Keras
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Allene from acetone and P₂O₅
Folks,
given than phosphorus pentoxide can dehydrate a *lot* of compounds, including strong mineral acids, can we expect that:
P₂O₅ + (CH₃)CO(CH₃) → (CH₂)C(CH₂) ?
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teodor
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It's interesting question. Dehydration of acetone with H2SO4 gives mesitylene, the yield is 15-20 percent. I was just wondering could another
dehydration agent be used with better yield.
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Keras
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Quote: Originally posted by teodor  | | It's interesting question. Dehydration of acetone with H2SO4 gives mesitylene, the yield is 15-20 percent. I was just wondering could another
dehydration agent be used with better yield. |
But that’s an aldol condensation in this case, taking place because sulphuric acid promotes the enolisation of acetone, followed by a nucleophilic
attack of the enolate ion on the carboxyl group of another acetone molecule.
Enolisations can take place either in acid or basic conditions. I have always used NaOH to produce mesitylene out of acetone, though I never bothered
to measure the yield, given the relative inexpensiveness of both reactants.
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Tsjerk
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I think at least a part of the formation of mesitylene is based on the protonation of acetone, for which H2SO4 of course is a good source. But P2O5
will not protonate, and only after obtaining a couple of water molecules the hydration products will become acidic. Even then I don't know in what
order phosphoric acid well protonate acetone. So P2O5 might be a good candidate to dehydrate acetone.
Edit: What Keras says
[Edited on 20-2-2022 by Tsjerk]
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Fery
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http://npcomplete-solutions.com/Projects/Research/PhoroneSyn...
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Keras
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TBH, as an aside, I’m more interested by mesitylene, that can be used as an intermediate to phloroglucinol, the other route being trinitrobenzene,
which I would neither synthesise nor handle.
Mesitylene can be oxidised into trimesic acid, itself reduced to phloroglucinol.
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unionised
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Is allene stable in the presence of strong acids?
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Keras
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That’s a good question. Double bonds are nucleophilic, of course, but I’m not sure they would bind to H⁺ the way you form, say, bromonium ions
(if only because H⁺ would have to form two bonds, which seems unlikely).
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Triflic Acid
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It might: https://patents.google.com/patent/US4301319A/en
There wasn't a fire, we just had an uncontrolled rapid oxidation event at the power plant.
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Keras
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So
I poured a small quantity of phosphorus pentoxide into a small beaker, and added acetone dropwise to it.
Of course, it didn't work as expected.
My acetone was taken raw from a commercial bottle, and it must've contained water. So what happened? The phosphorus pentoxide went from powder to a
sort of gooish solid, and from white to yellow/ochre and eventually light reddish brown. There was a strong smell of what I think is mesityl oxide.
Probably, the water and the phosphorus pentoxide reacting have given rise to pure phosphoric acid, which has catalysed the enol condensation of the
acetone with itself into mesityl oxide.
There were some bubbles, but I’m unable to know if that is caused by acetone boiling when heated by the reaction between water and phosphorus
pentoxide, or acetone being dehydrated into propadiene.
In any case, the reaction is impossible to assess unless the acetone is 100% water free, which was to be expected, in hindsight.
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Fery
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Keras, the acetone could by dry. H2O is a product of condensation of 2 molecules of acetone into 1 molecule of mesityl oxide.
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Keras
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| Quote: Originally posted by Fery | | Keras, the acetone could by dry. H2O is a product of condensation of 2 molecules of acetone into 1 molecule of mesityl oxide. |
That’s right, but could the enol condensation have taken place spontaneously without prior water addition?
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Fery
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Keras you asked an interesting question.
Here they used CaC2 for the condensation of acetone.
https://sci-hub.3800808.com/10.1039/C8RA05965A
Read the right column on page 30611:
| Quote: | The refluxing reaction of acetone on CaC2 surface
About 100 g of acetone was added to a 250 mL three-necked
round-bottom flask and 40 g of granular CaC2 was put on the
porous glass support of the distillation column, for details
please see Fig. S1 (ESI).† The flask was heated to the boiling
state of the liquid by an electric heater and the temperature of
the liquid and vapor in the column was recorded by two thermocouples.
In the boiling state, the acetone vapor went up to
the Allihn condenser through a side pipe, and condensed
therein to its boiling liquid and dropped back to the surface of
CaC2 catalyst. In order to reduce the evaporation loss of acetone,
we used a 40% glycol aqueous solution (10 C) as the coolant
of the condenser connected to a cryogenic thermostatic bath
(DFY-10/10, Shanghai Lingbiao), and all the connecting joints
were securely sealed by PTFE sealing tape and parafilm. The
condensation reaction occurred constantly on the surface of
CaC2. The as-formed products were removed instantly from the
catalyst by the refluxing acetone to the flask, and remained
there in the whole reaction process due to their much lower
volatility than acetone and easier condensation in the side pipe
by air cooling. As such, deep condensation of acetone can be
avoided significantly. Once steady reflux had been established,
the temperatures in the flask and the reaction zone were
recorded and then the start timing for the reaction was recorded.
A syringe was used for sampling the liquid mixture at set
time intervals. The resulting water from condensation of
acetone reacts with CaC2 immediately, forming C2H2 and
Ca(OH)2. The acetylene gas was collected by a draining method
from the top of the condenser, and the Ca(OH)2 formed on the
surface of CaC2 was washed away dynamically by the down-
flowing liquid. |
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Keras
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| Quote: Originally posted by Fery | | Keras you asked an interesting question. Here they used CaC2 for the condensation of acetone. |
It never occurred to me that CaC₂ could be acting as a Lewis base. Thanks for that, it opens quite a lot of possible uses besides making acetylene.
But, to turn back to the thread subject, could P₂O₅ work as a Lewis acid and catalyse the formation of mesityl oxide even without the presence of
water (probably).
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