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Sauron
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PBr3 -> PCl3 "New" Old Method
We have previously discussed the conversion of PBr3 to red P using Hg in the manner described in the P section of Brauer, q.v.
That method is do-able butrather long, and gives a product contaminated with Hg and Br2.
Now it seems that method is superfluous, at least for the purpose of turning the P into PCl3 or PCl5.
According to Mellor, citing J.H.Gladstone, Cl2 displaces Br2 from PBr3
2 PBr3 + 3 Cl2 -> 2 PCl3 + 3 Br2
The boiling points of the trichloride and tribromide are about 100 C apart so with any sort of column, Br2 will come off first, then PCl3 at 74-78 C
and any unreacted PBr3 will remain in the pot
HgCl2 also reacts to form solid HgBr2 and PCl3. This method is however unduly expensive, unless you are making your own HgCl2.
2 PBr3 + 3 HgCl2 -> 2 PCl3 + 3 HgBr2
A TEXTBOOK OF INORGANIC CHEMISTRY Vol VII Part II (1934) by Prideaux, makes the same assertion that chlorine's affinity for P is greater than that of
Br or I but the author uncharacteristically does not give a lit. reference.
@Formatil, does Gmelin's vhapter on PBr3 have anything to say about this, especially literature wise?
I have a liter of PBr3, and can buy more anytime, so I am really quite interested.
The paper by Gladstone cited by Mellor is J.Chem.Soc 3, 5, (1851) which by the way was during a brief period when that journal was quarterly.
At first glance the paper concerns the reaction of sulfur with PCl5 and I have yet to find any mention of halogen exchange with PBr3. It is anyway
attached. @woelen, you may find it interesting and perhaps confusing.
[Edited on 16-2-2009 by Sauron]
[Edited on 16-2-2009 by Sauron]
Attachment: qj8510300005.pdf (643kB)
This file has been downloaded 564 times
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gsd
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Passing Cl2 thru' Bromides is a general method of displacing Br by Cl.
In fact industrially, Bromine is made (recovered to be precise  ) by displacing it
from the inorganic bromides dissolved in sea water concentrate (called "bittern") in a counter current contacting column.
gsd
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Sauron
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Isn't it nice to have support for a specific application?
I regard my PBr3 as too precious to chance on a guess. Reactions often stated to be general often have some fine print that a Boston lawyer would be
proud of.
Case in point
3 CCl4 + AlBr3 -> 3 Cl3CBr + AlCl3.
Br ought not to displace Cl - but it does.
I would like to do the reverse, to make CCl4 from my Cl3CBr
because I can and have bought the latter but the former is prohibited. (Montreal)
I have yet to see any lit. on displacing Br with Cl in this specific instance. So much for general reactions.
[Edited on 16-2-2009 by Sauron]
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Sauron
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In a PM a member commented that Br2 liberated in this fashion will combine with unreacted PBr3 to form PBr5.
However the lit. states that Cl2 also decomposes PBr5 to PCl3 and Br2.
As the reaction proceeds, with increase of PCl3 and decrease of PBr3, free Br2 will separate as a lower layer because it is only partly miscible with
PCl3. Indeed, it is reported that Br2 dropped into PCl3 falls through without reaction.
There are known bromochlorides or chlorobromides of phosphorus but these do not form at ordinary temperatures or higher. They form in the cold. See
Mellor, also Prideaux.
It has also been advanced as a criticism that the bps of Br2 and PCl3 are too proximate for frationation. Bromine boils at just under 60 C and PCl3 at
74-78 C. Given that the bulk of Br2 will be removed as a separate layer and that only a minor amount will be present in the PCl3, given an efficient
column I do not see why 1 or 2 distillations will not clean this up.
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Nicodem
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| Quote: | Originally posted by gsd
Passing Cl2 thru' Bromides is a general method of displacing Br by Cl. |
Actually, it is only general to solutions of ionic compounds, that is Cl2 oxidises the bromide ion. Such redox reactions generally does not occur on
covalent compounds unless allowed by alternative mechanisms. For example, you can't "displace" Br from ethyl bromide with Cl2 and so on. The reason
why it works on PBr3 is not in that Cl2 oxidises the bromide ion since there are no bromide ions in PBr3, but because Cl2 oxidises PBr3 to the
transient species PBr3Cl2 which being highly unstable decomposes to Br2 and PBrCl2 and this further reacts the same way (or disproportionates) until
you have only PCl3 and Br2. It is a (III)/(V) phosphorous valence thing, not a ionic redox reaction.
