Fery
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2,6-dimethyl-gamma-pyrone = 2,6-Dimethyl-4H-pyran-4-one
Acetic anhydride undergoes cyclisation reaction in polyphosphoric acid into 2,6-dimethyl-gamma-pyrone (2,6-Dimethyl-4H-pyran-4-one)
Polyphosphoric acid Preparative Methods: by mixing x mL of Phosphoric Acid (85%, d 1.7 gmL−1) with 2.2 x g of Phosphorus(V) Oxide (P2O5) followed by
heating to 200 °C for 30 min.
Attachment: Polyphosphoric Acid 1.pdf (118kB)
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Experiment:
15,0 g H3PO4 85% (lit. density 1.6845 g/cm3 but so much viscous liquid is better to weigh than to measure its volume) was pipetted into 250 ml flat
bottomed flask and then 35,0 g of P2O5 added in small portions (at least 33,0 g should be used according the above method). The addition was
accompanied by evil hissing but nothing splashed / spattered, then the neck was loosely covered with a piece of plastic sheet of high melting point
secured with rubber circle to reduce H2O absorption from the air. The flask was heated on free flame for 30 minutes until P2O5 dissolved (only a
little of it didn't), the T was checked by an infrared thermometer and the T was 200-210 C.
Then cooled below 60 C at which it became more viscous than honey.
5,0 g of acetic anhydride added by weight (it should be circa 4,6 ml by volume) which immediately produced dark side-products by carbonization (the
acetic anhydride used was analytical grade, also P2O5 analytical grade, also 85% H3PO4 analytical grade G.R.).
Stir bar added (it ended on the top of viscous polyphosphoric acid and did not sink), reflux condenser attached using PTFE sleeves for joints.
Flask was heated on oil bath while stirred using magnetic stirrer. The stirring commenced only after polyphosphoric acid lost its heavy viscosity at
some elevated temperature after some heating. At the beginning the acetic anhydride refluxed a bit while the temp of oil bath circa 160 C, but later
no condensation from reflux condenser anymore. The reaction mixture was heated at 200 C on oil bath (oil bath temp. 200-205 C) for 3 hours while
magnetically stirring.
Then cooled down to room temperature and 150 ml of very cold water added, initially dropwise, later at few ml, the second half at once which produced
a lot of heat. Then it was heated and stirred by glass rod until the thick mass from the bottom dissolved, the dissolution produced some heat too as
the initial addition of cold water dissolved only some of the cold very viscous polyphosphoric acid. Original method states to pour the content into
200 ml of cold water but the mass was so viscous that it was not possible and pouring reaction mixture while still liquid at circa 100 C hot seems not
to be good idea.
Extracted with 100 ml of chloroform (maybe perchlorethylene could be used instead? density 1,63 g/cm3 b.p. 121 C vs. chloroform density 1,49 g/cm3
b.p. 61 C, or maybe dichlormethane density 1,33 g/cm3 b.p. 39,6 C, density of 45% H3PO4 1,293 g/cm3, density of 50% H3PO4 1,335 g/cm3) ???
The extraction was repeated 4 times totally, reusing chloroform by distillation from boiling water bath, the product collected incrementally in
distillation flask.
The first extraction was a quite difficult and with bad result as the layers did not separate well (due carbon and tar ???) and the bottom chloroform
layer contained a lot of water and thus also certainly H3PO4... Bottom organic phase had 150 ml... not good, because only 100 ml of chloroform used
for extraction = too much water phase contamination. Moreover both layers were so dark that they were not distinguishable on common light, a LED
handlamp helped in identification.
Bottom organic phase distilled on boiling water bath (boiling stones were not used due to impurities the solvent boiled smoothly without bumping),
collected 100 ml of chloroform which was used to reextract the upper water phase repeatedly again and again. The water content (150 ml of bottom
organic phase minus 100 ml of chloroform used for initial extraction = 50 ml of water + H3PO4 in the bottom organic phase) caused the chloroform to
boil as a mixture with steam and water was collected in the collection flask on top of chloroform.
In the distilling flask a dark product collected. Then 100 ml of water added and the extraction process repeated again with 100 ml of chloroform,
reusing it by distillation on boiling water bath (boiling stones were not used due to impurities the solvent boiled smoothly without bumping) and
topping loses with fresh solvent, product collecting in distillation flask
So for the second time again 4 extractions repeated, now good separation of chloroform bottom phase (volume of chloroform bottom organic layer always
a bit less than initial volume of chloroform), chloroform from bottom phase distilled out in boiling water bath and reused for subsequent extraction +
topping small loses of the solvent.
The last time the chloroform was not distilled out, but 1 g of active charcoal added and 1,0 g of anhydrous MgSO4 and after 15 minutes it was gravity
filtered through filter paper into a 150 ml beaker.
Solvent was evaporated out at room temperature.
The dark product was vacuum sublimed. Unfortunately it spattered and splashed in the vacuum sublimation apparatus (due to impurities and thus low m.p.
??? it first melted thus distilled and spattered instead of direct sublimation from solid phase into gas phase due to not strong enough vacuum of the
cheap chines 2-stage oil rotary vacuum pump ??? - it vacuum grade was able to lower b.p. of 2-nitrotoluene from 220 C to 70 C, pump in good condition,
new oil in it). Splashing and spattering contaminated the product on condenser part of the apparatus so the sublimation had to be repeated 5 times
(yes, five times, its not typo in the number).
Yield 0,7 g (damn too low, literature claims 1,67-1,97 g) of yellow product (damn literature claims white needles), m.p. 107-115 (lit. 132 C) - damn,
too impure.
Loses in yield maybe due to repeated sublimations? Could it undergo further condensations during workup due to H3PO4 (first extraction used 100 ml of
chloroform and obtained 150 ml of organic phase) / tar formation / carbonization ??? Or more extractions should be used or with more chloroform (what
is the distribution coefficient of the product in water/chloroform) ???
How to purify it further ??? Too low m.p. and too wide range.
Should it be colorless ??? What sideproduct colored it yellow ???
https://pubchem.ncbi.nlm.nih.gov/compound/2_6-Dimethyl-4H-py...
3.2.1 Boiling Point 251.0 °C
3.2.2 Melting Point 132.0 °C
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Fery
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reactants:
polyphosphoric acid preparation:
    
