modifying the stoichiometric reaction as follows:
1. Keep CS2 in substantial excess, and employ cooling. This means charge 2-3 X the required amount of CS2 to a flask equipped with a mechanical
stirrer and an efficient reflux condenser, and add SbCl5 slowly dropwise so that no CS2 rises more than halfway in the condenser. SbCl3 and S will
precipitate throughout. After addition of the SbCl5 is complete, continue to cool and stir the slurry for two hours, then discontinue cooling.
Continue stirring and flow of coolant to reflux condenser until no more boiling is evident.
Change setup for fractional distillation. Separate the solids from the mixed liquids. SbCl3 can be recovered and chlorinated back to SbCl5 for next
run. The liquids will be CS2, CCl4, and sulfur chlorides (SCl2, S2Cl2). Some SbCl3 will be recoverable from the S2Cl2 fraction.
By refluxing CS2 and SCl2, much more CCl4 can be made and S precipitated. This is an equilibrium reaction and CCl4 formation is favored by excess CS2
and by removal of S.
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