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Author: Subject: Definitive POCl3 prep
njl
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[*] posted on 2-7-2020 at 07:34
Definitive POCl3 prep


After UTFSE I've seen several claims that P2O5 and NaCl should react at high temperatures to form POCl3. The only ref I've found will be linked below, but in their experiments they always used a fluoride salt as well as NaCl. Can anyone weigh in on this?





Attachment: Tarbutton.POCl3.from.P2O5.NaCl.pdf (802kB)
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DavidJR
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[*] posted on 2-7-2020 at 18:54


See Lerner, Leonid (2011). Small-Scale Synthesis of Laboratory Reagents with Reaction Modeling. Boca Raton, Florida: CRC Press. pp. 169–177. ISBN 9781439813126. PDF available from libgen.


Quote:

19.3.1 Preparation from P2O5

The best reaction vessel for this preparation is a test tube with a quickfit joint, a capacity of at least 100 mL, and at least 22 mm in diameter. If this is not available, a 100-mL, single-neck flask can be used instead with a slight loss of yield. Scaling the reaction requires proportionally larger vessels.

First, 35.1 g (0.25 mol) of phosphorus pentoxide is rapidly introduced into the reaction vessel, ensuring that as little atmospheric moisture as possible reacts with the P2O5, as it subsequently generates HCl and HPO3, which attacks the glass. The transfer is best done by inverting a powder funnel inserted into the reaction vessel, over the mouth of the reagent bottle, and tilting until approximately the right amount of reagent is transferred into the flask. Next, 34 g (0.58 mol) of finely ground and thoroughly desiccated (1 h at 250°C) NaCl is added, and the reagents thoroughly mixed. The approximately twofold excess of NaCl improves the yield by about 10%. Finally, about 10 g NaCl is poured in a layer on top of the mixture, which serves to convert unreacted P2O5 subliming from the reaction zone (bp 360°C). The reaction vessel is placed in an air oven and connected to a bend leading through a Liebig condenser to a receiver flask immersed in cold water. The outlet from the flask is vented through a CaCl2 protection tube with a bubbler optionally attached for observation of reaction progress.

The oven is rapidly heated to 270°C, from where the temperature is raised more slowly to 450°C at about 65°C/h. A distillation conducted more rapidly than this will serve both to reduce yield by excessive sublimation of P2O5, and increase the possibility of the reagent vessel bursting due to excessive buildup of HCl and POCl3 pressure inside the reactor, as these do not have sufficient time to percolate through the viscous sodium metaphosphate medium (the reagents liquefy during the reaction). About 8.2 g (0.053 mol) POCl3 collects in the receiver at the end of the reaction while the reactor tube loses 9.3 g (the difference is due to losses in the distillation setup), corresponding to a yield of 76% (86% with respect to reactor weight loss) based on P2O5.


[Edited on 3-7-2020 by DavidJR]
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njl
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[*] posted on 3-7-2020 at 07:32


Perfect, Thanks.

Edit: In the interest of not starting a new thread, could P2O5 be used to dehydrate a secondary amine salt to an amide (akin to formamide synthesis). I know that excess dehydration of primary amides can give nitriles, but it seems like a substitution on the nitrogen would prevent this. I can't find any work on this so I'm not expecting a definite answer, but unless anyone can see any glaring issues with this logic I think I might experiment with this idea.

[Edited on 3-7-2020 by njl]
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