reaction:
   
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Fery
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first extraction cycle - 4 repeated extraction totally
first extraction with 100 ml of chloroform - just after shaking, waiting for a separation to occur
both phases too dark and impossible to identify the border, but LED handlamp helped in the identification of the border, bottom chloroform phase,
upper water phase
unfortunately, from 100 ml of chloroform used for the first extraction circa 150 ml of bottom organic phase resulted (with a lot of water and which is
worse phosphoric acid contamination, what else could cause the volume increase) - the bottom phase was real chloroform phase and not water phase as
addition of water did not mix with the organic chloroform bottom phase and stayed floating on top of it
chloroform recovered by distillation on boiling water bath to be reused for subsequent reextractions, product collected incrementally as a remainder
in distillation flask, the collected chloroform contained a lot of water on top of it (well visible on the photo) due to volume expansion of the
separated organic layer = massive water content in the organic phase layer
  
second extraction = good separation, good visual identification of layers, no water contamination of bottom chloroform organic layer
 
third extraction
fourth extraction
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Fery
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second extraction cycle - 4 repeated extraction totally
1st extraction of the second cycle of extractions
  
2nd, 3rd, 4th extractions of the second cycle of extractions
   
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Fery
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crude product to be vacuum sublimed, it shows some crystalline structure
 
1st vacuum sublimation
         
2nd vacuum sublimation
  
3rd vacuum sublimation
     
4th vacuum sublimation
 
the result of 5th vacuum sublimation
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Bedlasky
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So you manage to get some product from this black mass, awesome!  It's pity
that it is impure, but I think that even impure product is still success, it was initially contaminated with lot of tar.
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Fery
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Hi Bedlasky, yes I got something. But the m.p. does not match, the color does not match. The experiment and workup lasted 11 days with some
interruptions and resting days... At least I got much more skilled in vacuum sublimation
A lot of tar / black / carbon / sideproducts - well visible in the picture where I removed the condenser part of the vacuum sublimation apparatus and
cleaned the remainders in the apparatus - the part with 45/40 size naked male joint and black something inside.
And that was done with polyphosphoric acid. I cannot imagine the power of P2O5. I also stopped to be afraid of P2O5 which I had in stock for years but
only now I was brave enough to do my first experiment with it.
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Boffis
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Well done! I am impressed that you managed to extract anything from that black tar! Do you thing that the work-up would be easier if you worked on a
larger scale or is the sublimation stage hard to scale up. Could you use column chromatography given the small amount of material as an alternative to
sublimation.
Do you have any plans for the product? I understand that pyrone will exchange the O for N and yield pyridine derivatives.
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Fery
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Hi Boffis, maybe using stronger vacuum could cause direct sublimation. In my case (vacuum level which I'm unable to measure exactly but which lowers
b.p. of 2-nitrotoluene from 220 C to 70 C) the tar melted and then started to boil in the sublimation apparatus which was not good as that melt
produced bubbles which contaminated the product on condenser bottom which was only few cm above the melt. After repeating the sublimation for 5 times
the product was free of tar anyway.
I found the reason of low yield, I did only 4 extractions with chloroform in separatory funnel (and moreover I had to perform 2 cycles of 4
extractions as in first cycle the first extraction was not good), but in this reference they did this:
The cooled mixture was diluted with ice-water, and continuously extracted with chloroform for 24 hr.
Attachment: J39670000828.pdf (271kB)
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Condensation with malononitrile or thiobarbituric acid seems to be interesting as it produces interesting chromatofores:
https://sci-hub.wf/10.1021/ja01539a041
Attachment: ja01539a041.pdf (343kB)
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https://www.sigmaaldrich.com/EN/en/product/aldrich/516880
(2,6-Dimethyl-4H-pyran-4-ylidene)malononitrile
Dye intermediate used in the synthesis of an electrooptical chromophore.
condensation with malononitrile and with thiourea:
https://www.researchgate.net/publication/295546905_Synthesis...
Attachment: Synthesis_and_characterization_of_novel_4H-pyran-4.pdf (699kB)
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Complexes with SbCl5, HBr, BF3, ZnCl2, HgCl2 could be also prepared
Attachment: v61-150.pdf (267kB)
This file has been downloaded 269 times
Attached also a method of preparation by microwave irradiation (which is quickly done in 2 minutes instead of the method I did in 3 hours)
Attachment: download.pdf (69kB)
This file has been downloaded 205 times
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