woelen
Super Administrator
Posts: 7977
Registered: 20-8-2005
Location: Netherlands
Member Is Offline
Mood: interested
|
|
Preparation of PBr3?
I want to make some PBr3, just a small amount, which I want to use for a few small experiments with organic acids and alcohols.
I took 0.5 ml of bromine and mixed this with 3 ml of CHCl3.
I took a big spatula full of red phosphorus and added this to 2 ml of CHCl3.
Next, I added the solution with the bromine in small portions, swirling after each addition.
Each time, when some bromine, dissolved in CHCl3 is added, the liquid boils a little bit (boiling of the volatile CHCl3) and the color of the bromine
quickly fades. Finally, when all bromine is added, then the liquid remains pale yellow and somewhat turbid. Excess red P settles at the bottom.
I decanted the 5 ml of liquid from the excess red P and put this in a closed vial. After half an hour, a dirty beige/yellow precipitate has settled
and a clear pale yellow liquid is above the precipitate. This pale yellow liquid is fuming intensely, when exposed to air.
I expected a somewhat different outcome from this experiment. I expected that the liquid would become colorless and no precipitate would be formed. I
used excess red P and with the excess red P I expected all of the bromine to go to PBr3. According to literature, PBr5 is a bright yellow solid and
PBr3 is a colorless liquid.
What could this flocculent dirty beige/yellow precipitate be? Is this PBr5?
The problem is that I do not have any literature, which describes production of PBr3 from red P. All well known procedures start from white P, but I
do not want to spend that little amount of (impure) white P on this synthesis, while I have a fairly large amount of red P.
If I have a well worked out procedure, then I intend to make 10 grams or so of PBr3 and I also would like to make some PBr5, but I have not seen a
single procudure for making this, which is based on red P.
When I drive off all CHCl3 from my liquid, then a single turbid pale yellow blob of fuming liquid remains behind. It certainly is not a clear
colorless liquid. I have the impression that it is a mix of PCl3, PCl5 (and possible POCl3), but I do not have the equipment and analysis tools to
investigate its precise nature. And before I spend larger amounts of bromine, I first want to be more sure about what happens in this experiment.
Maybe one of you (Sauron?) can help out with this?
[Edited on 23-11-08 by woelen]
|
|
|
Fleaker
International Hazard
Posts: 1252
Registered: 19-6-2005
Member Is Offline
Mood: nucleophilic
|
|
Woelen, I'm of the opinion that you made PBr5 only. I've only read that PBr5 (and PCl5) can be obtained by reaction of the halogen with red
phosphorus. We've seen garage chemist attempt the chlorination of red phosphorus, and all he reported was the pentachloride. The pale yellow colour of
the chloroform might just be PBr5 dissolving in it, which could explain the intense fuming. I think you truly do need white phosphorus (good thing it
is very easy to make!).
Surely there's a way to test for the hydrolysis product of PBr3?
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
|
|
|
gsd
National Hazard
Posts: 847
Registered: 18-8-2005
Member Is Offline
Mood: No Mood
|
|
| Quote: | Originally posted by woelen
I want to make some PBr3.........
The problem is that I do not have any literature, which describes production of PBr3 from red P.........
|
Inorganic Syntheses Vol - II page 147-151, describes the procedure for PBr3 with Red P. The copy of this particular procedure is attached.
The major difference between the procedure described by you and the IS appears that you have not taken any particular precaution to keep moisture out.
The scale on which you are operating, even a trace amount of water will ruin your experiment. The IS procedure gives extra emphasis on very dry
reagents and apparatus.
Hope it helps
gsd
Attachment: IS-2_PBr3_Procedure.pdf (1.6MB)
This file has been downloaded 781 times
|
|
|
S.C. Wack
bibliomaster
Posts: 2419
Registered: 7-5-2004
Location: Cornworld, Central USA
Member Is Offline
Mood: Enhanced
|
|
Vogel's obscure text happens to detail the preparation in carbon tet that is vaguely mentioned in OS CV2.
|
|
|
Jor
National Hazard
Posts: 950
Registered: 21-11-2007
Member Is Offline
Mood: No Mood
|
|
The fuming of these phosphorus bromides is truly amazing.
20 minutes ago, I prepared the same solution as woelen in chloroform. After adding the solutions to eachother in small steps, my liquid starts to
boil. At this point the liquid is strongly fuming, and it looks like as there is stream of steam coming out, a lot of fumes!