Similarly the reaction of CCl4 with AlBr3 that Sauron cited above is also not comparable to Cl2 redox reaction with the bromide ion (actually it is
not even a redox reaction). It appears to be a simple S<sub>N</sub>1 reaction driven by the extreme acidity of AlBr3.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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Sauron
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Thanks, Nicodem, and very interesting.
How would you suggest accomplishing the replacement of Br on BrCCl3 with Cl?
Excessc AlCl3 at reflux?
Cl2 and hv? Danger here is C6Cl6 formation via the pesky trichloromethyl radical.
Or do I need to go to some more extreme chlorinating agent like (shuffer) SbCl5?
The literature is silent on this, probably because it is, on its face, unsound economically.
However, given that I am allowed neither CHCl3 not CCl3, but am allowed BrCCl3, it is a viable route for me if I can make it work.
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Nicodem
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The problem with the reverse reaction, that is 3 CCl3Br + AlCl3 <=> 3 CCl4 + AlBr3, is in that it goes against the thermodynamics (from AlCl3 a
stronger acid forms, AlBr3; even though the higher thermodynamic stability of CCl4 vs. CCl3Br gives a bit of a compensation). So you end up with
praising for fast kinetics and guestimation of the equilibrium constant being approximately bellow 30 which would enable you to remove CCl4 as it
forms via fractionaction from the slurry of CCl3Br with a large excess of AlCl3 (and nevertheless, once too much AlBr3 forms even a fractionation
column would not help to remove the ever more tiny equilibrium amount of CCl4). Though it might be very much worth a scientific experiment, I do not
see it as preparative method.
In principle you can calculate the equilibrium constant since, I think, most thermodynamic parameters of the components of this reaction are known.
Yet you can not calculate its kinetics - if the kinetics are too slow you can not remove CCl4 as it forms because it forms too slowly.
A photochemical homolytic cleavage of CCl3Br in a Cl2/CCl3Br solution should in principle give a Br2/CCl4 solution. The C-Br bond is more easily
homolysed than C-Cl and both the CCl3* and the Br* radicals are easily oxidized with Cl2, yet I'm no expert in photochemistry so I would not put a bet
on it.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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Sauron
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Yes I actually dug up papers on the UV photolysis, using special Kopp Glass filters to avoid the side reaction, and it was all done on a tiny scale
for p-chem purposes, so difficult to turn into a prep process. I identified a filter superior to the one used by the authors (syill made) and
discussed this on the forum.
What about the brute force option, SbCl5, that sort of thing? Still SN1 I think.
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panziandi
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I hate to keep raising my head on the CCl3Br > CCl4 topic but I have a few ideas
1) it may be possible to use a chlorinating agent such as SbCl5, or even HgCl2 AgCl based on the same reaction working with these fluorides.
2) it may be possible since the Br in this compound is reactive, to preferentially remove the Br over Cl with moist Fe to give chloroform. Ok it is
not a reaction to give carbon tet but it does give you chloroform which for Sauron is impossible to purchase like carbon tet.
I have spoken to Sauron with regards these ideas and I think it is safe to say that neither of use are sure if they will work. Given the bp difference
of CHCl3, CCl4 and CCl3Br I do not see why these reactions could be performed to fractionally separate the product as it forms driving the reaction
forward if need be.
I do not have CCl3Br to try either of these reactions I have been proposing, but in light of Saurons post with regards the reaction between CCl4 and
AlBr3, I may be able to make a small quantity of CCl3Br and give these reactions a go.
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Sauron
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Thanks. I have not forgotten your suggestions..
HgCl2 MW 271.5
HgBr2 MW 360
CCl4 MW 152
BrCCl3 MW 197
Stoichiometry
2 BrCCl3 + HgCl2 -> 2 CCl4 + HgBr2
Merck makes no mention of HgCl2 solubility in CCl4, though it is slightly sol in CS2.
The liquid phase will lose 45 g/mol, the solid white crytals gain 90 g/mol. The d of the liquid decreases sharply from 1.997 to 1.59 and the bp
decreases to 76 C from 105 C C from . CCl4 can probably be stripped off during the reaction but the column will need to be efficient and the reflux
ration high. I bet this will be a slow process.
