Sciencemadness Discussion Board

Thionyl chloride from sulphur dioxide and phosphorus pentachloride?

Keras - 7-4-2019 at 04:28

Well, guys, all is in the subject :p
Has anyone attempted this reaction?

PCl5 + SO2 → SOCl2 + POCl3

Thanks!

(PS: I found out another thread dating back 6 or 7 years about the same subject, but it petered out before anything substantial was published.)

[Edited on 7-4-2019 by Keras]

Assured Fish - 7-4-2019 at 14:17

To my knowledge the only person on the forum to make thionyl chloride was magpie, who made it by way of reacting SCl2 with SO3.
His yeilds werent great but there is lots of room for modification, the thread is in prepublications.

Otherwise there is also this: http://www.prepchem.com/synthesis-of-thionyl-chloride/

Magpie - 7-4-2019 at 20:13

len1 made it first, also using SO3/SCl2, also in Prepublication.

Keras - 8-4-2019 at 04:16

Quote: Originally posted by Magpie  
len1 made it first, also using SO3/SCl2, also in Prepublication.


Alright. But I do have PCl5, so that’s why I asked.

Also, thanks for prepchem, yeah, I checked it there. They seem to say the product is impossible to purify when made that way. No a very encouraging signal… :/

woelen - 8-4-2019 at 04:35

I have seen preps, based on sulfites, but these yield a mix of POCl3 and SOCl2 (and NaCl). It will be difficult to separate these two liquids. I imagine that preps based on SO2 yield POCl3 and SOCl2 as well.

I never did it myself, but if I recall correctly, distilling SOCl2 is quite difficult. SOCl2 is not that stable and it partially decomposes at its boiling point. You have to be very careful not to overheat. If you mix with POCl3, then most likely you need to heat a little bit further than with pure SOCl2 and then you may decompose too much. It might be necessary to use (partial) vacuum distillation, but this may decrease the separation of the two even more.

Keras - 8-4-2019 at 07:23

Quote: Originally posted by woelen  
I have seen preps, based on sulfites, but these yield a mix of POCl3 and SOCl2 (and NaCl). It will be difficult to separate these two liquids. I imagine that preps based on SO2 yield POCl3 and SOCl2 as well.

I never did it myself, but if I recall correctly, distilling SOCl2 is quite difficult. SOCl2 is not that stable and it partially decomposes at its boiling point. You have to be very careful not to overheat. If you mix with POCl3, then most likely you need to heat a little bit further than with pure SOCl2 and then you may decompose too much. It might be necessary to use (partial) vacuum distillation, but this may decrease the separation of the two even more.


Prepchem states: “When the reaction is complete the crude thionyl chloride is purified by fraction distillation. The products are separated by repeated careful fractionation, using an efficient column. The boiling point of thionyl chloride, 77° C and phosphorus oxychloride boils at 108° C. This preparation is generally not completely free of phosphorus compounds. The yield of thionyl chloride is 50% of theoretical.”

So yeah, it appears that at least ½ is lost, which doesn’t make it a very efficient way to prepare SOCl2…

Keras - 8-4-2019 at 09:30

Oh, I found that pdf about purifying thionyl chloride. The keyword “thionyl chloride purification” brings up a certain number of references on Google, which tends to prove it is hard to isolate indeed.