Stefan - 14/12/2006, 00:29
Title: Phosphorus pentachloride
This method of making phosphorus pentachloride, invented by me, uses a new approach to solve the problems in the
unification of the elements.
The problems with conventional reaction control (conducting chlorine onto red phosphorus) are, first, the extreme heat generated during
reaction, which must be dissipated, and second, the formation of the phosphorus pentachloride as a hard sublimate crust in the flask which must be
must be scraped off, with atmospheric moisture reducing the quality of the product by partial hydrolysis to phosphorus oxychloride.
My approach circumvents these problems by performing the reaction in an inert medium (solvent) in which chlorine is good and
Phosphorus pentachloride moderately soluble. The medium boils under reflux due to the heat of reaction, which eliminates the problem of
temperature limitation and heat dissipation is elegantly solved in one.
I used specially purified (dry and alcohol-free) chloroform as the inert medium. Unsuitable are all solvents which
react with the reactants and products.
Chloroform reacts with chlorine to a small extent to form carbon tetrachloride, but this reaction takes place only to a small extent,
which is why chloroform is suitable as a reaction medium.
Carbon tetrachloride would be the ideal choice. Dichloromethane will probably work as well, possibly taking place the
chlorination of dichloromethane occurs more readily than that of chloroform, however, so increased chlorine consumption may be detectable.
Chemicals:
Chlorine (from TCCS and HCl).
Chloroform
phosphorus pentoxide
conc. sulfuric acid
red phosphorus
Safety Instructions:
Chlorine is toxic and hazardous to the environment.
Chloroform is harmful to health.
Phosphorus pentoxide and sulfuric acid are corrosive.
Red phoshor is highly flammable.
Phosphorus pentachloride is very toxic and corrosive.
Caution: purified and unstabilized chloroform poses a risk of phosgene formation during storage. Once the
stabilizing ethanol has been removed, the chloroform should be used as soon as possible (within 1-2 days).
tightly closed and protected from light. Handle only under the fume hood.
After the chloroform has been recovered and purified, it is stabilized again with 1% ethanol to keep it shelf stable. It
contains a small amount of carbon tetrachloride after the reaction and should therefore not be poured back into the storage bottle,
but should be stored separately.
Caution. Phosphorus pentachloride reacts violently with water! Inform yourself about the properties of the substance beforehand.
reacts with many substances, sometimes unexpectedly, e.g. phosgene is formed with sodium carbonate!
Purification of the chloroform for synthesis:
The chloroform (110 ml) is first mixed in the separating funnel with about half of its own volume of concentrated sulfuric acid.
several times vigorously and for a long time, leaving it to stand for some time in between.
The chloroform is then very carefully separated from the sulfuric acid (the chloroform forms the upper phase) and stored in a
(the chloroform forms the upper phase) and mixed with phosphorus pentoxide in portions in a sealable vessel (round-bottomed flask or similar) and
shaken. Phosphorus pentoxide is added
phosphorus pentoxide is added until it remains powdery and no longer clumps together when added further. It is left to stand for a few hours
with occasional shaking.
Then the chloroform is decanted into a dried column flask, a little fresh phosphorus pentoxide is added and distilled off.
distilled off. The still should be protected against the ingress of moisture.
Procedure:
Work is to be carried out in the fume hood!
A gas generator is set up, followed by a frit washing bottle with concentrated sulfuric acid (for drying).
A 250 ml three-neck round bottom flask is fitted with gas inlet, reflux condenser and stopper and clamped over a magnetic stirrer.
clamped in place. A stirring fish is placed inside.
The gas generator is filled with trichloroisocyanuric acid (used: 84g, this was not sufficient as was shown later) and 23% hydrochloric acid (used:
150ml).
hydrochloric acid (used: 150ml, not sufficient), the reaction flask is filled with 10g red phosphorus and the purified chloroform.
The magnetic stirrer is set in motion at high speed and the chlorine introduction is started.
Due to the heat of reaction, the chloroform immediately becomes hot and soon begins to boil under reflux.