The refractive index of CCl4 is 1.459 and that of BrCCl3 is 1.5055
[Edited on 18-2-2009 by Sauron]
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panziandi
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Yes Sauron the only trouble with my suggestion is the expense and also the m.w. of both AgCl and HgCl2. On a mass for moles basis it is a hideously
large reaction. I will try and make some CCl3Br at somepoint and then give this a go. Both AgCl and HgCl2 are easy to make from scrap metals if this
reaction was to be scaled up with reduced cost in mind.
Anyway back onto the original thread topic I think... I don't see why a couple of fractionations of the PCl3 wouldn't purify it of Br2 either. What is
the solubility of Br2 in PCl3 anyway? Also we all know just how volatile Br2 is perhaps a brisk current of warm dry nitrogen could be blown through
the PCl3 to remove the Br2?
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Sauron
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Better make that Ar. N2 often reacts with P compounds.
Br2 has very limited solubility or miscibility with PCl3. Added to PCl3, liquid bromine falls through and forms a bottom layer without reacting. I
have not seen any quantification of the miscibility, though.
I have both HgCl2 and PBr3. So I can try this but I want to get a new pair of gauntlets for my glove box. This is going to be an Ar atmosphere job
and, I do not want to make POCl3 out of my PCl3 by expositiong it to atmospheric O2.
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panziandi
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Mmm I was going to say Ar then I thought "nah N2 would do" ... really I never thought it would react with nitrogen.
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Sauron
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Hold on, you are going to do BrCCl3 to CCl4. That is fine with N2. I am talking PBr3 -> PCl3. and for that I would use Ar.
That's what I get for posting at 4 a.m.
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Formatik
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| Quote: | | Originally posted by Sauron @Formatik, does Gmelin's vhapter on PBr3 have anything to say about this, especially literature wise?
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I had only some info from the 8th edition on the chlorides, which synthesis info was posted in the acetic anhydride thread. The 7th edition does
mention that mercury chloride and PBr3 get PCl3 and mercury bromide according to Gladstone in: Phil. Mag. [3] 35, 345; J. pr. Chem. 49, 40; J.B. 1849,
243.
The Refs.: Phil. Mag. = Philosphical Magazine and Journal; J. pr. Chem. = Journal für praktische Chemie; J.B. = Jahresbericht. The prime reference
should be Phil. Mag. and the J.B. likely referencing the two refs since it is similar to Centralblatt.
Basically, Gladstone from another angle. I suspect the 8th edition likely has additional references, but can't confirm it at this time.
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Sauron
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I do not know where (or if) I can lay hands on Philosophical Magazine in syberspace.
J.prakt.Chem. is another matter.
Unfortunately the free BnF archive for this journal starts 20 years too late.
But Wiley to the rescue.
Ueber die Verbindungen der Halogene mit Phosphor (p 40-51)
J. H. Gladstone
J.prakt.Chem 49, 40 (1850)-51
DOI: 10.1002/prac.18500490102
Thanks, @formatik, I am requestion this from the angels and demons of References.
For the benefit of other members: J.prakt.che, can be found at Wiley as a previous title of Advanced Synthesis & Catalysis
And from 1870 till mid 1930s it is in BnF Gallica for free.
[Edited on 18-2-2009 by Sauron]
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Sauron
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PS to above
Philosophical Magazine, started in 1798, is the flaship publication of Taylor & Caldwell who ballyhoo it as the oldest continuously published
scientific journal.
However the digital archive extends back only to 1956.
This sucks'
BnF does not have it.
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panziandi
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Well yes I will try the CCl3Br if and when I get a chance to (involves making CCl3Br first from CCl4 just to see if I can then convert it back to
CCl4!!!)
My post about flushing Br2 out with a current of warm gas was in relation to PBr3 > PCl3 and I never realised nitrogen would react with PCl3!
Besides if the Br2 contamination was low it wouldn't require much of a brisk current of gas to remove any. But I suppose it is best to be safe than
sorry and use Argon, then fractionate/distill the PCl3.
I like the sound of this method Sauron. You say you can get PBr3 but you also said it was too precious. Am I correct in assuming you can get hold of
PBr3 unlike PCl3 but that PBr3 is exceedingly expensive? If so I suppose you don't want to risk losses in the protocol and using Argon would certainly
reduce any potential losses.
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Sauron
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I have a liter of PBr3. I pay approx 2X the ex works price, which is shipping and import costs. I do want o have confidence in the method, and yes I
will use Ar atmosphere to prevent atmospheric oxidation. Just opening a sealed bottle of PCl3 starts the process of oxidation, slowly at ambient.