After I poured the red liquid in another test tube, I took the test-tube with the residues in it, and clamped it, but I tightened the clamp too much
and it shattered  . At that point a HUGE could of HCl-vapour rised up in my fume hood, truly amazing. As a comparison, i broke an ampoule of AcCl and
poured this in a small bottle, and there was little fuming here.
The remaining liquid is red, and when shaken with equal volume of water, it becomes light yellow, and a little turbid, and the water layer is light
yellow. How is this? If the red color is bromine, either layers should become dark red right? They both were almost colorless after adding a few mL
of water.
After, I collected the chloroform as chlorinated waste, and the water was flushed down the drain. No smell of bromine was observed when pouring down
the drain.
So why is the red color? I also cannot observe any bromine vapor above the chloroform-solution.
|
|
|
woelen
Super Administrator
Posts: 7977
Registered: 20-8-2005
Location: Netherlands
Member Is Offline
Mood: interested
|
|
Jor, the result of your experiment differs from mine. I ended up with a pale yellow solution, with a beige/yellow flocculent precipitate. I used
excess phosphorus. Did you use excess phosphorus as well? If you used excess bromine, then of course the liquid remains red.
I also can explain that mixing with water gives a pale yellow color and not the red color of bromine. As soon as water is added, any excess bromine
instantly is destroyed:
PBr3 + 3H2O --> 3HBr + H3PO3
H3PO3 + H2O + Br2 --> H3PO4 + 2HBr
The phosphorous acid, formed on hydrolysis of PBr3 immediately reduces any excess Br2. This experiment from you is an indication that PBr3 is formed
and not only PBr5. If only PBr5 were formed, then the red color of Br2 would not disappear when mixed with water.
@gsd: Thanks for the attached document. Although I will not try the experiment with such large amounts of Br2 and P, I'll certainly try to follow the
directions, given in the document. Next time, I'll assure that the reagents are really dry.
[Edited on 23-11-08 by woelen]
|
|
|
Fleaker
International Hazard
Posts: 1252
Registered: 19-6-2005
Member Is Offline
Mood: nucleophilic
|
|
gsd, thank you for the paper; apparently excess phosphorus is key. I wonder why garage chemist had poor luck with PCl3? It appears that Woelen did
everything right except for precluding moisture from entering the reaction mix. I will give this reaction a try.
Neither flask nor beaker.
"Kid, you don't even know just what you don't know. "
--The Dark Lord Sauron
|
|
|
garage chemist
chemical wizard
Posts: 1803
Registered: 16-8-2004
Location: Germany
Member Is Offline
Mood: No Mood
|
|
I have done the synthesis of PBr3 quite some time ago and remember that it worked very well for me.
I proceeded according to a procedure I found somewhere that used CCl4 as the reaction medium.
I used dichloromethane instead because of the easier fractionation of this solvent from the PBr3. This worked perfectly.
I placed dry red P (slight excess) in a flask fitted with a reflux condenser, dropping funnel and CaCl2 drying tube on top of the condenser, covered
the P with DCM (previously distilled from P2O5 in a still protected from aerial moisture) and put dried bromine (shaken with conc. H2SO4 and
distilled) into the funnel.
The DCM started refluxing by itself upon slowly adding the bromine.
After all was added, condenser and funnel were removed and the flask was set up for distillation (no column is necessary when using DCM as solvent),
the DCM distilled off and the PBr3 collected as soon as the correct boiling point was reached. Yield nearly quantitative.
No PBr5 is formed when red P is present in slight excess and the mixture allowed to reflux during the bromine addition.
This is a big contrast to the analogous reaction of red P with chlorine in chloroform that I have done as well. Here, PCl5 and red P coexist despite
refluxing and PCl3 can't be obtained (and even if PCl3 formed, it would be very difficult to separate from the solvent due to the similarity in
boiling points).
I explain this difference in reactivity between Pbr5 and PCl5 by the markedly lower stability of PBr5. The latter is dissociated considerably into
PBr3 and Br2 already in the solid state, shown by its yellow/orange color, the dissociation becoming nearly complete at 100°C.
PCl5, on the other hand, is much more stable, and requires heating in the solid state with red P to react to PCl3.
If you want to make PBr3, you should switch to proper apparatus instead of beakers and bottles. Working with, and especially preparing, phosphorus
halides isn't "test tube chemistry".
Your dirty beige/yellow precipitate is probably the hydrolysis product of PBr3, H3PO3, which is insoluble in the solvent and in PBr3.
I got a viscous "ring" of H3PO3 on the walls inside my PBr3 storage bottle after several months due to reaction with traces of moisture.
