Sciencemadness Discussion Board

Synthesis of PBr3

woelen - 8-8-2011 at 13:30

I made some PBr3 with a yield of appr. 70%, directly from bromine and red phosphorus. These two chemicals react very violently, but with some care and patience it is possible to make quite pure PBr3 (without PBr5) from these two chemicals. The procedure takes quite some time and you need to distill the raw product, but the final product is quite good.

The only issue remaining is that the final product is slightly turbid. This turbidity is not strong, as the final picture on the web site shows, but it is noticeable. Any idea what causes this turbidity?

The nice thing of this synthesis is that it uses red phosphorus instead of white phosphorus. Most descriptions I have seen require the use of white phosphorus, which is added to bromine, dissolved in a solvent (the solvent dilutes the bromine and makes the reaction less violent). Red phosphorus is easier to obtain (at least for Europe-based people) and much safer to handle than white phosphorus.


PBr3 is a very interesting chemical. I now have appr. 17 ml and this is sufficient for a few demonstration experiments (some of these will follow soon).

[Edited on 8-8-11 by woelen]

Endimion17 - 8-8-2011 at 14:47

Wow, that's a lot of reagents. :)

Turbidity could be something like phosphoric acid or hydrobromic acid mist. After all, your system was opened, and water could get inside. An azeotrope could've formed, so the distillation was useless for removing it.

I don't know, it's late here, and I wrote the first thing that came on my mind.

redox - 8-8-2011 at 15:15

Great job woelen, I love reading your website. :P

You could try redistilling the PBr3 from calcium chloride (or some inert drying agent), just to see if it is a trace impurity of water or an azeotrope.

But if I were you, I wouldn't worry about it. I'm sure if would still work just fine for brominations or whatever you plan to do with it (I'm eager to see some demonstrations.!).

Endimion17 - 9-8-2011 at 04:53

No, drying must be done using anhydrous calcium bromide. Chloride will probably react if there's any water, which there probably is.

Retard-3000 - 9-8-2011 at 05:12

If there was any remaining water in the PBr3 it would have reacted with it.

redox - 9-8-2011 at 05:31

Quote: Originally posted by Retard-3000  
If there was any remaining water in the PBr3 it would have reacted with it.

Correct you are Retard ;)! I can't believe I overlooked that reaction...

PBr3 + 3H2O --> H2PO3 + 3HBr

Retard-3000 - 9-8-2011 at 06:14

:D, could the turbidity be due to the acids formed from PBr3 reaction with water ?, also redox, your equation doesn't balance :P

gsd - 9-8-2011 at 07:44

Quote: Originally posted by redox  

PBr3 + 3H2O --> H2PO3 + 3HBr

I think it is just a typo

PBr3 + 3H2O --> H3PO3 + 3HBr


woelen - 9-8-2011 at 08:43

@redox: I do not really worry bout the turbidity, my question is more of an academic nature. Most likely the turbidity is caused by not more than 0.1% of impurity. A remarkably small amount of impurity can make a chemical look very different (e.g. think of a liqud with only 0.1% of precipitate in it, which is totally opaque like milk).

Endimion17 - 9-8-2011 at 13:55

Quote: Originally posted by Retard-3000  
If there was any remaining water in the PBr3 it would have reacted with it.

If there was any pentoxide inside, and chances are there were, almost 100% phosphoric acid would've been created, and that molecule holds water to it very strongly. It is possible that even PBr3 isn't strong enough to rip it away in a chemical reaction. I'm not sure, just guessing.

Nevertheless, his product is very pure because, as woelen noted himself, even very small impurities can create large turbidities. It might be a bad thing to perfectionists, but it's a quite useful thing in a whole lot of analysis out there. :)

Lambda-Eyde - 9-8-2011 at 14:38

Woelen, why don't you use a solvent for the reaction? Carbon tetrachloride would be the ideal solvent for this, I reckon. Attached is a paper which might be of interest to some.

Attachment: IS-2_PBr3_Procedure.pdf (1.6MB)
This file has been downloaded 544 times

woelen - 11-8-2011 at 01:05

Maybe this indeed works somewhat better with a solvent. In my case that would be CH2Cl2 or CHCl3. CCl4 is more problematic. I have some, but not much and it is difficult to obtain. Using a low boiling solvent like CH2Cl2 has the added advantage that it is more easily separated from the high boiling PBr3.

For now, however, I have sufficient PBr3 to do some experiments, but if I make this again, I'll consider using CH2Cl2. I'll try this on a test tube scale first.

benzylchloride1 - 11-8-2011 at 17:34

Pure n-heptane will work for the solvent if you have a fractionating column. I have prepared several hundred grams of phosphorous tribromide at a time using this solvent. I have always been able to get a good separation of the solvent and the desired product. Wait for the distillation flask to cool to room temperature before disassembling the distillation apparatus or else the residue in the flask will spontaneously ignite!

woelen - 29-8-2011 at 01:43

I have a nice and interesting observation.

The slight turbidity, which I mentioned at the start of the thread is gone. The liquid is totally clear and has a very pale yellow color, no (not even a very thin layer) precipitate is formed. When the liquid is swirled in its container, then no solid particles can be observed. It took between one and two weeks before the turbidity was gone. One week after preparation it still was turbid, two weeks after preparation it was clear. In the meantime I did not check the liquid.

Another strange thing is that the liquid hardly wets glass anymore. It just flows over the glass without leaving a thin layer of liquid. Only some small droplets are left behind at a few places, but most of the glass is completely dry after some PBr3 has moved along it.

Maybe the turbidty was indeed due to the presence of water in the PBr3, which slowly reacts with it. I tested some PBr3 by adding it to water and it does not react violently like e.g. sodium. It even does not react like SOCl2. It just sits at the bottom of the test tube, an oily blob under water, which very slowly dissolves.