Bubbling dry air or O2 through PCl3 will convert it over time to POCl3 and if moist, H3PO4. Kind of expensive for H3PO4.
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panziandi
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I know that PCl3 oxidises to POCl3 but what about reaction with nitrogen? I keep my PCl3 is a full bottle, I originally had a litre and when I used
half I moved it into a 500mL bottle to save the headspace since it's not a chemical I often use!
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Sauron
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I would be wary of distilling it or refluing it under anything but Ar.
At ambient, any reactions are probably too slow to matter.
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S.C. Wack
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| Quote: | Originally posted by Sauron
I do not know where (or if) I can lay hands on Philosophical Magazine in syberspace. |
Others can. I posted a direct link to the very article in question when I returned to the "Something Else to Do with Your Red P" thread 2 days ago.
Here it is again for those not listening.
http://books.google.com/books?id=g08EAAAAYAAJ&pg=PA345
However, I
| Quote: | Originally posted by Sauron
can go to hell and take google books along with you. |
Since I already tried to help him view these pages and was inexplicably violently rebuffed, someone who desires to do so will have to convert this to
a Sauron-approved format.
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Sauron
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Do you prefer a private apology or a public one? I will do either or both.
That link works. I can read the Gladstone paper from Phil.Mag.
However, I cannot save it, nor can I print it.
I downloaded Google Books Downloader and tried to install it, it crashed at once. Well, it is an alpha after all. That however does not get the job
done.
The paper states that "chloride of mercury" (without specifying Hg(I) or Hg(II)) displaces Br from PBr3, and that similarly I2 is displaced from PI3
by either chlorine or bromine.
So far I have seen no mention of the use of the elemental halogens to effect this, although Mellor syayes it does claim so.
I do not know why the link you provided earlier failed, nor why the one not_important provided also failed. I can try getting to this in archive.org
as I know how to d/l from there.
Anyway thank you for the assistance, I regret the earlier unpleasantries, I withdraw my remarks, and will endeavor to avoid any repetition.
[Edited on 19-2-2009 by Sauron]
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not_important
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Try this, a grab and slight cleanup of the text version displayed by Google Books. U2U me with questions on text and I'll try to fix them from the
G.B. image version.
I'm happy because it confirms something I suspected
3 PBr5 + 5 PH3 = 8 P + 15 HBr
in two steps:
3 PBr5 + PH3 = 4 PBr3 + 3 HBr
PBr3 + PH3 = 2 P + 3 HBr
which gives a medium temperature route to elemental phosphorus from on of the commercially available phospides through reaction with halogen for
PX3/PX5 and with aqueous acid to PH3 (yeah, toxic)
Attachment: On the Compounds of the Halogens with Phosphorus by Gladstone.txt (26kB)
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Sauron
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Thanks but I already learned how to get the image files, by a roundabout method.
1. Go to the book you want
2, Now go to Documents and Settings/Administrator/Local Settings/Temporary Internet files and delete all
Now return to the google book and slowly page through it.
Then Refresh
Now back to the temp internet files folder and Refresh.
Save all the PNG files. Those are the page images.
This is not without ptoblems and a price.
The price is all temp settings are lost
That includes logins, such as email, this forum etc
The problems are that Acrobat Pro 8.1 barfs on about half the PNGs. When that happens, I use Photoshop to open the PNG and save as a PDF. Acrobat then
recognizes it. I then renumber the pages and compile the pdf.
All right for a short article, a nightmare for a BOOK!
Goodle Book Downloader does not work.
Leech-something does not work.
Printing to PDF does not work.
So, I must still regard Google Books as being of questionable utility except when the files can also be found at archive.org from which they can be
downloaded by the FTTP function.
But I do appreciate your efforts once again.
Finally, the scan quality is really rather poor so I doubt that OCR was of too much use.
Yes phosphine will take PBr3 down to P. Now calculate how much % wt P is of a Kg PBr3 - rather expensive. OK you can generate PH3 from phosphid but
it is not without hazard,. Still, that is WP not RP.
The tidbit I found was that the mercury halide is (I) not (II). based on balanced equations in the text. Mercurous chloride. Mellow got this detail
wrong, or at least was ambiguous about it.
The procedure is to distill the trihalide from the mercury salt. Thus, PBr3 + 3 HgCl -> PCl3 + 3 HgBr, Gladstone said nothing about Cl2 gas
accomplishing this reaction, at least not in this paper.
[Edited on 19-2-2009 by Sauron]
Attachment: PhilMagGkadstone.pdf (998kB)
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