Also, PBr3 is reactive with oxygen as well, slowly giving POBr3. For optimum results, fractionate before use, or use directly after preparation. This
is also the case for PCl3- it can turn into POCl3 when exposed to oxygen during storage.
Converting alcohols to alkyl bromides with PBr3 requires following a specific procedure that includes keeping the alcohol at -10 to -5°C during the
slow addition of PBr3 to keep the intermediate HBr gas dissolved in the reaction mixture- if it escapes, you only get the alkyl phosphite.
|
|
|
woelen
Super Administrator
Posts: 7977
Registered: 20-8-2005
Location: Netherlands
Member Is Offline
Mood: interested
|
|
I think I have another problem with my CHCl3. The liquid is stabilized with appr. 1% ethanol. I added a pinch of PCl5 to the CHCl3 in order to check
the effect of ethanol on this. The PCl5 dissolves, but while doing so, bubbles of gas (most likely HCl) are produced and I think this is from the
ethanol. I think that this ethanol will give rise to formation of POBr3 and/or H3PO3 when making PBr3 from red P and Br2.
So, next time I have to switch to CH2Cl2 instead of CHCl3. Could this reaction also be done in low-boiling ligroin, in a dark place, such that the Br2
does not react with the ligroin? Ligroin is much easier to obtain for me than CH2Cl2 and also a lot less expensive. Ligroin is low-boiling (40..60 C)
and can easily be separated from PBr3. I hope to find some time next weekend to experiment with this reaction, but if someone knows the answer in
advance or has some advice in using ligroin, then that would be nice.
[Edited on 27-11-08 by woelen]
|
|
|
S.C. Wack
bibliomaster
Posts: 2419
Registered: 7-5-2004
Location: Cornworld, Central USA
Member Is Offline
Mood: Enhanced
|
|
3 methods of removal of ethanol from chloroform are in Vogel, and also Perrin and Armarego.
|
|
|
Jor
National Hazard
Posts: 950
Registered: 21-11-2007
Member Is Offline
Mood: No Mood
|
|
The bubbles you observe not only HCl, also ethyl chloride.
I would not remove the ethanol from the chloroform, as it will then form quite some phosgene in with air and light.
I would use DCM. It's NOT expensive 
Labstuff sells for 10 EUR a liter, reagent grade.
I have a liter myself.
By the way, check your eMail
|
|
|
panziandi
Hazard to Others
Posts: 490
Registered: 3-10-2006
Location: UK
Member Is Offline
Mood: Bored
|
|
You could safely remove the ethanol from chloroform prior to use with anhydrous calcium chloride for example. If the chloroform is used straight away
it will not be dangerous due to phosgene formation.
|
|
|
DJF90
International Hazard
Posts: 2266
Registered: 15-12-2007
Location: At the bench
Member Is Offline
Mood: No Mood
|
|
Slightly off topic but how does the CaCl2 remove the ethanol? I know it does, but is it a chemical reaction or just some kind of selective absorbtion?
Or a completely different method altogether?
|
|
|
woelen
Super Administrator
Posts: 7977
Registered: 20-8-2005
Location: Netherlands
Member Is Offline
Mood: interested
|
|
A good idea to use anhydrous CaCl2. This stuff is dirt cheap and can be purchased in any hardware store.
@DJF90: I think that this reaction is similar to hydration of CaCl2. Most likely the ethanol molecules coordinate to the calcium ions, just like water
molecules do. So, instead of hydration we have 'alcoholation' 
|
|
|
DJF90
International Hazard
Posts: 2266
Registered: 15-12-2007
Location: At the bench
Member Is Offline
Mood: No Mood
|
|
Oh right, that makes sense. But what about other anhydrous salts? Why dont they "react" with the ethanol (and other alcohols?)?
|
|
|
12AX7
Post Harlot
Posts: 4803
Registered: 8-3-2005
Location: oscillating
Member Is Offline
Mood: informative
|
|
Others do. Probably anhydrous MgCl2 is another. Alcohol isn't a very strong nucleophile, so you need a pretty strong electrophile to subtitute it;
sodium amide would react to form sodium ethoxide and ammonia. NaOH will form an equilibrium of ethoxide and water, hydroxide and alcohol, which may
be enough to precipitate it at least.
You could also oxidize the ethanol to acetic acid (or all the way to CO2!) and precipitate that with a base.