A small amount of hydrogen chloride escapes from the top of the cooler, which is due to reaction of chlorine with the chloroform.
In order to reduce this reaction somewhat, I have tried to cool the thing by water cooling:
But this turned out to be very disadvantageous, because then the inlet tube was clogged by crystallizing PCl5. Do not use
use external cooling!
Despite working at the boiling point, the inlet tube clogged several times due to PCl5 especially towards the end of the reaction. This is very
dangerous, since pressure then builds up and some component, usually the wash bottle head or the dropping funnel, can be catapulted out.
can be catapulted out. You have to keep a close eye on things and often pull out the inlet tube a little (but leave the gland
left in! No humidity should get in!) to see if it is still open. A sudden drop in the
a sudden drop in the gas injection rate, which can be seen in the wash bottle, is an alarm signal.
must be loosened.
One then pushes a rigid wire into the discharge pipe and pokes out the blockage.
The chlorine injection lasted three hours, with chloroform boiling continuously under reflux! This gives an impression
of the enormous amount of heat released by the reaction. Without a solvent, nothing really works, at least not on such a large scale.
large scale.
The chlorine developer had to be reloaded once in a while, since the frequent loosening of blockages caused a
a significant part of the chlorine escaped into the fume hood.
The feed pipe should definitely be flared at the bottom next time to avoid this!
When the reaction is complete, any red color has disappeared and the chloroform above the PCl5 takes on a green color due to dissolved excess
chlorine.
chlorine (important to avoid PCl3 formation).
This is what it looks like (still hot, magnetic stirrer turned off):
The flask was left tightly closed until the next morning (it would be better to work up immediately, but it was already
late at night).
The still dissolved PCl5 crystallized out and filled the whole flask, no liquid was visible at all:
The flask was prepared for distillaton, heating must be done with water bath.
Chloroform was distilled on the sidende water bath until nothing remained. The contents of the flask liquefied
partially.
This is how it looked when nothing more passed over:
Then the mass was crushed a little with a spatula (the flask must only be opened very briefly, because of humidity), the
distilled chloroform/carbon tetrachloride mixture was removed and the template was replaced.
Then vacuum was applied and the whole mass was slowly heated with the water bath. Some solvent escaped again (through the water jet pump, recognizable
by the smell). Heating was continued (up to almost 100°C) until the last residual solvent was removed and the PCl5 began to sublime.
Then the heating bath was removed, the vacuum was taken away and the
mass was coarsely crushed (in the flask, with the spatula) after cooling and immediately transferred to a tightly sealable glass vessel with a Teflon
seal that had been weighed empty beforehand.
glass vessel with Teflon seal.
It is weighed to determine the amount of product.
I obtained 65g, an almost 100% yield. This must also be obtained, otherwise one has worked uncleanly (air humidity
was added).
The distilled off solvent is washed thoroughly with aqueous sodium carbonate solution (still contains some
phosphorus compounds, such as POCl3 and co-distilled PCl5) and then dried (e.g. with CaCl2 or sulfuric acid) and distilled
(observe boiling range to see how much CCl4 is in it).
It is mixed with 1% ethanol for stabilization and labeled and stored as recycled chloroform (contains carbon tetrachloride).
Uses:
Phosphorus pentachloride is the strongest chlorinating agent and can generally convert almost all acids to their acid chlorides.
It can even be used to chlorinate sulfuric acid to chlorosulfonic acid! Neither PCl3 nor SOCl2 can do this.
However, only one chlorine atom is utilized. The by-product is always phosphorus oxychloride POCl3.
H2SO4 + PCl5 ---> ClSO3H + POCl3 + HCl
The acid chlorides can also be produced directly from the salts of oxygen acids by the action of PCl5. Example is
the preparation of phosgene from sodium carbonate:
Na2CO3 + 2 PCl5 ---> COCl2 + 2 POCl3 + 2 NaCl.
You can also make PCl3 from PCl5 by mixing it dry with an excess of red phosphorus and heating it. The
mixture liquefies and PCl3 can be distilled off.
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