Sciencemadness Discussion Board

Preparation of phosphorus halides from phosphorus pentoxide

Elemental Phosphorus - 29-12-2016 at 11:08

I am new here and mostly talking without experience, but if dried HCl gas was passed over phosphorus pentoxide, would it produce phosphorus pentachloride? This would provide an easier way to make useful phosphorus halides without actual phosphorus. It also seems thermodynamically favorable, but would the produced water react with the remaining pentoxide to produce H3PO4? Also, if it would work, then to produce the trichloride, could P4O6 be substituted?

Magpie - 29-12-2016 at 17:45

Heat of formation of PCl5 = -399 kJ/Mol
". ". ". " P2O5 = - 3382 kJ/Mol

Why do you say the reaction is thermodynamically favorable?

PirateDocBrown - 29-12-2016 at 18:41

You can reduce calcium phosphate in a crucible with carbon to calcium phosphide, which you can then chlorinate to PCl3.

Eddygp - 30-12-2016 at 07:23

P=O bonds tend to be very strong, which makes many reactions, like the Wittig reaction, thermodynamically favourable.
In fact, the reason why you can use phosphorus chlorides to chlorinate alcohols is because you create these bonds to oxygen.

Therefore, unless you oxidise P directly forming P-Cl, making PCl3/5 will be very difficult. From phosphorus oxides, if we avoid reducing them, there is no feasible method (most probably not, anyway).

EDIT: apparently, phosphoryl chloride can be formed by reaction of phosphorus pentoxide with sodium chloride at very high temperatures, so maybe there IS a way to get the pentachloride.

[Edited on 30-12-2016 by Eddygp]

Melgar - 30-12-2016 at 19:36

Quote: Originally posted by Eddygp  
P=O bonds tend to be very strong, which makes many reactions, like the Wittig reaction, thermodynamically favourable.
In fact, the reason why you can use phosphorus chlorides to chlorinate alcohols is because you create these bonds to oxygen.

Therefore, unless you oxidise P directly forming P-Cl, making PCl3/5 will be very difficult. From phosphorus oxides, if we avoid reducing them, there is no feasible method (most probably not, anyway).

EDIT: apparently, phosphoryl chloride can be formed by reaction of phosphorus pentoxide with sodium chloride at very high temperatures, so maybe there IS a way to get the pentachloride.

[Edited on 30-12-2016 by Eddygp]

This is still the oxychloride, that is, POCl3. I believe the thermodynamics work because you're starting with P2O5, then getting a mixture of sodium phosphates and phosphoryl chloride. Only the POCl3 would be volatile in that mixture. Temperatures aren't even that ridiculously high, if I remember right.

I've heard that using aluminum powder to reduce P2O5 to phosphorus is fairly easy, although I haven't done it myself. Still, if you want a phosphorus halide other than POCl3, you're going to need to start with either phosphorus, a phosphide, or a related phosphorus halide.

[Edited on 12/31/16 by Melgar]

Magpie - 31-12-2016 at 16:07

Quote: Originally posted by PirateDocBrown  
You can reduce calcium phosphate in a crucible with carbon to calcium phosphide, which you can then chlorinate to PCl3.


This post has piqued my curiosity. I might try this when the weather warms up. Meanwhile I took a look at some real old-school chemistry here:

https://www.youtube.com/watch?v=ZvDppuCkoew

Phosphine anyone? Note how non-challant these old gals are. :D

Elemental Phosphorus - 1-1-2017 at 06:01

Quote: Originally posted by Magpie  
Heat of formation of PCl5 = -399 kJ/Mol
". ". ". " P2O5 = - 3382 kJ/Mol

Why do you say the reaction is thermodynamically favorable?

Guess I was an idiot and screwed up somewhere.

Magpie - 1-1-2017 at 10:11

Quote: Originally posted by Elemental Phosphorus  

Guess I was an idiot and screwed up somewhere.


You are not an idiot when you take ownership and learn something. ;)

Elemental Phosphorus - 4-1-2017 at 18:41

Quote: Originally posted by PirateDocBrown  
You can reduce calcium phosphate in a crucible with carbon to calcium phosphide, which you can then chlorinate to PCl3.

Thank you for telling me. I will have to try this, as soon as my new furnace is built and maybe I can get a clay crucible (dont want to ruin my graphite crucible). I assume this would also work to produce PBr3? Before anyone says " why don't you just make phosphorus?" I dread having to work with white phosphorus, and the legality is questionable. It would suck to give the impression that we cook meth.