Tim
|
|
|
panziandi
Hazard to Others
Posts: 490
Registered: 3-10-2006
Location: UK
Member Is Offline
Mood: Bored
|
|
I don't know how great an idea it would be to add hydroxides to chloroform may get some carbene formation. You could oxidise the ethanol but then you
would have an oxidiser in the chloroform which would require separating. Using calcium chloride to complex the alcohol and then filtering it off is
dead easy and much cheaper option. CaBr2 and CaI2 would also work but am unsure about MgCl2.
|
|
|
woelen
Super Administrator
Posts: 7977
Registered: 20-8-2005
Location: Netherlands
Member Is Offline
Mood: interested
|
|
I now tried the same reaction on a somewhat larger scale with better removal of moisture. I mixed some dried bromine (dried by shaking with H2SO4)
with dried CH2Cl2 (dried by shaking with powdered P4O10) and in another flask I made a suspension of red P in CH2Cl2 (also dried with P4O10). Then I
dripped the Br2/CH2Cl2 mix into the suspension of red P in CH2Cl2. Each time when a drop is added, there is some boiling and the liquid heats up
considerably.
This time the reaction mixture remains clear. I used excess red P and when all of my bromine was added, a clear orange/yellow liquid was obtained,
with still quite some red P at the bottom. I did not have the turbidity due to moisture as I had in my previous test tube experiment.
Next, I poured the yellow/orange liquid into a spherical flask and carefully heated this flask such that the CH2Cl2 boils off. When this is done, an
amazing amount of white fumes are produced as well, apparently also some PBr3 boils off. At a certain point, the material becomes somewhat viscous and
the temperature rises sharply. I stopped heating at this point. Crystals of yellow material are formed inside the spherical flask and a yellow liquid
remains as well. I transferred some of this liquid to a test tube with some water and it reacts fairly vigorously and a lot of heat is produced. The
resulting liquid is reducing, it quickly reduces dichromate to green chromium(III) without formation of bromine. So, there must have been a lot of
PBr3 in it, whose hydrolysis products easily are oxidized to phosphorus(V) compounds.
Still, I am not really happy with my results. I have the strong impression that I have a mixture of PBr3 and PBr5 and despite the excess amount of red
P used in the experiment, I still am left with quite some PBr5. I tried heating some of the liquid to a much higher temperature than the boiling point
of CH2Cl2 and when this is done, the air above the liquid becomes light brown. Apparently, some bromine is formed on heating. On cooling down, this
color disappears again.
Why don't I get pure PBr3? I did use excess red P and allowed this to be in contact with the yellow/orange liquid for several hours, before I drove
off the CH2Cl2.
|
|
|
benzylchloride1
Hazard to Others
Posts: 299
Registered: 16-3-2007
Member Is Offline
Mood: Pushing the envelope of synthetic chemistry in one's basement
|
|
I have synthesized phosphorous tribromide several times using a procedure similar to that in Vogels Textbook. I was using the PBr3 to synthesize
phosphine ligands for coordination chemistry and catalysis. I used heptane obtained by the distillation of automotive starting fluid and did not have
a problem with the separation of the solvent and PBr3 using a Snyder column with 3-partitions which is commonly used for removing solvents prior to
analysis of pesticides via GCMS. The yield was always around 75%. Red phosphorous usually contains some impurities which appear after reaction with
with the bromine. The solution must be filtered through a fluted funnel before distillation. Do not disassemble the distillation apparatus until it is
cold, spontaneous ignition of the residue can occur! This was experienced in one of my syntheses of PBr3. An interesting aside; phosphorous
trichloride can be easily produced by passing dry chlorine over red phosphorous, use a wide gas inlet tube to avoid plugging. The reaction requires a
little bit of heating to get it started and the PCl3 distills over in nearly quantitative yield. Upon distillation, very little phosphorous
oxychloride was noted. I have ran this synthesis twice also and it works. A literature procedure can be found in Inorganic Syntheses Collective Volume
II. This information may be of use to many who have access to the required chemicals. My interest in these compounds lies in the fact that they can be
converted to many interesting ligands and also their use in general synthetic organic chemistry.
Amateur NMR spectroscopist
|
|
|
len1
National Hazard
Posts: 595
Registered: 1-3-2007
Member Is Offline
Mood: NZ 1 (goal) - Italy 1 (dive)
|
|
You dont get much POCl3 that way because there is no where for oxygen to come from, but you do get substantial amount of PCl5 dissolved in the PCl3,
since equilibrium favours its formation in the Cl2 flux.
|
|
|
mario840
Hazard to Others
Posts: 229
Registered: 20-1-2010
Member Is Offline
Mood: No Mood
|
|
Say that my friend have PBr3 and PBr5 can he make PCl3 or POCl3 ??
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
